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168
AND CHAMCTERIZATION
PROPERTIES
FOODPROTEINS:
teins such as milk, egg, and meat proteins. It is estimated that about 8 kg of
plant proteins is neededto produce one kilogram of animal proteins.' The low
efficiency of conversion of plant proteins to animal proteins not only increases
the cost of production of animal proteins, but also decreasesthe net availability
of proteins for human nutrition. However, as the demand for food proteins
increasesowing to population growth, and as the demand for fabricated foods
increasesas a result of socioeconomic-cultural changes,and as the demand from
the food industry for cheaper protein sources increases, the critical need for
improving the functional properties of novel proteins (e.9., plant proteins, whey
proteins, microbial proteins) for direct use in foods also will increase.
The major impediment to increasing the use of novel proteins is lack of understanding of the molecular bases of protein functionality in foods. The sensory
quality of a food does not emanate from a single functional attribute; rather it
is a product of complex interaction of multiple attributes. This implies that for
a protein to perform well in a food product, it should possessmultiple functionalities. This requirement fuither complicates proper understanding of the structure-functionality relationship of food proteins. Most of the traditional proteins,
such as milk, egg, and meat proteins, are mixtures of several proteins with a
wide range of physicochemical properties. These traditional proteins are capable
of performing multiple functions in a wide variety of foods. For example, the
multiple functionalities of egg white (e.g., foaming, emulsification, gelling, heat
coagulation, binding/adhesion properties) make it the most desirable protein in
many food applications. Although the molecular bases for the multiple functionalities of egg white are not well understood, they may be related to complex
interactions of the various protein components present in egg white.
4.2 Functional Properties
Several definitions for functional properties of food proteins have been proposed.
Kinsella2'3has defined functional properties of proteins as "those physical and
chemical properties which affect the behavior of proteins in food systemsduring
processing, storage,preparation and consumption." Pour-Ela defined functional
properties as "any property of a food or food ingredient except its nutritional
ones, which affects its use." The various functional properties performed by
proteins in various food systemsare listed in Table 4.1.5
The functional properties of proteins are related to their intrinsic physical,
chemical, and structural properties. These include size, shape, amino acid composition and sequence,net charge and distribution pattern of charges,and hydrophobicity/hydrophilicity ratio. Secondary structure, tertiary and quaternary structural arrangements, inter- and intrapeptide cross-links (disulfide bonds),
molecular rigidity/flexibility in responseto changesin environmental conditions,
and the nature and extent of interactions with other components are also
important.
Several functional properties of proteins are affected by their amino acid
ruNCTIONALPROPERTIES
Table 4.1 FunctionalRolesof I
Function
Solubility
Viscosity
Water binding
Gelation
Cohesion
adhesion
Elasticity
Emulsification
Mechanism
Hydrophihcin
Water binding.
hydrodynamrc sr
shape
H-bonding. ion hrc
Water entrapmenl i
immobilization.
network formatir
Hydrophobic. ionrc
H-bonding
Hydrophobic bond
disulfide cross-l
Adsorption at inter
film formation
Foaming
Interfacial adsorpti
film formation
Fat and flavor
binding
Hydrophobic bond
entrapment
Source: Ref. 5, reproducedwith permissron
composition. Whereas the reli
decide the net charge of a prott
philic and hydrophobic amino
binding potential, and surfacta
teins significantly affects their
thermal processing,thesesulfh
molecular disulfide bonds. Mc
sulftrydryl-di sulfide interchang
linking. These successiveunfi
the hydrodynamic size of prot
properties.For instance,the vir
to be due to intermolecular dir
tions betweengluten molecules
heat treatment is also often col
e
of globular proteins.T
Proteins that have very higt
For example, 35 out of 209 an
residuesin a"1-caseinare prolir
in gelatin are either proline or I
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I7O
FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
of these residues in the primary structure of these proteins effectively precludes
formation of a-helical or B-sheet structures.Because of high molecular flexibility, both caseinsand gelatin exhibit multiple functionalities (gelation,,emulsification, foaming, curd formation, etc.) in several food systems. In contrast,
most of the globular proteins, which possesshighly ordered structures and lack
molecular flexibility, often exhibit inferior functional behavior in formulated
food systems.It should be cautioned,however, that molecular flexibility alone
does not confer functionality to a protein; other molecular properties are also
essential.
Many physicochemical properties of proteins are related to their hydrophobic
and hydrophilic amino acid content and their distribution in the primary structure. The primary sequenceof a protein dictates not only the final three-dimensional structure and its thermodynamic stability, but also the characteristics of
the protein surface that comes into contact with its surroundings. Many empirical
approaches to correlate the hydrophobic and hydrophilic amino acid ratio of
proteins to their physicochemicalpropertieshave been suggested.rl'12
Bigelow12
proposed that the averagehydrophobicity and the charge frequency are the most
important molecular parametersthat have the greatest influence on such protein
physicochemical properties as solubility. Proteins with lower averagehydrophobicity and higher charge frequency are expected to have higher solubility.
Although phenomenologically, this empirical correlation seems to be true for
severalproteins,there are exceptions.r3It has been pointed out that the solubility
of a protein is more related to the nature of the protein surface and the thermodynamics of its interactions with the surrounding solvent than to the global
average hydrophobicity and charge frequency of the whole molecule.'3 Specifically, protein solubility is affected by the folding pattern of the polypeptide and
the physicochemical properties of the resulting protein-water interface.
Many functional properties of food proteins are affected by the properties of
the protein surface. The folding of a protein is guided by two thermodynamic
considerations: that a majority of the hydrophobic residues are buried inside,
and that a majority of the polar residues,especially the charged residues,necessary to keep the global free energy of the protein molecule at the minimum,
are at the surface being exposed to the solvent. In keeping with this general rule,
globular proteins tend to have the hydrophobic residues in the interior and the
hydrophilic groups at the exterior of the protein molecule. However, in most
proteins, while almost all the hydrophilic residues are located on the surface,
not all hydrophobic residues are buried in the interior. Because of steric constraints imposed by the polypeptide chain, it seemsthat it is physically impossible to bury all hydrophobic residuesin the interior of the protein. As a result,
in many globular proteins, about 40-50% of the protein surface is found to be
nonpolar patches,distributed uniformly on the surface.'aThe extent as well as
the pattern of distribution of hydrophobic patches on the protein surface would
significantly affect the functional properties of food proteins. For instance,the
surface hydrophobicity will have a direct bearing on the solubility characteristics, the tendency to form dimers, oligomers, and micelles (as in soy proteins
FUNCTIONAL
PROPERTIES
and caseins),and the surfacta
of exposureof hydrophobic rr
stability of the protein. protein
surfacesare less stable than t
buried in the interior. In esse
tribute to structural stabilin. t
bility and interfacialpropertie
In addition to the foresoine
erties of proteins are affeJted i
isolation(e.g., ion exchange\
perature,the extent of purifica
method of functionality measu
nents.Some of the major facto
are summarizedin Table 4.2.t:
of the extrinsic factors on the 1
ifestationsof alterationsdue to
icochemical properties of the p
In a broader sense,the vario
regardedas manifestationsof r
namic properties and the surfa
ties are predominantly relatedt
protein. In contrast, the surfac
phobic, hydrophilic, and steric <
otherphasesin a system.'oThe
thickening, and texturization ar
namic properties,and the functi
solubility, foaming, emulsificat
the properties of the protein su
more influencedby the physica
Table 4.2 FactorsThat Influenc
Intrinsic
Composition of protein(s)
Conformation of protein(s)
Mono- or multicomponent
pl
o:
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Source: Ref. 15. reproducedwith permission
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172
AND CHARACTERIZATION
FOODPROTEINS:PROPERTIES
composition, the properties of the protein surface are affected exclusively by the
amino acid composition, distribution and the folding pattern, and less by the
actual shape and size of the protein molecule. Understandably, there might be
several exceptions to this general rule.
The fundamental relationship between the conformational, hydrodynamic,
and surface properties of food proteins and their functional behavior in food
systems is poorly understood. Most of our current knowledge of the functional
properties is based on the behavior of individual proteins in simple model aqueous systems. More often than not, the results obtained from model aqueous
systems fail to predict the functional behavior of proteins in real food systems
piepared under industrial processing conditions. The extensive conformational
changesthat occur in proteins under industrial processing conditions, as well as
multilateral interactions of the protein with other food constituents, make it
impossible to translate the results of model systems studies into predictions of
behavior in real food systems.tt'tt Another reason for insufficient understanding
of the structure-functionality relationship of food proteins is the lack of standardized methods for measuring the functional properties.3It has been reported
that the quantitative measurementof functional properties is affected by type of
equipment used, sample size and geometry, method of sample preparation and
Although the literature contains a wealth of data on the funcother factors.3'1e'to
tional properties of various food proteins, variations in the methodologies and
proceduresused from laboratory to laboratory seem to prevent meaningful evaluation of the results. Despite these shortcomings, considerableprogresshas been
made in recent years toward understanding the role of various physicochemical
properties of proteins in the expression of functional properties.
Proteins are capable of performing a variety of functions in various foods.
However, a simple survey of processed foods would reveal that a majority of
them are gels, emulsions, or foams. In this respect, the gelling, foaming, and
emulsifying properties of proteins are preeminently important for the use of these
substances in food products. Therefore, this chapter focuses on the various
molecular factors and physicochemical principles that are involved in gelation,
foaming, and emulsifying properties of food proteins. Since the gelling, foaming,
and emulsifuing properties of proteins are very much influenced by their solubility characteristics,this chapter also discussesprotein-water interactions and
solubility.
FUNCTIONAL
PROPERTIES
4.2.1.1 Hydration and
Thebindingof waterby dry p
usually exhibitsa sigmoidalsh
ity. For a typical protein. the
humidities (i.e., at a* < 0.05
followed by a slow increasetr
humidity the water uptake incrr
protein concentrateis shown in ,
isotherm shows that water binc
steps.23The four regions in the
states of water in hydrated prot
egories of water in hydrated p
water refers to the water molec
ture. Thesewater molecules.cht
ing, might be critically importa
water molecules are neither ar
terms of food protein functiona
trJ
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a
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4,2.1 Protein-Water Interaction
Several important functional properties of proteins (e.g., solubility, wettability,
dispersibility, thickening, foaming, emulsification, and gelling properties) are
afficted by the extent of interaction with solvent water.s For instance, the rheological properties of wheat dough and the tenderness of meat and meat anaIn contrast, the solvation and
logues are affected by water-binding capaciry.21'22
gelling, foaming,
thickening,
proteins
their
affect
of
characteristics
dissolution
properties.s
emulsification
and
Figure 4.1. Sorptionisothermsf
ref. 24, reproduced
with permissi
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FOOD PROTEINS:PROPERTIESAND CHARACTERIZATION
t74
Table 4.3 Categoriesof Water Associated with Proteins at Progressively
IncreasingValues of a*
FUNCTIONALPROPERTIES
Table 4.4 Water Uptakeat 90%
Structural Water
Water H-bondedto specificgroupsin the protein;participatesin stabilizationof protein
structure;l0-20 moleculesper protein;very difficult to remove.
Monolayer Water
The first monolayerof highly structuredwatermoleculeschemisorbedvia hydrogenbondingand
ion-dipole interactionsto polar and chargedgroups,and hydrophobichydrationofnonpolar
groupsof the proteinin a* range0.05-0.2;this water (2-10 g/100g protein)haskinetic and
thermodynamicpropertiesdifferentfrom bulk water and is unavailableas solventfor most
chemicalreactions.
Multilayer
Vl/ater
Layersof H-bondedwater with progressivelydecreasingstructurethat surroundthe structured
water layer in a* range0.3-0.7; the "average" thermodynamicpropertiesare intermediate
betweenthoseof structuredand bulk water.
Protein
Bovine serum albumin
Casein
Chymotrypsinogen
Collagen
Cytochrome c
Gelatin
Hemoglobin
Insulin
B-Lactoglobulin
Lysozyme
Unfreezable Wster
Includesall orderwater up to c* 0.9 that doesnot freeze;the amount(50 g/g protein)varies
with amino acid content:mav be availablefor somechemicalreactionsand as solvent.
Capillary Water
Waterheld physicallyin clefts,voids, or cavitiesby surfaceand capillaryforcesin the protein
in gels);similarto bulk waterin physical
moleculeat a* 0.5-0.95(e.g.,waterentrapped
properties,availableas solvent,and for chemicalreactions.
Myoglobin
Ovalbumin
Ribonuclease
Salmin
Source: Ref. 23, reproducedwith permission.
Hydrodynamic Hydration ll'ater
with the proteinat a* )0.99 that is transportedwith the protein
Water "loosely" associated
during diffusion;haspropertiesof normalwater,but affectsviscosityand diffusion of the
protein.
structural water varies from protein to protein depending on the folding pattern;
typically, it is between l0 and 20 water molecules per protein molecule.2T
The monolayer water refers to the water molecules tightly bound to the protein
surface via dipole-induced dipole (hydrophobic hydration), ion-dipole (ionic
hydration), and dipole-dipole (hydrogen bonding) interactions with polar groups
on the protein surface.23The formation of this initial layer of water on the protein
surface occurs in the water activity range of 0.05-0 .3.24At the saturatedmonolayer coverage,most proteinsadsorbabout 0.3-0.5 g of water per gram of protein
(Table 4.4)." This monolayer water is unfreezableand does not participateas a
solvent in chemical reactions.In most literature,this monolayer water is referred
to as "bound" water, which connotesirreversiblebinding to the protein surface.
However, several lines of evidence indicate that this perception of "bound"
water is technically invali6.zs-:oIt has been shown that at room temperature,the
Moreover,
monolayer water freely exchangeswith those in the bulk phase.3r'32
it is highly mobile comparedto that of the water moleculesin ice3t'33un6 has a
heat capacity close to that of br
the adsorption of water to lysoz
also support this hypothesis.It I
adsorptionup to 0.07 glg proteir
fold greater than the thermal er
the hydration range of 0.07-0.
adsorption drops precipitously
majority of the water molecules
and not "tightly bound" to the
At the monolayer coverage.a(
binding of water molecules to b
Table 4.5 ThermodvnamicPara
Hydration Range
(g water/g protein)
I. 0.38- m
rr.0.27 - 0. 38
( 0. 32s)
( 0. t 7)
rrr.0.07- 0. 27
rv. 0
0 . 07( 0. 3s)
Source; Ret 26. reproducedwith permission
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NOIJVZIUSJJYUVHJ CNV SSIJU
AND CHARACTERIZATION
FOODPROTEINS:PROPERTIES
176
and nonpolar groups on the protein surface. The water-binding capacities of
various amino acids are given in Table 4.6.24Amino acid residueswith ionized
side chains bind the greatestamount of water. Kuntz3s showed that the hydration
capacity of a protein is related to its amino acid composition, and that it can be
obtained using the empirical equation:
(4.1)
o: f.+0.4fe+0.2f"
where a is grams of water per gram of protein, ,f" is the fraction of the charged
residues,/o is the fraction of polar amino acid side chains, and f , is the fraction
of nonpolar residues in a protein. The hydration capacities of various proteins,
calculated from equation 4.1, reasonably agreed with the experimental results.35
This is rather surprising, becauseneither the conformational differences among
proteins nor the degree of accessibility of nonpolar and polar residues, nor the
Table 4.6
Proposed Amino Acid Hydrations Based on Nuclear Magnetic
Resonance Studies of Polypeptides
Amino Acid Residues
Hydration
(mol water/mol amino acid)
Ionic
Asp
Glu
Tyr
A.g*
HisLyr*
6
7
7
J
4
4
Polar
Asn
Gln
Pro
Ser,Thr
Trp
Asp
Glu
Tyr
Arg
Lys
2
2
Source; Ref. 23. reoroducedwith oermission.
topographicaldifferencesamo
the hydration capacity of prr
reported that even upon dena
increasesonly by a small am
additional binding sites.3sInr
taining varying amounts of nz
binding capacity was not affe
samples.38Although the intrin
acid composition, extent of de
hydration capacrty,,several exr
ture, particle size of protein I
capacity of proteins.'4,3n'oo
Mos
at isoelectric pH,35presumabl
and enhanced hydrophobic intt
Water binding to proteins a
tilayer formation. Unlike the n
for chemical reactions,but son
sharp increase in water uptake
of the protein, which refers ro
capillaries and crevices on the p
able and have properties simil
events at the molecular level d
illustrated in Figure 4.2.26
In terms of protein functio
meats, and simulated meat pro
in the form of capillary water
monolayer hydration. The grea
textural and mouth-feel qualiti,
water, should be able to form z
via noncovalentinteractionsun
J
2
2
2
2
(3)
a
J
4
Nonpolar
Ala
Glv
Phe
Val
Ile, Leu, Met
FUNCTIONALPROPERTIES
I
I
I
(0)
I
I
4.2.1.2 Solubility
Solubility is often consideredto
in several food applications.: F
foods that require gelation, eml
uble protein is desirable.Comr
solubility characteristicsas a r
processing.For example, moist
lipoxygenaseand inhibitors of
proteins.arSolvent extraction f<
also reducessolubility. Althoug
highly soluble even at their iso
in commercial whey protein co
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LLI
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pezruor glr^{ senprseJpr33 ourr
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NOIJVZIUSJJVUVH
J CNV SSIJ
178
FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
_>
<__
+--
-->
Figure 4.2. Sequential steps in the protein hydration process: (A) dry protein, (B) initial
hydration at the sites of ionizable groups, (C) formation of water clusters near the polar
and charged protein surfaces, (D) completion of hydration at the polar surfaces, (E)
hydrophobic hydration at the nonpolar surfaces;completion of monolayer layer coverage,
(F) bridging of water associated with protein with the bulk water, (G) completion of
hydrodynamic hydration. (From ref.26, reproduced with permission.)
FUNCTIONAL
PROPERTIES
This property is mainly attribu
of the whey at elevated press
of the solubility of food prote
understandthe factors. both i
solubility.
The solubility of a protein r
the thermodynamic manifesta
and protein-solvent interaction
from the interactions of hvdrc
with the surrounding aqueous
of a protein is related to the ar
and the charge frequency of th
and higher the charge frequen
tionship is shown in Figure ,l.:
to be valid, it fails to explain
For instance,myoglobin and se
quency (0.34 and 0.33, respec
al bumin( 1120 callm ol r esidue
residue).12Based on these r.a
albumin to be lower than that
myoglobin is insoluble at its isc
in the pH range 3-9. Similarly
isoelectricpH, the whey proteir
pH range 3-9.44These obsen'a
proteins may depend not simp
frequencybut on the physical ar
and the thermodynamicsof its i
In other words, the surface h.t.d
tein may have a direct bearing ,
The water-soluble proteins
amino acid residuesand a high
all the charged and hydrophilic
hydrophobic residuesare buried
30% hydrophobic amino acid r
internalize all the hydrophobic
presentat the surfaceexposedto
attractionbetweenthe nonpolar
and repulsion between the pro
solubility.
The solubility of proteins is
types, temperature,solvent poll
affect the solubility of proteins
hydrophilic, and hydrophobic int
of most proteins at their isoele
aS;eqc Jo uor]€zlleJlneu ol enp sr slurod crJlceleosrJreql le sureloJd lsoru 3lo
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'Atrlrgnlos
Je,/Y\oluqrqxs IIr,/y\urelotd eql 'selnceloru urelord eq] uae.{\}eq uorslnde"rpue
uorlerp,(q cruor eqt u€ql req8rq qcnu sr seqclzd reloduou eql uoo^UequorlJpr]le
crqoqdorpr(qeqlgl 'surelo;dqcns uJ ']uellos eqt o] pasodxeocqrns eq] ]e tuese;d
ers sonprsor oseq] Jo or.uosoroJoreql lsenprser crqoqdorpfq aql IIe azrletuelur
01 elqrssodrur ,,(llecrrelssr 1r 'rene,rrroq'senprsal prce ourru€ crqoqdorp Ktt olrOt
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ellq^\ ]€ql 'relervroq 'punoJ sr lI 'urqo13o.(urJo 1€q] u?ql ra./y\o[oQ ot urlungle
ruruos go flrpqnlos or{l lcedxe plno,^Aeuo 'sen1u^eseq} uo pes?g .,'(enprsa-r
Iou/lec 0601) urqolEo,(uJo tur{l uerl};atear8 sr (enprserlor.u/lec0Zl) urunqle
'gE'g
ruluos go flrcrqoqdorpfq a8ereneeql lnq '(,(ye,ulcadse-r
pue
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fir1rqn1oseq] ]et{l pelseEEns.,,Lro1a8rg
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Sutsuee8ueqcr(8rauaoo{ teu eq} o} pe}Blersr }I 'suol}cere}urluollos-ura1o:d pue
utelord-utelo;d uoo,t.rlequrnrJqrlrnbe eqt Jo uorlelseJruerucruuu,(pouJeql aqt
sr suorlrpuoclelueruuoJrlueJo les uenr8 e Japun urelo.rde go ,{1r1rqnlo-s
eqJ
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ol lueurued sr 1r 'suorlecrlddepoog fueu ur surelord poog yo ,frlrqnlos eq] Jo
ecueuodruraql Jo ^\ern ul zr'eJnleraduolpue eJnssardpelenalo l€ ,(eqm eql Jo
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r80
AND CHARACTERIZATION
FOODPROTEINS:PROPERTIES
FUNCTIONAL
PROPERTIES
>€
=
-
o
.=
J
$
AA
u
o
q
E
u')
Figure 4.3. Relationshipbetweensolubility, chargefrequency,and hydrophobicity.
with permission.)
(From ref.43, reproduced
repulsion among the protein molecules; this promotes aggregation via hydrophobic interactions. In proteins that show no or least precipitation at their isoelectric pH (e.g., serum albumin, whey proteins), the extent of hydrophobic
patches on the protein surface is very low, and the hydration repulsion forces
apparently more than overcome any tendency for aggregation via hydrophobic
interactions.
Both pH and ionic strength affect solubility of proteins via their effect on
electrostatic forces. Shena6studied the effects of pH and ionic strength on the
solubility of soy protein isolate (Figure 4.4). At pH 6.8, the solubility of soy
protein isolate decreased very slightly at 0.05 ionic strength and remained
unchangedat higher ionic strength.At pH 4.7, however, the solubility increased
with increaseof ionic strength up to 0.6, whereas at pH 2.0 the solubility progressively decreasedwith increase of ionic strength, until reversing direction at
an ionic strengthof 0.6 (Figure 4.4). The data in Figure 4.4 representthe classic
Figure 4.4. Effectsof pH and i
(From ref. 46,reproduced
with pe
example of the complementary
point of proteins. At pH 2.0 tl
ionic strength is increased. prt
moleculesprogressivelydecre
net chargeis progressivelydec
becauseof enhancedhydropho
pH 4.7, which is close to the i
equal numbers of positive and
is increased,becausethe bindi
Na* to COO-, the chloride io
-ord r{crq,tr'urelord eq} o} spulq fllequere;erd uot optrolt{o aq} '_OOJ o} *eN
'paseeroul sI
Jo t€q] ueqt re]BerEsr fg51 ol _lJ go ,firug;e Surpurq eqt esnsceg
'se8reqcenrleEeupue e^IlISod;o sraqlunu
lenbe
IJeN Jo uor]?rluecuoceql sV
serilec urelord aql 'surelord .{osJo Hd clJ}celeoslogt ol osolc sI qclq.& 'L', Hd
'solncelotu
pecuer{uo
uorlcuJelur
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eqt
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'sutelord;o
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(gV 'Jeruorg)
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NOIJVZIUSJJYUVHJCNV SSIJU3
182
FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
gressively increasesthe electronegativityof the protein. The increasedelectrostatic repulsion between protein molecules destabilizeshydrophobic aggregation, which results in increasedsolubility. At pH 6.8, at which both the carboxyl
and amino groups are fully ionized, despitethe binding of Na* and Cl- ions to
the counterionson the protein, the protein retains a net negativecharge and thus
maintains its solubility.
In addition to the ionic strengtheffect, certain salts exert ion-specific effects
on the solubility characteristicsof proteins.4T'48
Whereas the ionic strength
effects of salts act on the electrostaticforces in proteins, the ion-specific effects
of salts are related to their effect on hydrophobic forces.oT'a8
The solubility profiles of soy protein in various sodium salt solutions are shown in Figure 4.5.4e
The solubility of soy protein decreasedin all salt solutions up to 0.15 M. Above
0.15 M, while I-, Nol, Br ', and cl- increasedthe solubility, so?- further
decreasedthe solubility. A similar behavior was also observedin the caseof the
solubility of gluten in various salt solutions.tuThese salting-in and salting-out
effects of various neutral salts on protein solubility are believed to be related to
their chaotropicand cosmotropiceffects,respectively,on bulk water structure.aT
Denaturation of proteins during processing significantly alters the hydrophobic/hydrophilic balanceof the protein surface.Subtle conformationalchangesin
N H 4N O I
d'e
-l
FUNCTIONAL
PROPERTIES
.=
E
.ct
5
-
o
a
Figure 4.6. The pH versussolu
whey (!), and sulfonatedsweetr,r
w i th 0. 1M EDTA at pH 4. 5 ( A) .
N H oB r
960
proteins also can cause dramat
ple, Gonzalez and Damodaran
bond per 43,000g'mol of prote
method causeda dramatic cha
sweet whey (Figure 4.6).
f
(n
( N H o1 , 5 9 0
4.2,2 Interfacial Prope
l S a l tl a d d e d(Ml
Figure 4.5. Effectsof varioussaltson the solubilityof soy proteinisolate.(Fromref.
49, reproduced
with permission.)
A majority of fabricated and
gel-type systems.An emulsior
dispersedphaseand a continur
of the interfacial tension betwe
resulting in phase separation
suited than small molecular w(
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aJor.u,{11eepr
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NOIJVZIUSJf,YUVHJCNV SSI
184
FOODPROTEINS:PROPERTIES
AND CHARACTERIZATION
tants in emulsion- and foam-type products. This is because,in addition to lowering the interfacial tension, proteins can form a continuous, highly viscous film
at interfaces via complex intermolecular interactions. The high viscosity and
high dilatational modulus of protein films impart the ability to withstand mechanical shocks, thus rendering foams and emulsions stable for longer periods of
time than can be achieved with small molecular weight surfactants.
Although all proteins are amphipathic, proteins differ very remarkably in their
surface active properties. Since an examination of the amino acid composition
of various proteins reveals that they differ only within a narrow range, differencesin their surface active properties cannot be attributed simply to differences
in amino acid composition. Furthermore, the average hydrophobicity values of
several food proteins as well as biologically important proteins differ by only a
small percentage.5tTherefore, the remarkable differences in the surface activities
of proteins must arise from conformational differences as well as differences in
the physicochemical characteristicsof their exterior surface, the area that comes
into contact with other phases in a food system. In a broader sense,the factors
that enormously influence surface activity are conformational stability and/or
flexibility, adaptability of the protein conformation with respect to changes in
the environment, and symmetric or asymmetric distribution of hydrophilic and
hydrophobic surfaces.These differences in molecular properties will collectively
influence the surface active properties of proteins. Knowledge of the fundamental role of each of these molecular properties on adsorption and film formation
of proteins at interfaces is highly critical; because of the interdependence of
these molecular properties, however, such elucidation has been difficult to attain.
Changes in one property cause de facto changes in other molecular properties
and thus confound systematic analysis of various factors that affect the behavior
of proteins at interfaces.
The dynamics of protein adsorption and film formation at an interface is very
different from that of simple low molecular weight surfactants.While in the case
of small molecules the entire molecule adsorbsand instantaneouslyorients itself
between the aqueousand nonaqueousphases,the adsorption of proteins proceeds
through sequential attachment of several polypeptide segments. In most cases,
a greater portion of the molecule remains suspendedin the aqueousphase in the
form of "loops" and "tails." The retention of the adsorbed molecule at the
interface against thermal motions depends on the number of segmentsinvolved
in the attachment and the sum of the free energy of adsorption of all segments.
Since the first step in the formation and stabilization of protein-stabilized foams
and emulsions involves transport of the protein from the bulk phase to the interface, much attention has been directed toward understanding the molecular factors that affect the kinetics of adsorption of proteins at the interfaces.A summary
of some important studiesto date is presentedin Section 4.2.2.1.
4.2.2.1 Kinetics of Adsorption
Because proteins are amphiphilic, they tend naturally to migrate toward interfaces, where the global free energy of the protein is lower than at either of the
FUNCTIONAL
PROPERTIES
phases.Ward and Tordaisr fi
other amphiphilic molecules
irreversibleprocessit is gir en
dr
Do \''
^(
C o [:
;:
" \ 3 . 14 l 6 t/
dr
I
or
/
D o t\
-^l
f : 2Co[ :
I
\J.l4l6l
where f is the surface concen
the diffusion coefficient, and I
f versus /1/2would be linear.
In several investigations,hou'
surfacepressuresfor most prot
tion of proteins, calculated frc
nificantly from that of a diffu:
has been observed even in the ,
as fatty alcohols and alkyl sulf
Tordai52 first proposed that al
the interface. MacRitchie and
sure increases(i.e., interfacialt
the nonlinearity of f-1r"2 plotr
surface pressure barrier. That
the rate of adsorption would b
df
/ - rL4'
- : KC6 exp[ *
d
t
\
f
t
/
or
/ar\
l n l; l : l n ( K C o ) - -
r
\dr /
where rr is the surface pressur
for a protein molecule to adso
Boltzmann constant, and f is t
adsorptionis irreversible.Bas
surface pressure rr, it should I
Assuming that rr is proportion
can be transformedto:
^(#):rn(KC6)-
According to equation 4.6, a 1
slope of A,AlkT.
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186
FOODPROTEINS:PROPERTIES
AND CHAMCTERIZATION
The A'A values for several proteins at the air-water interface are listed in
Table 4.7.seNo data could be found in the literature for the oil-water interface.
The experimental LA values are much smaller than the cross-sectional area of
proteins. For most proteins the values are between 50 and 200 A and seem to
be relatively independent of the size, shape, amino acid composition, and other
physicochemical properties of the proteins. Since the cross-sectionalarea of an
amino acid residue is about 15 A2,the L,A values in Table 4.7 suggestthat for
the initial penetration and anchoring of a protein at the air-water interface, a
peptide segment of only about 4-10 amino acid residues, strategically located
on the surface of the protein, will suffice.
Recently, several investigators have crittcized the validity of using equation
4.5 or 4.6 to explain the energy barrier to adsorption and have questioned the
physical meaning of L,A values obtained from such treatments.55'5e-6t
DeFelter
and Benjamins55have pointed out that it is physically impossible for a globular
protein to bind itself to an interface through a hole as small as 60 A2. Recent
studies6ron the adsorption of raC-labeled B-casein and bovine serum albumin
(BSA) at the air-water interface have also questioned the validity of equation
4.5.In the treatment of the energy barrier theory, it is assumedthat equation
4.2 ts valid only for the very early stages of adsorption (i.e., when r < 0.1
mN/m).53 Above 0.1 mN/m surface pressure,becauseof surface pressurebarrier
Table 4.7 Area (AA) Clearedby ProteinsDuring Adsorptoin at the Air-Water
Interface
Adsorbate"
Myosin
Human 7-globulin
Human albumin
Ovalbumin
Lysozyme (native)
Lysozyme (acetylated)
Bovine albumin (native)
Bovine serum albumin (acefylated)
BSA structural intermediatesb
I
1
L
Co(wt%)
0.003
0 .0 0 1
0.002
0.003
0 .0 0 1
0.005
0.001
0 .0 0 1
4
5
6
7
Native
"
'
Values from Ref. 59, with permission,except as noted.
Values from Ref. 56, with permission.
Mol wt
(x l0-3)
8s0
180
70
44
l5
l5
70
70
70
70
70
70
70
70
70
Al (nm2)
1.45
1.30
1.00
|.75
1.00
2.4 -r0.20
1.00
0.5 -F0.08
1.35
0.52
0.49
0.65
0.57
0.60
0.77
Approximate
Number of
Residues
t0
9
6
t2
6
t6
6
3
9
4
a
J
4
4
4
5
FUNCTIONAL
PROPERTIES
buildup, not every collision c
results in the nonlinearity of r
disagreewith the adsorptionb
tt/2 was linear up to z- : 1,5m
of surface pressurebarrier. th
(Figure4.7A), suggestingthat
the adsorptionof native bor ine
a nonlinearf-r'/2 kinetics(Fig
of surfacepressurewas lorr er
an energy barrier to adsorptio
difference might be that the ar
small compared to that of ser
exponential term in equation
high surfacepressure.Hou'er t
shows that for the exponentia
Al has to be less than I A:.
amino acid residue(Al : 15
interface. These inconsistenc
that the energy barrier to ads
physicochemicalconstraintso
the interface.
The mechanism of adsorp
and eludes proper understan
adsorption, it is assumedthat
at the subsurfaceimmediatel
centration at the subsurface
subsurfaceand the bulk phas
bulk phase to the surface. Up
cules are immediately and in
course of adsorption, the sub
zero.In this treatment,it is as
ecule is always lower at the su
bulk phase and that every coll
basic premise may be an o\'(
adsorption.
In a recent study on the ad
interface it was observed that
positive adsorptionto the inter
interface,the native lysozynter
migrated away from the freshl
face) during the first hour of a
stages.62
On the basisof these
ical potential 6ptl5( (where {
which drives the molecule fror
the concentrationsradient but
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NOIJVZIUSJJYUVHJCNV SSI
PROPERTIES
FUNCTIONAL
The chemical potential of a P
hydrophobicity, surface hydro
F:
whefg
.C
th
s'
1.5
I
E
g
o
.E
g l.o
o
I
6
o
o
o
qt
E 0.5
Q
50
Tit"tP
100
("tE)
Figure4.7. Plotsof surface
concentration
f versust'''. (l) ForB-casein
adsorption
at
the air-water interface. D, 5 x l\-so/o; L, l0-4o ; o,2 x l04oh. (B) For native BSA
adsorption at the air-water interface: A, 5 X l\-so/o; o, l\aYo; a, 2 x l0-4oh; A, 5 x
10-ao/o;o, l0-3o/o.(From ref. 61, reproducedwith permission.)
188
llsa,
-
lJ
lJ-"
4.2.2.2 Influence of P
E
0
ltrideat, flco.,f'
ll.,,,'r
entropy, hydrophobic, electro
tively. Whereas the chemical
adsorption of proteins, the Po
to adsorption.Proteins (e.g.. ,
pH6 are much greater than th
energy barrier to adsorPtionr
lysozyme) in which the sum o
and ps6 may experiencean er
words, the energy barrier to at
hydrophilic, and conformatio
barrier at the interface.
40
tw,
Fia.ul *
To explain the influence of P
Graham and Phillips and co-'
adsorption at the air-water ir
albumin, and lysozyme. The
followed the order B-casein
explained in terms of differen<
B-caseinis a flexible randomzyme is a highly structured n
random) with four intramolec
upon adsorption at the air-w
spreads,and occupies a grea
the interfacial tension. In col
lysozyme adsorbsslowly and
face, which minimizes its abil
surface coverage. The kinetic
rate of change of surfacepres
coverage but is also affected
Theseearlier studieshave sug
possessthe following molec
adsorption to the interface. (
interface,and (3) the ability t,
interactions at the interface.
Damodaran and Songs6'6
mation on adsorption can be
structural intermediates of a r
acid composition and sequen
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190
FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
protein, the differences in the adsorption behavior of the structural variants can
be attributed unambiguously to conformational differences alone. Using this
rationale, Damodaran and Songs6'6'studied adsorption of native and several
structural intermediates of bovine serum albumin at the air-water interface. The
unfolded BSA intermediates exhibited a much higher rate of change of surface
pressure as well as higher steady state surface pressure than the native BSA.
Among the intermediates studied, a positive correlation was observed between
the rate of change of surface pressureas well as the steady state surface pressure
and the extent of unfolded state of the intermediate. Estimation of the apparent
diffusion coefficient according to equation 4.3 showed that the apparentdiffusion
coefficient increasedprogressively with the extent of unfolded state of the BSA
intermediate.56Similar observationswere also made in a related study.6t
The observations just mentioned raise several interesting questions. For
instance, according to the Stokes-Einstein equation, D : kTlf (where frZ is the
thermal energy of the molecule, and f is the frictional coefficient), the diffusion
coefficient of a molecule is inversely proportional to its frictional coefficient,
which is related to the hydrodynamic radius. Since the hydrodynamic radii of
the BSA intermediates were larger than that of the native 85,{,6I the apparent
diffusion coefficients of the BSA intermediates should be lower, not greater,
than that of the native BSA. The diffusion coefficients of several other proteins,
calculated from surface adsorption studies, also have shown some
abnormalities.55
Several studies have reported that under comparable bulk phase concentrations, the rates of adsorption of several proteins were greater in the denatured
statethan in the native state,63'6a
suggestingthat the initial structure of the protein
in the bulk phase influences the kinetics as well as the equilibrium adsorption
at an interface. These observations also indicate that the mass transport of a
protein from the bulk phase to an interface is not simply dependent on its diffusivity; instead, it is critically affected by its conformation and the physicalchemical nature of its molecular surface. One possible reason for the higher rate
of adsorption of denaturedproteins might be the greater exposureof hydrophobic
patches, which were initially buried in the interior of the native protein. The
availability of a greater number of hydrophobic patches in denatured proteins
might increasethe probability of successof each collision of the protein at the
surface/interface,leading to its irreversible adsorption (Figure 4.8). Thus, in the
simplest case,equation 4.3 can be rewritten as follows:
t:2coP^(#h)"
(4.8)
where Pu is the probability factor, which in essenceis directly proportional to
the number of hydrophobic patches on the protein surface. It should be borne in
mind, however, that if the percent fraction of hydrophobic patcheson the surface
of a protein exceedsa critical level,,it might impair the solubility of the protein
and thus affect its surface activity. Therefore, an optimum ratio of hydrophilic
PROPERTIES
FUNCTIONAL
Hydrophilic
represe
Figure 4.8. Schematic
ability of adsorptionof protein
permission.)
to hydrophobic patches on the
maximum surface activity.
4.2.2.3 Electrostatic I
It has been pointed out that tht
forms of adsorption of protei
charge of the protein is zero
values away from the pI, prote
The net chargeon a protein c
to adsorption. First, rf e is thr
dielectric constantsof the aq
electrostatictheory'uuan imag
the low dielectric phase.If { it
electrostaticrepulsive potenti
F c l e:
ee'
zg"n:
e'
t11
zd,i
It hasbeen suggestedthat in th
is clean), this repulsivepoten
the interface.utIf protein mo|
interface,there would be a ten
tial, to desorbaway from the i
'puoceg 'eceJJolul
qJosopo1 '1et1
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tuor;
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e,rtslnderstql '(uee1cst
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Gv)
e 1 oe os]7
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(
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eql 'ecegraluloql ruo4 utetord eqlJo ecuslslp eql sI | 31
ur reedde lllr\\ (s + os)l@ - os)a : ,o eEreqc eEeurt ue nn'floeql cll€]sorlcole
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']srlC 'uorlfuospe o1
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NOIJVZIUSJJWVHJCNV SsI
192
FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
has been adsorbedto the interface, the adsorbedprotein molecules will create
an additional electrical potential barrier65:
pp : eQ
(4.10)
where r/ is the potential in the two-dimensional plane of charged proteins at the
interface.
MacRitchie and Alexander6s studied the influence of the electrical barrier on
the adsorption of lysozyme by spreading monolayers of various negatively and
positively charged substratesat the air-water interface. It was shown that the
rate of adsorption of lysozyme was faster when the surface potential of the
monolayer was negative and slower when positive. Song and Damodaran6T
reported that the rate and extent of adsorption of B-lactoglobulin at the air-water
interface decreasedprogressively with increase of the extent of succinylation
(Figure 4.9). The rate of increase as well as the equilibrium surface pressure
increased with increase of ionic strength (Figure 4.10) The equilibrium surface
pressuresof both the native and 99o/osuccinylated B-lactoglobulins approached
^ 2 0
E
z
E
q)
l-
o
U'
o
e to
o
o
(U
U)
^
E
z
?E 2 0
E
()
=
o
o
o
ct
o
o
z
l-
E
l-
o
b
=
g,
(t,
o
$t
L
o.
o
o
t-
=
a
l0
(U
o
T i me (h)
Figure 4.9. Time-dependent
increasein surfacepressureof succinylatedB-lactoglobulin (B-lg) solutions;protein concentration,2x l0-4oh: o, native F-lg; o, 29o/osiccinylatedB-lg;2,50% succinylatedB-lg;t,69% succinylated9-lg L,83% succinylated
with permission.)
F lg; L,99% succinylated
B-lg. (From ref. 67, reproduced
Figure 4.10. Effect of ionic str
native and (B) 99%osuccinYlate
i o n i c s t r e n g t ho
: , 0 . 0 3 8 ;r , 0 . 0 5
permission.)
(uorssrured
q l r , ^ p e c n p o ; d e ; ' L g ' J eur r o r g ) ' ? ' 0 ' l i Z ' 0 ' o l l ' 0 ' V l S 0 ' 0 ' t : 8 E O ' 0 ' e : L l l b u e r l sr t u o l
,.6/n.,01
palelfurccns o 66 (g) pue enlleu
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'61'7 arnS;g
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r94
FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
each other as the ionic strengthwas increased,indicating that the poor adsorption
of the 99oZ succinylated B-lactoglobulin at low ionic strength was due mainly
to high electrostaticpotential energy barrier to adsorption.
Although the electrostaticpotential barrier affects the rate of adsorption,once
the protein molecule has been adsorbedat the interface,the electrostaticforces
positively contribute to the surface pressure of the protein film. The surface
pressureof a protein film at any given surface concentrationis given byuo'
tt aw
-
,, kin
I
'
,, ele
I
|
( 4 . 1l )
/l coh
where rrkin, Tete,and Tt"o6zre the surface pressure contributions from kinetic,
electrostatic and cohesive forces. The electrostatic contribution to the surface
pressureof the protein film is given byun'
r e t:e 6 . 1 c21
rinh'(#)
lrorh
- r
]
(4.t2)
where C is the concentration of the electrolyte in the subphase and A"1. is the
area per charge in the protein film. To elucidate the influence of protein charge
on the surface pressure of protein fllms, Evans et a1.70studied the surface pressure-area isotherms of native, acetyl, and succinylated B-casein films at the airwater interface. The net charge of these B-casein derivatives were -11, -19,
and -27, respectively. It was found that at a given surface concentration,the
relative values of surface pressurefollowed the order succinylated B-casein )
acetyl B-casein ) native B-casein. Recent studies on the surface behavior of
native and succinylatedB-lactoglobulin disclosedsimilar behavior.6TIn the surface concentrationrange of 0.5-2.0 mglm2, succinylatedB-lactoglobulin exhibited higher surface pressurethan the native B-lactoglobulin (Figure 4.1 I ). The
results of these studieshave also suggestedthat in films of highly chargedproteins, the surface pressure may arise mainly from the electrostatic and kinetic
forces rather than from cohesive forces.67
4.2.2.4 Configuration of Proteins at Interfaces
As mentioned earlier, proteins adsorb at interfaceswith multiple contact points.
The number of residues or segments in contact with the interface is dependent
on the degree of flexibility of the polypeptide chain. Experimental evidences
indicate that only a fraction of the polypeptide chain is in direct contact with
the interface. The configurations of a polypeptide chain at an interface can be
classifiedinto three groups: trains, loops, and tails (Figure 4.12). The trains are
segmentsin direct contact with the interface, the loops are the polypeptide segments that lie between the trains and are suspendedinto the bulk phase, and the
tails are the segments at the N- and C-terminals of the polypeptide chain. The
relative distribution of trains, loops, and tails in an adsorbedpolypeptide depends
on the flexibility of the chain as well as the surfacepressureof the film. Proteins
that predominantly assume the train configuration at an interface will exert
PROPERTIES
FUNCTIONAL
A { t r
E r v
z
E
o
tf
|/N10
o
o
L
CL
o
o
(u
L.
=
3
U'
S u r fa c e
Figure 4.11. Surfaceequationo
toglobulin;brokenlinesreferto th
monolayers
with z : -8 (thenetr
690/osuccinylated
F-lg.) (Fromre
higher surface pressurethan tl
loop configuration. It is often
dilute monolayer), proteins un(
peptide chain assumesa train
assumption is questionable.b
would be some degreeof loop
actions in proteins, and the cor
chain groups on the polypeptii
Severalpieces of indirect e'
tional change upon adsorption
peuoder r/IoC[ pue o{e]eqe}1;1'oo€Jroluru€ o} uor}drospe uodn e8ueqc
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-relur rBlnsoloru€Jlureql
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196
FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
Phase 1
fnterface
1
loop
Phase2
Figure 4.12. The variousconfigurations
of a flexiblepolypeptideat an interface.(From
ref. 13,reproduced
with permission.)
that while the four cysteine residues of native ovalbumin did not react with 5,5'dithiobis(2-nitrobenzoic acid) (DTNB), two cysteine residues in the foamed
ovalbumin reacted with DTNB, indicating that a conformational change had
occurred in ovalbumin during foaming. Clark et a1.72
studied the conformational
changesin foamed BSA using circular dichroism and fluorescencespectroscopy
methods. It was found that while the a-helix content of the protein decreased
by a small amount, major changesin the tertiary structure of the protein occurred
during foaming, as indicated by changes in the intrinsic fluorescenceproperties
of the protein.
PROPERTIES
FUNCTIONAL
I n t his r es
B S A > lysozym e. Ts
macroscopicfoaming system c
at the air-water interface in un:
foamabilities of theseproteins I
adsorptionas well as to differe
at the air-water interface. Altt
either BSA or lysozyme, the f
BSA and lysozyme,tt suggest
for good foamability do not im
The stability of the lamellar
factors. These include film visr
nitude of disjoining pressurebt
fi1m.76In general, factors that
decrease foam stability. The re
equation:
- dh
v _ _ : l
dr
/ zh3 \
- l l l
\,3pR- /
where ft is film thickness. I is
the bubble, and AP : rtn - ri,
and n6 is the disjoining pressu
tigations on thin films have sh<
ment of thermodynamic equilit
ning and collapse of the bubbk
4.2.3 Foaming Properties
Thefoamingproperties
of proteinsencompass
the abilitiesto producea large
interfacial arca of foam per unit weight of protein (i.e., foamability) and to
stabilize the interfacial film against internal and external forces. The foamability
of proteins is fundamentally related to their film-forming ability at the air-water
interface.73"74
In general, proteins that rapidly adsorb at the newly created airliquid interface during bubbling or whipping, and undergo unfolding and molecular rearrangementat the interface, often exhibit better foamability than proteins
that adsorb slowly and resist unfolding at the interface. On the other hand, the
stability of a protein-stabilized foam is affected by the rheological properties of
the protein film: proteins that form a viscous, gel-like cohesive film with high
elasticity often produce highly stable foams.
Studies on the foaming properties of B-casein, BSA, and lysozyme showed
that the relative foamability of these proteins followed the order B-casein )
\
ptateau
noraer)/
I
Figure 4.13. Structureof a poh
with permission.)
('uorssruuedqtr.,rn
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NOIJVZIUSJJVUVHf,CNV SSIJ
198
FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
drainage show shearresistance,low elasticity, and low viscosity. The poor stability of B-casein foam is related to its inability to form a cohesive elastic film
and its poor capacity for binding and retaining water. The higher stability of
globular protein films, such as lysozyme and BSA, is attributableto their ability
to form cohesive,elastic,,and viscous films.
According to the Reynolds equation(eq.4.13), the rate of thinning of lamellar
film is proportional to the difference between the capillary hydrostatic pressure
and the disjoining pressurebetween the protein layers. If the disjoining pressure
is equal to or greater than the hydrostatic pressure, the foam should be stable.
The magnitude of the disjoining pressure between the two protein layers is
affectedby steric (r,), electrostatic(zr.), and dispersionforces (including hydrophobic, zr") between the protein layers. The contribution of ru to the disjoining
pressureis negative becausethe attractive van der Waals and hydrophobic interactions between the protein layers will tend to decreasethe disjoining pressure.
In general,both rr"and rr, contribute positively to the disjoining pressure.Since
proteins are either positively or negatively charged at a given pH, the net electrostatic repulsion between the protein layers will increasethe disjoining pressure. However, excessiveelectrostaticrepulsion between protein molecules in
the layers will impair the integrity of the protein layers and thus collapse the
film. The steric effects arise mainly from the loops and tails of the adsorbed
protein. In general, lamellar films in which zr.uis greater than the sum of r" and
z'. will thin rapidly. Because the hydrostatic pressure is usually higher than the
disjoining pressure, most protein foams never reach equilibrium conditions.
Even when they attain apparent equilibrium, they are in a metastable state.Te
External perturbations, such as vibration, thermal fluctuations, or exposure to
dust particles, upset the equilibrium and promote film rupture.
The mechanismof rupture of protein films is believed to follow a nucleation
process.The forces that initiate the nucleation processare not well understood.
However, retardation of the growth of a hole in the film is influenced by film
thickness (the most critical parameter),interfacial tension, and the elasticity of
the film.7e Below a critical thickness of about 500 A, the growth of the hydrodynamic surfacewaves acceleratesthinning of the lamella.8o'81
The elasticity or dilatational modulus of protein films is defined as r :
-A(dnldA), where I is the area of the film and rr is the interfacial pressure.The
elasticity of protein films indirectly helps the stability of the film by slowing the
rate of liquid drainage.The elasticity of protein films is affectedby the molecular
flexibility and/or rigidity of the adsorbedprotein. Proteins that are highly flexible
(e.g., B-casein)exhibit low elasticity, meaning that during contractionor expansion of a foam bubble, the change in interfacial pressure(or tension) per unit
change in the interfacial area of the film is minimal.
The dilatational modulus of B-caseinmonolayer at the air-water interface is
about 5-30 mN/m.7e This low rate is attributed to rapid configurational fluctuations of the polypeptide chain from trains to loops and from loops to trains
upon compressionand expansion,respectively,of the film. The relaxation time
for this configurationalfluctuation is about 10-8 second.teBecauseof theserapid
FUNCTIONAL
PROPERTIES
configurational changes. local
expansionof a foam bubble dr
changesin the interfacial tensi
uous expansionof the bubble.r
resulting in liquid drainageanc
In contrast,the films of _slo
higher dilatationalmoduli. The
are in the ranges of about 60-J(
becausethey resist and/or lack
urational changesfrom trains t
expansion of the film. Becaus
the film cause greater chanse
interfacial tension as a foam bu
spontaneousflow of the prot
regions. Such rapid movemen
beneath or adjacent to the filr
thinning. Thus, the high elasti
fundamentalreasonsfor the ste
At the molecularlevel. then
damentallyrelatedto the molec
ular flexibility is important for f
tant for the stability of the foi
might be neededto produce go
Severalstudieshave shou'n
by the rheological properties o
mum intermolecularinteracti on
network often form highly stab
casein film at the air-water int(
lysozyme film was about 1000 r
the foamability of lysozymeis'
molecular interactionsto form
the foam. The extent of interm
mation of the protein at the int
for network formation. German
by native soy I I S protein collal
with dithiothreitol was stablefo
to improved rheological prope
more extensiveunfolding and 1
Apart from molecular flexib
inexplicably related to their h1
erties. The hydrophobicity of
These are surfacehydrophobic
hydrophobic residueson the pr
amino acid residuesof the prot
phobicity is determined from t
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NOIIVZUSIJVUVHJ CNV SsIJ
AND CHARACTERIZATION
FOODPROTEINS:PROPERTIES
from an aqueous phase to an organic phase (e.g., ethanol). The surface hydrophobocity is usually determined from the extent of binding of hydrophobic flu8a
orescentprobes such as ci s -parinaric acid and I -anilino- 8-naphthalensulfonate.
Studies have shown a positive correlation between Bigelow's average
hydrophobicity and foaming capacity of various proteins8s(Figure 4.14). However, no such strong correlation has been found between the surface hydrophobicity of proteins and their foaming capacity. For instance, the surface hydrophobicity of a-lactalbumin was lower than that of lysozyme, yet the former
produced better foam than the 1atter.86However, when individual proteins (e.g.,
*-
FUNCTIONAL
PROPERTIES
lysozyme, ovalbumin, B-lactog
progressivelyheat-denaturedto
hydrophobicity exhibited a cun
4.15). This curvilinear beharror
surface hydrophobicity and foar
simply means that a surface hvd
adsorption of the protein at the i
tein has been adsorbed, its abi
groups at the interface is critica
bility). The strong correlation be
power indicates that most proter
most of the nonpolar residuesat
the air-water interfacial tension
foam are limited not by the sur
hydrophobic groups in the prote
Although the hydrophobicity
150
E
Y
F
O
q.
100
'3:o
fJ
-oz5
o
cro
=
=
o
lr
ol
800
1000
H 6'
OYQ
1200
1400
( hool/ ?Qrlduc )
Figure 4.14. Correlation befween foaming capacity and average hydrophobicity of
selectedproteins: 1, ribonuclease;2, ovomucoid; 3, lysozyme; 4, trypsin; 5, serum albumin; 6, ovalbumin; 7, conalbumin; 8, pepsin; 9, rc-casein;10, B-lactoglobulin; ll, Bcasein.(From ref. 85, reproducedwith permission.)
0
t000
SURF
Figure 4.15. Relationship
betr,r'e
teins:O, ovalbumin;G,
soy75 glob
albumin.Numbersindicatefinal ter
87, reproduced
with permission.)
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r0z
AND CHAMCTERIZATION
PROPERTIES
FOODPROTEINS:
202
foamability of proteins, proteins that are highly hydrophobic are likely to
undergo interfacial coagulation and precipitation upon interfacial denaturation,
thus forming less stable foams. On the other hand, proteins that are highly
charged and less hydrophobic may not be able to form a cohesive network
becauseof strong electrostatic repulsion at the interface. In fact, an inverse relationship is found between Bigelow's charge frequency and foam stability for
severalproteins8s(Figure 4.16). Hence, it is apparentthat the foaming properties
of proteins depend on an optimum balance of hydrophobicity and charge frequency, as well as other noncovalent interactions.
Along with the inherent physicochemical properties of proteins, several additional external factors, including protein concentration, ionic strength, pH, temperature, and the presence of other food constituents (e.g., sugars, lipids) affect
E
.s
E
>40
F
J
dl
F
a
=
O
otLL,n
.t!
o
2
4
CHARGE DENSTTYx 1Ci2
units.res.l
6
betweenfoam stabilityand chargefrequency:l, ribonucleFigure 4.16. Relationship
ase;2, ovomucoid;3, trypsin 4,lysozyme;5,pepsin;6,conalbumin;7,ovalbumin;8'
(From ref. 85,
bovine serumalbumin;9, x-casein;10, B-lactoglobulin;ll, B-casein.
with permission.)
reproduced
FUNCTIONAL
PROPERTIES
the foaming propertiesof food pr
increasesboth the foamabilitl a
higher viscosity, which produc
have shown that the formabilitl
icantly improved in the neighbo
protein is not insoluble at that pl
BSA foam, the interfacial area
was minimum in the range of pl
electrostaticrepulsion is minimr
to zero. Therefore, cohesive int
via noncovalentforces is marirr
trostatic repulsion also allou.s gr
face. This increasesfilnr thickr
properties of the 61tn.z:'ozThe I
drainageT3;
the surfaceyield stre
these factors cumulatively cont
should be pointed out, how,ever
electrostaticforces to the disjoir
the decreasedrate of thinning o1
the electrostaticcomponentof th
by the increasedviscoelasticitr.z
steric factors.
Processing-inducedchangesi
teins affect foaming properties.l
preparedby various methods dil
in their functional properties.De
protein composition of WPCs r r
preparation:while the WPCs pri
protein composition very simila
from ion exchangemethods con
proteins. The fat content of men
WPC preparedby ion exchanger
has beenreportedto causewide'
studied the foaming propertieso
pasteurizedand nonpasteunzeda
ultrafiltration/diafiltration and s
were very similar in composition
greatly. This paradox has been a
state of proteins in these prepar
the free sulfhydryl content and 1
the lipid and ash content had ne
WPCs.
Heat denaturation of prote
improve.e3'e6'ee
This improvemer
phobicity,roowhich decreasesth
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AND CHARACTERIZATION
PROPERTIES
FOODPROTEINS:
interface.Thus, heatingof WPC dispersionsat 65-85oC for 30 minutes improved
foamability compared to the unheated control,e2 whereas heating above 80'C
causeda decreasein foamability. This behavior showed some correlation with
the undenaturedprotein fraction of WPC: while the undenaturedprotein fraction
decreasedfrom 80% to 62o/oin the temperature range of 65-80'C, it decreased
to 40o/oat 85"C. Perhaps, then, the precipitous drop in foamability at 85"C is
related to a drop in soluble protein below a critical level. Conversely, above a
critical level, the insoluble protein particles and the high molecular weight polymers may adversely affect foamability of heat-denatured proteins. It was
reported that when heat-coagulablewhey proteins were removed, the remaining
solution showed excellent foaming properties.l0l The critical ratio of undenatured to denatured proteins that imparts better foamability may not be same for
all proteins because,while this ratio seemsto be about 40:60 for WPC, several
proteins exhibit better foaming properties only when the solubility index is about
20o .8sTherefore, optimum heating conditions need to be established for individual proteins to improve the foaming properties of each one.
Lipids, especially phospholipids, adversely affect the foaming properties of
food proteins. For example, addition of egg yolk to egg albumin at levels below
0.03% decreasedthe stability of egg albumin foam.ro' Cooneyro3reported that
addition of small amounts of phospholipids to WPC increased foamability but
decreasedfoam stability. However, Joseph and Manginol0o found that addition
of milligram quantities of fat globule membrane protein (MFGM) to a commercial egg white preparation and whey protein concentrate cause a dramatic
decreasein both foam oveffun and foam stability. These authors claimed that
the adverse effects of MFGM on the foaming properties were due to the protein
component of the MFGM, not to the lipid component.
Since commercial protein preparationsare mixtures of various proteins, recent
research has focused on the foaming properties of protein mixtures in model
systems. Studies have shown that the foaming properties of acidic proteins can
be improved by mixing with basic proteins such as lysozyme and clupeine.105t08 Addition of lysozyme up to a level of 0.loh to a solution of 0.5o/oBSA
dramatically improved the foaming propertiesof BSA at pH 8.0 (Figure 4.17).105
This effect has been attributed to electrostatic interaction between the negatively
charged BSA and the positively charged lysozyme. Both the stability and the
foamability of the BSA-lysozyme foams were decreasedby increase of ionic
strength. The optimum pH for better foaming properties was found to be about
8.0; above and below this value, the foaming properties decreased.These observations suggested that electrostatic interactions were responsible for foam
enhancementby basic proteins. The foaming properties decreasedat higher lysozyme-to-BSA ratios, possibly as a result of precipitation of the lysozyme-BsA
complexes.totSimilar resultswere also obtainedwith B-lactoglobulin and whey
protein isolate foams.tOe1n addition to having better foaming properties,acidicbasic protein mixtures have been found to overcome the destabilizing effect of
lipids on protein foams.rr0Thus, while addition of corn oil to a 4o/o(w/v) whey
protein isolate solution inhibited the foamability of WPI, addition of clupeine
s
a
400
I
q,
E
o
o
300
200
rOO
(a)
F
qn
:
ao
; 4 0
o
t
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q
o
(b)
Figure 4.17. Effectof pH on foa
o, 0.5%B:
aqueous
BSA solutions:
reproduced
with permission.
)
902
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NOIJVZIUSJJVUVHJCNV SSII
AND CHARACTERIZATION
PROPERTIES
FOODPROTEINS:
206
o
o
PROPERTIES
FUNCTIONAL
phase,or water-in-oil emulsion
oil is quite high, emulsionsare I
occurswith time. However. the
amphiphilic surface active mol
reducethe interfacial tension. I
as macromolecularsurfactants
teins can form continuous col
impart mechanical stabilit) to r
The emulsifyingpropertiest
activity index (EAI), emulsion
(EC). EAI is usually determine
W r t hc t u p e i n e
W i t h o u tc t u p e i n e
EAI:
-
10
15
15
l 00
3
? 55
20
Y o C o r no i t( i n f i n o t m i x )
r.0
Figure 4.18. Effect of corn oil concentrationon the foam expansion(FE) of whey
with
proteinisolate(4%) with and without 0.4ohclupeine.(From ref. 110,reproduced
permission.)
(0.4% w/v) dramatically increased the foamability of WPI-corn oil mixtures
(Figure 4.18). In fact, very good foams were obtained up to an oil content of
33%.rr0 Transmission electron micrographs of BSA-clupeine-corn oil foam
revealed a novel foam system in which the oil droplets coated with protein film
were attached to foam cells.
4.2.4 Emulsifying Properties
An emulsion is a two-phaseliquid systemin which one of the liquids is dispersed
as droplets in the other liquid. Food emulsions can be divided into two types:
systemsin which an oil is dispersedin an aqueouscontinuous phase,or oil-inwater emulsions,and systems in which water is dispersedin an oil continuous
2T
.
co
where c is the protein concentr
fraction of the oil phase,and Z
nm.ttt Since,accordingto Mie
interfacial area of emulsion dr
facial area of emulsion genera
sion is usually expressedas I
turbidity of the emulsion is ha
is defined as the maximum voh"
solution up to the point of invt
The factors that affect the ,
those that affect the foaming
adsorption at the oil-water inte
of conformational realrangeme
facial tension, and the ability t
via both covalent (disulfide bo
To elucidate the influence c
at the oil-water interface, Gral
lysozyme, and B-casein at the
few qualitative differences, the
face was very similar to that at
tendency to form loops at the c
In contrast,lysozyme and BSI
to a greater extent than at the
BSA and lysozyme films wer
interface.ToThe surfaceviscos
1, 10, and 5000 mNs/m, respe
and more rigid film than BSA
decane-water emulsions stabi
correlation was found betwee
and the interparticle force reqt
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NOIJVZIUSJJYUVHJ CNV SSIJ
PROPERTIES
AND CHAMCTERIZATION
FOODPROTEINS:
the force required to coalescethe emulsion droplets followed the order BSA >
lysozyme ) B-casein, indicating that properties other than the film viscosity
played a dominant role in emulsion stability. Similar behavior has been observed
I 16
I5
in the casesof casein,whey, soy, and blood proteins,l and B-lactoglobulin.
These reports offer evidence that the rheological properties of protein films may
not be critically important for the stability of protein-stabilized emulsions.Te
Similar conclusionswere made by others.l17
With respect to the stability of foams, on the other hand, the rheological
properties of the protein layers in the lamellar film play a crucial role in the
retardation of liquid drainage and the eventual stability of the foam. One of the
most convincing arguments in support of the unimportance of the rheological
properties to emulsion stability is that despite the similarity of the rheological
properties of protein films at the oil-in-water and water-in-oil emulsion interfaces, the former are more stable than the latter.117It appearsthat other factors,
such as the disjoining forces arising from electrostatic,solvation, and steric interaction between the loops cf the protein molecule in the aqueous side of the
interface,play amajor role in the stability of emulsions.rr8Indeed,most proteinstabilized emulsions are stable when the pH of the emulsion is far away from
where the electrostaticrepulsion and
the isoelectricpoint of the protein,lr6'11e'120
hydration repulsion forces are maximum. It should be pointed out that in the
case of foams, however, most protein foams are stable at near the isoelectric
point of the protein.
Some reports in the literature claim correlation between coalescencestability
11 is not
of emulsions and rheological properties of interfacial fi1ms.121-t23
known, however, whether such correlations are directly attributable to interfacial
rheology per se or to other changes in the interfacial film.r2a
The chemical properties of the oil phase seem to affect the rate of adsorption
of proteins and the rate of decreaseof interfacial tension. Parkerl2sreported that
while the net reduction in the tension at the decane-water interface due to
adsorptionof BSA, lysozyme, and B-caseinfollowed the order B-casein> BSA
) lysozyme, the relative order was BSA ) B-casein ) lysozyme at the myristyl
tri glyc eride-water interface.
The emulsifuing properties of proteins are affected by the hydrophobicity of
proteins. However, unlike the foaming properties of proteins, which show strong
correlation with Bigelow's average hydrophobicity, the emulsifuing properties
show strong correlation with surface hydrophobicity. Keshavarz and Nakail26
first reported that the ability of various proteins to decreaseinterfacial tension
at the water-corn oil interface was strongly related to the surface hydrophobicities of proteins as measuredby the retention coefficient of the proteins on butylepoxy- and hexylepoxy-sepharose columns. Similarly, other workersso'tt2'127
have reported a strong correlation between the interfacial tension, EAI, and surface hydrophobicity (as measured by the crs-parinaric acid binding method) of
various native and heat-denaturedglobular proteins (Figure 4.19). The poor correlation between the emulsifying properties and Bigelow's average hydrophobicity indirectly suggeststhat the adsorbedproteins at the oil-water interface do
FUNCTIONAL
PROPERTIES
tr
-o){
12
c
,6
11
q)
3o 1 0
(!
(u
-c
q
0
500
1000
Figure 4.19. Correlationbetu'e
naric acid fluorescentprobemetho
interface,and emulsifoingactivin
ducedwith permission.)
not undergo extensive denatura
interpretation is reasonable bec
interface is considerably lower
interfacial energy probably is
barrier for extensive unfolding
Although positive correlatio
fying properties of proteins hav
suggeststhat this relationship i
the surface hydrophobicity of I
7, the protein exhibits better er
than at pH 3.tte Studieson rela
whey proteins (lactofenin, sen
and a-lactalbumin) at different
face hydrophobicity and adsor
factors other than surface hydr
of emulsifying propertiesof prr
formational rearrangementat tf
presenceof hydrophobic patch
It is also questionablewhet
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;o
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relncolotu ler{t slsaSSns11nsers1{I 0.,'fllllqnqrosp? pue ,Qrcrqoqdotpfq ece;
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Jo oceJrns]€J paglslntuo oq] ]u ftrpqegrospe e^rleler uo sorpnls orr'€ Hd t€ u?ql
'L
1 Surpeecxe senlel Hd le sergedord Eurr{grslnruerol}eq s}rqrqxe ura}ord oql
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CNV SSIJ
NOIJVZIUSJJYUVHJ
210
AND CHARACTERIZATION
PROPERTIES
FOODPROTEINS:
fluorescent probes truly reflects the hydrophobicity of the protein surface that is
11was reportedthat although the surfacehydroin contact with the solvent.16'128
phobicity of reducedand denaturedBSA should have been very high, the extent
of binding of the hydrophobic fluorescentprobe l-anilino-8-naphthalenesulfonate (ANS) was very poor; however, the extent of binding of ANS increased
progressively as the denatured BSA molecule resumed a folded state.rtnThis
finding suggeststhat fluorescent probes bind only to well-defined hydrophobic
cavities on the protein surface rather than to nonpolar residues, randomly distributed on the protein surface.r6The cavities are accessible to fluorescent
probes,but not to the solvent. It is also possiblethat thesenonpolar cavities will
not bind to the oil-water interface unless the protein undergoesa conformational
reaffangement at the interface that renders the cavities accessible to the oilwater interface. Solubility, in addition to surface hydrophobicity, is important
for emulsifying propertiesof proteins.tt Highly insoluble proteinstend to exhibit
very poor emulsifying properties. However, although there is at least some
degree of positive correlation between surface hydrophobicity and emulsifying
properties, no such correlation exists between solubility and emulsiffing propi32
erties.r30 Nonetheless,since the stability of a protein film at the oil-water
interface requires favorable interaction of the protein with both the oil and aqueous phases, an optimum balance of hydrophilic and hydrophobic groups that
keeps the protein in solution is needed for better emulsifuing properties.
Several studies have shown that controlled heat denaturation of proteins that
does not result in protein insolubilizationcan improve the emulsifoing properties
tz't::
of proteins.s4'87,r 161r result has been attributedto an increasein the surface
hydrophobicity of heat-treatedproteins. The stability of lysozyme and ovalbumin emulsions increased proportionally with increase of surface hydrophobicity.t" Similar results were obtained in the case of progressively heat-denatured
soy lls.134 However, excessive heat denaturation of soy proteins at high
temperatures and longer heating times adversely affect their emulsifying properties.r3tThus, high temperature(170'C), short time heating of soy 7S and l lS
globulins impaired emulsifoing properties.t3tAggregation and loss of solubility
are primarily responsible for this degradation.
Although heat denaturation of proteins usually increases surface hydrophobicity, in some proteins the opposite behavior is observed. Thus, heating of Plactoglobulin causes a decreasein surface hydrophobicity compared to that of
the native protein; the result is impairment of the protein's emulsifuing
properties.8T
In addition to causing unfolding and an increase of surface hydrophobicity,
heat treatment of globular proteins invariably causes polymerization via sulfhydryl-disulfide interchange reactions.tt Although much effort has been spent
in relating the changes in emulsi$ing properties of heated proteins to changes
in surface hydrophobicity, no systematic study has been published on the effects
of heat-induced polymerization of proteins on their emulsifying properties.
Tornbergr3sreported that the amount of protein adsorbedto fat droplets in an
emulsion was dependent on the interfacial area of the dispersed phase as well
FUNCTIONAL
PROPERTIES
as on the type of protein. The
protein and low for caseinate
small (i.e., larger fat droplets).
The data sugge
the opposite.r35
phase
the bulk
as more interthc
proteins,the protein initialll ad
area was created.l3s11was also
chloride increasedthe amount c
surface,the opposite occuned ir
Food protein isolatescontain
Depending on their molecular
and/or differentially adsorbto th
mrzv et al.r20showed that thc-r
from acid whey exhibited selec
water. Moreover, this selective
proteins extracted from the em
lactalbumin in the film progress
l0o/oat pH 9, whereasthe fracti
at pH 3.0 to about 61ohat pH 9.
transferrin,and lactoferrin erhiL
tivity of adsorptiondid not corr(
in the pH range studied, indicat
conformational flexibility of the
Recently,Robson and Dalgle
ti on of d, r - , F- , and r - caseins
emulsions. It was found that e
hydrophobic (1335 callmol res
(1170 callresidue),there was nc
at the oil surface. The molar r
Table 4.8
Relative Abundance (
Fraction Adsorbed ol
Whey Proteins
a-Lactalbumin
B-Lactoglobulin
Casein * immunoglobulin,
light chain
Immunoglobulin, heavy chain
Serum albumin
Transferrin * lactoferrin
Source: Rei 120.reproducedwith permissio
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rulg eql ur urunqlelcel
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unrpos N z'0 Jo uorllpp€ elrq,&\leq] po^rasqo osls s3,&\]I ser'pol?erJ s?^\ sorB
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Eur,(;rslnue s,urelord aql Jo l
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gd
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STIJUSdOUd'IVNOIJJNNJ
NOIJVZIUSJf,YUVHJCNV SsIJ
212
FOODPROTEINS:PROPERTIES
AND CHARACTERIZATION
.E
T
o
9
tr 8
t
t
{g
_g
G'
T
. 9 .
-......L
6
o
o
0
2
3
4
3
wt gplatin in lkg ernltsur/g
Figure 4.20. Adsorptoin of gelatin at the oil-water interface in emulsions made from
mixtures of gelatin * caseinate (solid curve) and gelatin *
B-lactoglobulin (dashed
curve). Initial bulk protein concentration was 5 g pei kilogram'of emjsion. (From ref.
140, reproducedwith permission.)
PROPERTIES
FLINCTIONAL
content of 5 g of protein per
freshly formed emulsion cont
(Figure a.20); but at higher rz
adsorb at the interface.t+o1n t
lactoglobulin,however.a sign
even at very high ratios of P4.20;).This suggestedthat B-la
placing gelatin from an oil-r
sodium caseinateis added to t
gelatin at the interface is read
displace the gelatin phase dec
Furthermore.\
of caseinate.t38
ing of pure caseinatesolution i
that of the gelatin-caseinate r
was being continuously dispk
of mixed proteins undergo tir
tion; and such changesin the
may have an effect on emulsi
The chemical and enzyma
their foaming and emulsifyinl
cinylation, phosphorylation.
increasein the emulsifying pr
of proteins generally impror e
P
that of intact proteins.ras'ra6
the
emu
impaired
lysis) often
4.2.5 Gelation
interface immediately after emulsion formation was similar to that of sodium
caseinate.136
However, during aging, B-casein in the aqueous phase replaced
some of the 4,1-caseinat the interface. These observationsindicate thai since
both a"1- and B-caseinsare highly flexible coils, the lack of conformational
constraints enablesthese proteins to adsorb at the same rate and to spreadreadily
at the interface. During aging, however, because of its higher hydrophobicity
and surfaceactivity, B-caseinslowly displacessome of the a.1-caseinmolecules
from the interface. Thus, it seems that while conformational flexibility of the
protein is important for initial adsorption and spreading at the interface, hydrophobicity is quintessential for retention and stabilization of the protein film at
the interface.
Other studies also have shown that highly hydrophobic proteins readily displace.$qry liquid interfacesproteins that are less hydrophobic.t37-t4tDickinson
et al.t37-t4ostudiedthe propertiesof mixed protein hms of gelatin and caseinate
at the oil-water interface. These investigators showed that when the weight ratio
of gelatin to caseinatein the bulk solution was below 2:l (at a total protein
The propertiesof variousfo
+o
authors.lau-r l6atefore, this
that affect the gelation of prot
Gels are often consideredI
Ferrylt2 defined
liquid.r50'rs1
no
stateflow.'
steady
exhibits
either covalently or noncoval
of entrapping water and othe
links are not necessarilypoint
but usually involve segmentmolecules. In the case of pr,
transformation of the protein
other agents.In food systems
ertiesto the food, the three-di
for holding water, flavors, an
The steps involved in hea
in Figure 4.21.r6In heat-indu
an\C ul
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eperu SuoISInue',{pee13 8€r./''o}suleseJ,{q peceldstp ,(lsnonul}uooEureq sem
'sf,ep
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.srnoq urqlr,l\ enle^ unrJqrlrnba ue peqceeJ pue uollnlos eleulessc erndSo Eur
IIo
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o1 ,Qrpqe eql 'rene^loq ieleuresec,{q paceldsrp ,(ppeer sI ece#e}ul eql 1eurlele8
eql 'urlele? Kqpazqlq?1suolslntue epelu ,{1qsar;B o} peppe sI e1euleseotunlpos
ueq/\\ l€q] ul!\oqs 0SIB se./y\lI orr'e3eJJelulJele/r\-llo uP tuoJJturleleE Surceld
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e-rn8rg)eseqd {lnq eq} ur urleleE ol ulnqolflolcel-i Jo soller q?rq ,{ran }3 uene
'tenelrroq 'ur1nqo13o1ce1
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-d
eq] 1e qrospe
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2t4
FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
ThermalGelationof Proteins
(Coaguh:m type
ge1)
["],,
t
aggregation
and
coagulation
nP*--@+
nPo
s'/
f/
e"/
METASOL
(Partially
progel
refol-ded
state)
Figure 4.21. Heat-induced changes during thermal gelation of globular proteins: P*
and Po, native and denatured states of the protein; P*,, partially refolded state of the
protein; [Po],, translucent-typegel state; [Po,],, coagulum fype-gel state.(From ref. 16,
adaptedwith permission.)
state is converted to a progel state by heating above its denaturation temperature.
The sol-progel transition is usually an irreversible process, which involves dissociation and denaturation of the protein molecule.r53 The primary importance
of the denaturation process is to expose the functional groups which, under
appropriate conditions, interact to form a three-dimensional gel network.
Depending on their molecular properties,the unfolded protein molecules form
two types of gel networks:
1. Proteinsthat contain high levels of apolar amino acid residuesundergohydrophobic aggregation;when the protein concentrationis above a critical level,
these aggregatesform an irreversible coagulum-type gel network.
PROPERTIES
FUNCTIONAL
2. Proteins that contain lou I
aggregates,which set into
It is also very likely that the
temperature partially refold d
protein would affect the numb
ture formation. Generall,v.in
weaker than it would have be
t4
Hermanssont suggested
gation processesduring heati
formation. If the rate of aggr(
the rate of denaturation. rand
result in the formation of an r
elasticity and water-holding r
gation is slower than the rate
mobility of moleculesin the P
gel network with lower oPac
ity.tto This phenomenologic
relative ratesof various proce
of specific molecular propert
of gel network formation is P
and the translucent type of g
hydrogenbonding interactio
The tendency of a protein
gel should be fundamentalll
low's average hydrophobic
showed that proteins contain
and Trp (hydrophobic) residu
that containlessthan 31.5 mc
translucent-typegels (Figure
did not include other hydrop
hydrophobicity calculation. r
most cases,it might not be
ionic strength.For instance.
idues is about 32 mol % in P
included); the protein forms
denaturation temperature. In
0.05 M, however,B-lactoglo
although the mole percent o
high, charge repulsion amon
itates formation of a transluc
hydrophobic aggregationan<
vations suggestthat the phys
tallv bv the balance of the
crlutsorlJelo o^rslndor pue clqoqdorpfq enrlcerpe eq] Jo oou€leq oql ,{q f1ye1
-ueruspunJpelseJJ€sr lJo/yueu 1eE3r{}Jo a}€}sl?Jrs,(qd eqt leql lseEEnssuol}?^
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uI pel?eq ueq,/y\1eBluecnlsu€J] e suuoJ utalord eqt i(pepnlcut
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'sesec
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lsotu
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ftrcrqoqdo;pfq
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sror1}neeqt ,(qm 'renetroq 'lueptne lou sI \ (ZZ'V ern8rg) sleE ed,{1-}uecnlsusrl
ruroJ o] pue] porusu 1sn[senprsarrelode eq]Jo o lolur g'19 ue{} ssel ule}uoc teq}
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lJorvueu yeE3o
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lunocce o{e} ol slreJpue ue1s,(sEurlle8 eq} ur sessecordsnoIJeAJose}€r oll}?ler
eql uo srsuqdrua qcntu ool seceld uotleueldxe lecrEolouotuoueqd srql or,'{tl
-cedec Eurploq-rele^\ pue ,{1rcr1se1e
raqErq pue ,QrcedoJe/hol tlll^{ >lro^uau 1eE
aql uI selncelotuJo ,$rpqou
poropro u€Jo uorpuuoJ el€lrllc€J l{8ru e1e1s
leEord
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-ar33e Jo oler eq] JI 'pueq rel{to eql uO ',trcedec Eurploq-rel€,/y\puu flrcqsele
yeEpereproun u?
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1aBgo edfi eql Sururuuelep q elor e ,{e1dlqErur Eurleaq Suunp sassacorduotle?
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NOIJVZIU3JJVUVHJCINVS3II
216
FOODPROTEINS:PROPERTIES
AND CHAMCTERIZATION
,.I
Gelation tYPe
o
p
o ^
<UO.
o i
'E
o,
(u-c
&
,9
.clo
O =
o.:
O
O
E
g l -
,ts
O-
o -
- o o
o - >
$.o
Coagulation tYPe
,rI
trI
r
f
',L
,tI
Ovomucoid
Gelatin
Figure 4.22. Relationshipbetweenthe mole percentof hydrophobicamino acid residuesandthe type of gel networkformedin globularproteins.(Fromref. 156,reproduced
with permission.)
interactions. Therefore, the ratio of charge frequency to average hydrophobicity
might serye better to predict the gelation behavior of globular proteins than the
average hydrophobicity alone.
In addition to charge and hydrophobic properties, the extent of denaturation
during heating might affect the type of gel formed by globular proteins. An
assumption implicit in Shimada and Matsushita's empirical rule is that once a
protein has been heated above its thermal transition temperature,all hydrophobic
residues are totally exposed in the progel state and remain exposed when the gel
sets at lower temperature.This need not be true for all proteins. For instance,
bovine serum albumin, which has about 29 mol % of apolar amino acid residues,
forms a translucentgel when heatedin the absenceof reductants;in the presence
of reducing agents such as cysteine or dithiothreitol, however, it forms a coas-
FUNCTIONAL
PROPERTIES
ul um-t ype gel. r o't ssThis beh
unfolding of BSA in the abs
have been made in the caseo
The formation of protein
protein-protein and protein-s
formation of a self-supporti
mechanical motions is depen
and noncovalent) formed per
sum of the energiesof these i
gel network should be stab
the greaterwould be the gel st
gels are primarily noncovale
ing, and electrostaticinteracti
linkable functional groups per
proteins, the strengths of var
gelation conditions differ r e
related to differencesin certa
ular proteins. It has been prol
the ability of a protein to forr
ment is a prerequisite for ge
known to be the best gelling c
residues.This implies that di:
for protein gelation. It is pr
increasespolymer chain lenl
properties.
To elucidate the role of di
Damodarant58studied the effi
ethylmaleimide (NEM), whic
gelation of severalproteins. i
that for the proteins that did n
hardness of the gels at any I
additive > Cys ) NEM. For
bonds, the relative order was
also found that the gels form
geneousspeciesof disulfide r
in these gels indicated that v
same order as gel strength. Si
possible in the presenceof e
concluded that the difference
weight-averagemolecular we
It was also found that desl
of the proteins studied, the s
exhibited a linear relationship
peptides in these gels (Figur
the r-axis in Figure 4.24 migl
(elq€1s
p
ou
u€elu
urelord
relnqol8
rruoJ
uec
]eq]
lq8ru p7'p amfuC uI slxe-r eql
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uo ]decretur eq] ]eq] pozlsaqlodfq sB,/\4,
]I
-,{1odgolqEremrelncelotu eEerene-lq8remeq} q}I^\ drqsuorleyerreeull B pe}Iqqxe
sle8 urelord snouen Jo sseupreq aq] Jo loor erenbs eq] 'perpnls surolord eql;o
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's1eEeseql ur seprlded,(1odeql
Jo (ezrs) ]q8rern relncelotu eEerene-]qEre,l
eq] ur secuereJJrpot enp oro^\ qlEuerls 1eEeqt uI secuereJJlpeq] ]BI{}pepnlouoc
eJuoserd eql ur alqrssod
se.&\ 'WEN ;o eurelsr{cJog}re s}unorue ssacxe
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lou sr uorlerruoJ puoq epglnsrp Jelncelouuelur ecurs
1eBse repro otues
eq] ur pes€ercur lq8rem relncelotu eEerene-]qErem]eq] pelectpur sle8 eseq] uI
epglnsry go sercedssnoeue8
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-oreleq peurs]uoc I IEN -ro s,(3 peppe ou qtr./KperruoJ sleE eql 13q] punoJ osls
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ou JepJo eql pe.,!\olloJuorl?rluecuoc urelord uenrE Kue 1e sleE eql Jo sseupr?q
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eq plnoqs sercuedeJcsrpeseql ',{1}uecgruErs,ften reJJIp suorlrpuoc uorleleE
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-puoq ueSorp,(q 'crqoqdorp,(q ''zr,r) suorlceJelurluelelocuou flueurud e.reslaE
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1eE
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ryom1eu
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Jo requnu eql raq8rq eql
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e ecuo ]eq] sr olu IuJrrrdruas
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uorlernlsuepJo ]uelxe eql 'sar
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ase
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NOIJVZIUSJJWVHJCNV SSI
218
FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
PROPERTIES
FUNCTIONAL
50
20
c!
t o
15
x
E
{\t
F>-
ol
o
o
o
c
EI
L
(g
J\r
g
10
lc.c 20
5
I
6
8
10
12
14
16
Concentration (Y")
15
Figure 4.24. Relationship betr
gels and the weight-averagemol
work; open symbols correspon
135% protein concentration:prc
l, Cys-soy isolal,e;V and V. C
'-'and o,
Cys B:
Cys-egg white;
N
, o
x
10
E
o)
o
o
o
tr
!,
l-
(g
!
1 0
12
14
16
Concentration (Y")
Figure.4.23. (A) The effects of cysteine and N-ethylmaleimide (NEM) on protein concentration versus hardnessof BSA gels: o, no additive; D, 400 mM cysteine; A, 20 mM
NEM. (B) The effects of cysteine and NEM on protein concentration versus hardnessof
o, no additive; !, 50 mM cysteine; A, 20 mM NEM. (From ref.
9o_yI I S globulin gels:
158, reproducedwith permission.)
self-standing gel network at
unless the weight-average n
23,000.'t8This predictionag
trypsinolyzedphaseolin:natir
45,000 for each subunit; it hr
any free sulfhydryl groups.'n
unit of the protein is cut app
six polypeptides,each with a
native phaseolin was able to
protein concentration,the tn
prote inconcent r at ion
r ange.
that the primary imporlance
related to their ability to incr
chain length of polypeptides
Nearly a half-century ago.
condit ions.suchas pr ot einco
'suolllpuoJ
looJ oJenbsaql'qlSuaJlscruorpue'Hd'uorlerlueouoculelo;d se qcns
'o8e
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aql ro lq8re,,vr
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FOODPROTEINS:
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of the rigidity of gelatin gels was proportional to the molecular weight of the
gelatin. It was also shown that the minimum weight-averagemolecular weight
below which gelatin cannot form a self-supportinggel at 5oC was about 20,000.
This seems to be the case for globular proteins as well.'s8 Globular proteins
having a molecular weight of less than 23,000 can form gels, provided they
contain at least one free sulfhydryl group that can be oxidized to form a protein
dimer, with the result that the molecular weight of the dimer is above 23,000.
It is conceivable that the longer the chain length of the polypeptides, the greater
the molecular entanglement in the gel; this circumstance might restrict the relative thermal motions of the polypeptides in the gelling system and thus impact
on the stability of the noncovalent cross-links in the gel network. Enzymatically
hydrolyzed proteins produce weaker gels than the intact proteinsr68becausethe
short polypeptide fragments are unable to form a continuous gel network.
In addition to the chain length of the protein polymer, several other factors
also affect the number density of cross-links formed in a gel network. A critical
factor is the protein concentration. To form a self-supporting gel network, a
minimum protein concentration, known as the least concentration end point
(LCE) is required.r6eFerryr67pointed out that a continuous three-dimensional
network cannot be formed in a polymenzing system if the concentration of the
polymer is so low that the dispersedchains cannot reach each other. At or above
the critical concentration (i.e., LCE), the greater probability of intermolecular
contacts facilitates formation of a stable network. Below this critical concentration, instead of forming an ordered network, the thermally unfolded polypeptide
chains may undergo random aggregation: in the case of hydrophobic proteins,
the aggregatesmay flocculate and eventually precipitate. On the other hand, in
the case of less hydrophobic proteins, aggregation sometimes results in the for1s3'156'r70
mation of solublemacrocomplexes.
Under similar gelation conditions, the minimum concentration neededto form
a gel differs from protein to protein. For example, while the LCE is 8% for soy
proteinsr53and 3o/ofor egg albumin,rs6it is about 0.6% for gelatin.'tt The differences in the LCEs of various proteins are attributable to differences in the
molecular properties of the proteins (net charge, amino acid composition, molecular size, etc.). These factors affect the number of cross-links formed per unit
cell of the gel network. In addition, variations in experimental conditions affect
the LCE of proteins. Shimada and MatsushitarT2showed that the LCE for soy
protein gelation was lower at pH 9.0 than at 7.5
The relationship between gel strength and protein concentration usually follows a power law,toT'161'173
that is:
G x C"
(4.15)
where G is gel strength or rigidity, C is protein concentration, and n is a constant.
For most proteins the value of n lies between 1 and 2 and is dependenton protein
concentration.For example, in the case of myosin gels, the value of n varies
from 1.7 to 2.0 in the concentrationrange of 0.1-l .0o .t73For gelatin gels, the
PROPERTIES
FUNCTIONAL
value of n is about 2 in the i
50o/orange of gelatin concent
Fibrous proteins such as mr
ble of forming a gel net$'ork
proteins.however.requirea st
to set into a gel. For globularp
G:K(C-Co)"
where C6 is the LCE belou '
porting gel network, and K is
and K can be obtained by finir
The heating temperature al
ally, gel strengthincreasesu'itl
temperaturerangeis reached
is relatedto the extent of unfol
the extent of unfolding. the s
myosin formed a stronger ge
correlatedwith a decreasein
4oh at 70"C.
Although heating at a high
excessiveheating causesthen
network formation.ttt In som
unfolding promotes protein-g
interactions.The result is a g
Severalstudieshave indicated
is just above the thermal trans
protein exhibits highest gel st
to the thermal transition tempe
the optimum heating tempera
just above its thermal transitir
The rates of heating and c
slower the heating rate, the hi
more heat absorption, and exte
at a slower cooling rate exhi
strength than the gels forme
attributed to greater chain mo
probability of successfulcros
slow cooling may facilitate 1
known to act as junction zon(
Several environmental fac
impact on the ability of the p
is affected by the pH. At hig
charge.The strong electrosta
isoelectric pH, proteins have
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FOODPROTEINS:
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phobic interactions. This leads to formation of a coagulum-type gel with a
coarser network and lower gel strength. Only at the optimum pH, which permits
an optimum balance of protein-protein and protein-solvent interactions, can a
uniform gel matrix with high gel strength and water-binding capacity be formed.
Generally, the optimum pH is about 7-8 for most proteins. However, there are
exceptions. For example, whey protein gels formed at pH 6-7 were stronger
than those formed at pH 4.0.185In contrast, the gels of ovalbumin formed at
acidic pH (< 4.0) were strongerthan those that were formed at pH 6.0.186Obviously, the molecular forces involved in theseprotein gels under similar pH conditions vary in magnitude.
Reductants such as cysteine and B-mercaptoethanol,and sulfhydryl blocking
agents such as NEM, affect protein gelation becausethey affect the sulfhydryldisulfide interchange reaction. Schmidt et al.r87reported that the gel strength of
WPC increased with addition of up to about 9.7 mM cysteine and decreasedat
higher concentrations. WPC did not form a gel above 30 mM cysteine. In the
case of soy proteins, the gel strength decreasedas the cysteine concentration
was increasedfrom zero to 0.08%.188
At high concentrations, denaturants (e.g., urea, guanidine hydrochloride)
cause gelation of egg white and serum albumin.T Soy I I S globulin gels at 20"C
in urea and alcohol mixtures.ttn These gels melt at 50-60'C and set irreversibly
agarn at 70-80"C. Several investigators have studied the conformation of proteins in the gel network using infrared, Raman,reote2 and circular dichroism
spectroscopicmetho6t.la8'r60Wang and Damodaranr6oreported that while the
unheatednative BSA contained 575% a-helix, 42.5% aperiodic structure,and
no B-sheet,the BSA in heat-setgels contained about 42o/o o-helix,26.5%oBsheet, and 31.5% aperiodic structure. B-Sheet structure in the gel was formed at
the expense of a-helix and aperiodic structures. Investigations on soy proteins
revealed that the unheated soy isolate and soy I I S contained about 660/oB-sheet,
28.5% aperiodic, and very little a-helix structure.In heat-setgels, the p-sheet
content was about 26oh, and the remainder consisted of B-turns and aperiodic
structures.160These studies have indicated that formation and/or retention of a
critical amount of B-sheet structure is important for protein-protein interaction
and gel network formation in globular protein gels. The critical amount of Bsheet structure needed to form a gel network appearsto be about 25oh for globular proteittt.ras'160
It is probable that the B-pleated sheetsin globular protein
gels are intermolecular rather than intramolecular. The regions of intermolecular
hydrogen bonding between the B-sheets,oriented in either parallel or antiparallel
configurations, may act as junction zones in globular protein gel network.r60
This is in contrastto the caseof gelatin gels, in which the junction zones involve
partial re-formation of collagen triple helices.raT
4.3 Summary
Although the literaturecontainsa volume of qualitative information on the functional propertiesof food proteins,there is still a lack of fundamentalunderstand-
PROPERTIES
FUNCTIONAL
ing of the structure-functiona
ysis of the data in the literatur
are affected to alarge extent b
molecular flexibility, and ste
knowledge of the extent of i
expressionof a given functior
it is well understoodthat mol
most irnportant descriptors fo
teins, it is not evident hou' hy
it is to exhibit excellent foam
Although quantitation of m
age hydrophobicity is relative
ibility and steric propertiesof
Severalmethods have been d,
of proteins. These include tht
heptanebinding method.'" S
phobicity of proteins (as mea
several functional properties h
empirical analysesshow that
phobicity are very important
Table 4.9 RegressionEquati
RelationshiPsBetr
Functionalityof Fo
Functional Property
Emulsifying activity index (EAI) of
native and heatedprotcins
Emulsion stability index (ESI) of na
and heated proteins
Fat binding capacity (FBC) of natirr
and heated proteins
Foaming capacity (FC) of native
proteins
Water absorption (AMo) of minced
meat in brine
Coagulability (C) of native and hea
0.5% ovalbumin solutions
Gel strength (6) of native and heate
5.0% ovalbumin solutions
Abbreviations. ANS, hydrophobicitydetc
acid; CP.'\S
determinedusing c'is-parinaric
protein solutionby heatingin the presence
i:
a c c o r d i n gt o t h e m e t h o do f B i g e l o u r r . r
potential;.i. solubility
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FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
teins, it should be noted that the correlation coefficients (i.e., the R2 values) are
not very high in many cases.This could mean either that the quantitation of
surface hydrophobicity using the fluorescent probe technique is not precise or
that other molecular descriptors(e.g., molecular flexibility) affecting the functional properties are not included in the empirical equations.
In addition to developing a basic understanding of the influence of various
molecular factors on the expression of functional properties of proteins, future
research should attempt to develop better methods to quantitate molecular
descriptors that affect a given functional property. Recently the electron spin
resonancetechniquehas been used to probe the flexibility of caseins.re6
4 good
correlation was found between the reorientational frequency of spin-labeled
amino acid residues of the protein and the foaming properties.le6 Gekko and
YamagamireTreported that the adiabatic compressibility of proteins can be used
as an index of protein flexibility and showed a positive correlation between the
foaming capacity, free energy of unfolding, and proteolysis of several food proteins and their partial specific adiabatic compressibility. Further basic research
is needed to elucidate the relationship between molecular properties and functional properties of food proteins.
References
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PROPERTIES
FUNCTIONAL
H. E. Swaisgood: Chemistry of r
P . F . F o x ( E d . ) , p p . l - 6 0 . E l s e ri c
W. Kauzmann: Some factors in tl
14 (1es9) r-63.
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ar
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I 985.
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1 9 . C . V . M o r r ,J . B . G e r m a nJ. . E
A. Lewis,andM. E. Mangtno:,
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procedurefor me;
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gluten and other ProteinPoh'm
PhillipsandJ. W. Finley (Eds.)
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) 9-345.
2 8 , 1 9 7 4n
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25. P.-H. Yang and J. A. Ruple1'
watersystem.Biochemistn'.12
26. J. A. Rupley,P.-H.Yang,and(
acting with proteins.In Water
127,pp. 111-132.AmericanC
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Rev.Food Sci.Nutr.24 (1986
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226
FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
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lysozyme and some acetyl derir:
t67-t79.
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228
FOOD PROTEINS:PROPERTIESAND CHAMCTERIZATION
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66. M. F. Perutz:Electrostatic
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85. A. Townsend and S. Nakai: Con
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4a
tJ.
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74. T. Mita, E. Ishido,and H. Matsumoto:Physicochemical
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102. D. H. Bergquist: In Egg Science c
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I123-B.
1 0 4 . M . S . B . J o s e p ha n d M . E . M a n g i n
foaming and gelation properties
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230
FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
105. S. Poole, S. I. West, and C. L. Walters: Protein-protein interactions:Their importance in the
foaming of heterogencoup
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106. S. Poole, S. I. West, and J. C. Fry: Charge and structural requirementsof basic proteins for
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FUNCTIONAL
PROPERTIES
1 2 4 . D . F . D a r l i n g a n d R . J . B i r k e r r :I
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125. N. S. Parker: Propertiesand tunc
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T e c h n o l .2 4 ( 1 9 8 9 ) l 2 l - 1 3 7 .
126. E. KeshavarzandS.Nakai: Thc,
of proteins. Biochim. Biophls. ,1
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209-223.
109. L' G. Phillips,S. T. Yang,W. Schulman,andJ. E. Kinsella:Effectsof lysozyme,clupeine,
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S c i . 5 4( 1 9 8 9 )7 4 3 - 7 4 7 .
I10. S. Poole,S. I. West,andJ. C. Fry: Lipid-tolerantproteinfoamingsystems.
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I I l. K. W. Pearceand J. E. Kinsella:Emulsifyingpropertiesof proteins:Evaluationof a turbidimetrictechnique.
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p. 75. AcademicPress,New York, 1976.
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116. K. P. Das and J. E. Kinsella:pH-Dependent
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l2l. C.W.N. Cumperand A. E. Alexander:The surfacechemistryof proteins.Trans.Faraday
S o c . 4 6( 1 9 5 0 )2 3 5 2 5 3 .
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123. H. J. Rivasand P. Sherman:Soy and meatproteinsas emulsionstabilizers:
4. The stability
and interfacialrheologyof o/w emulsionsstabilizedby soy and meatproteinfractions.Coll o i d sS u r f .l l ( 1 9 8 4 )1 5 5 - 1 7 1 .
128. M. Shimizu, M. Saito,and K. \'
proteins. Agric. Biol. Chem. S0 t
129. S. Damodaran: Influence of se
Biochim. Biophys. Acta,9l4 ttgl
1 3 0 . L . P . V o u t s i n a s ,S . N a k a i . a n d \ '
and thermal functional propenics
185 190.
l3l.
K . H . M c W a t t e r sa n d M . R . H o l n
properties ofheat denatured prorc
132. H. Aoki, D. Taneyama, N. Onmt
on its emulsion stabilizing propr1 3 3 . A . K a t o , N . T s u t s u i ,N . M a t s u d o
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2760.
134. N. Matsudomi, H. Mori, A. Kato.
heat denaturedsoybean I lS globu
C h e m .4 9 ( 1 9 8 5 )9 1 5 - 9 1 9 .
135. E. Tomberg: Functional characten
ior of proteins in a valve homoqc
1 3 6 . E . W . R o b s o na n d D . G . D a l * e l e i
J. Food Sci.52(1987)1694 168
137. E. Dickinson, A. Murray, and G.
films of casein * gelatin auhe oil
262.
138. J. Castle,E. Dickinson,B. S. Mur
oil-water interface.ln proteinsur
L. Brashand T. A. Horben(Eds
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232
FOODPROTEINS:PROPERTIES
AND CHARACTERIZATION
FUNCTIONAL
PROPERTIES
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K . S a m e j i m a ,M . I s h i o r o s h i .a n d
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FOODPROTEINS:
PROPERTIES
AND CHARACTERIZATION
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IVa
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gel formationof
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5.1 Introduction
This chapter discussesthe inte
food proteins and subsequent
properties.Food proteins have l
5000 B.c., mainly for the purp
products.An example is the en
and cheese.
More recently, however. int
studying structure-function rel
enzymatrc methods used for th
intended for use as foods. The
structure by safe chemical and
physical methods are also used
applicationsas well as accepta
This broad subject has been
of this chapteris to outline the
ically modified. More recent sr
ships of proteins are also disc
performed by the author's rese
alkylation, attachmentof amino
bic acid. Thesereactionsare tr(
of the chapter deals with the n
tions of food proteins.