Exudate Gums Exudate gums Gum arabic

O
Exudate Gums
O
Exudate gums
Gum arabic
„ Gum karaya
„ Gum ghatti
„ Gum tragacanth
„
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–
Use has diminished over the years because of
uncertain availability and increased cost
Only gum arabic still has important food
applications
O
Gum arabic
„
„
An article of commerce 4,000 years ago
Source
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–
Acacia trees in the Sudan and other African
countries
In the trees, this material seals wounds and
gives protection from bacterial infection
1
O
Gum arabic production
Most important area is central Sudan
„ Trees are tapped in October
„ 2 foot x 2 inch strips of bark are removed
„ Gum (20-200 grams) is hand-collected
over a 4 to 8 week period
„ Total production is about 60,000-80,000
tons
„
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About half is shipped to the US
O
Gum arabic
O
Gum arabic production
“Tapping” the Acacia tree
2
O
Gum arabic exudate
O
Gum arabic harvesting
O
Sorting the gum
3
O
Packaging the gum
Packaged in 50 or 100 kg burlap bags
O
Gum arabic
„
Structure
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D-galactose, L-arabinose, L-rhamnose, Dglucuronic acid
MW 250,000 to 1,000,000
Very complex structure (slightly acidic, pH
4.5-5.0)
O
Gum arabic analysis
Ash = 3%
Nitrogen = 0.29%
„ Methoxyl = 0.25%
„ Specific rotation = -30o
„ Intrinsic viscosity = 13.4
„ Uronic acid = 16%
„ MWn = 250,000
„ MWw = 580,000
„
„
4
O
Gum arabic analysis
DPn = 1550
„ DPw = 3600
„ Polymer is polydisperse
„ Composition
„
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D-galactose = 44%
L-arabinose = 24%
D-glucuronic acid = 14.5%
L-rhamnose = 13%
4-O-methyl-D-glucuronic acid = 1.5%
O
Heterogenity
70% of weight of gum is polysaccharide
with little or no nitrogenous material
„ Remaining higher molecular weight part
has protein as part of its structure (a
protein polysaccharide)
„ Attached to polysaccharide through
hydroxyproline and serine residues
„ Protein content on average is 2 wt% but
specific fractions may have up to 25 wt%
„
O
Sources of heterogenity
Tree species
Variety
„ Part of the tree
„ Geographic origin
„ Climate
„ Processing
„ Molecular size*
„ Protein/polysaccharide ratio*
„
„
*Most important variations determining properties
5
O
DGlcpA
1
6
3)-D-Galp
1
R
Proposed structure
of gum arabic
6
3)-D-Galp(1
6
R
R
R
3)-D-Galp(1
1
3)-D-Galp
6
1
3)-D-Galp
6
1
4)-D-GlcpA
R = L-Rhap(1
, L-Araf(1
R
R
3)-D-Galp(1
6
1
3)-D-Galp
6
1
3)-D-Galp
6
R
, D-Galp(1
1
4)-D-GlcpA
3-L-Araf(1
, or
3)-L-Araf(1
L-Arap(1
D-GlcpA = D-glucopyranosiduronic acid
D-Galp = D-galactopyranose
L-Rhap = L-rhamnopyranose
L-Arap = L-arabinopyranose
L-Araf = L-arabinofuranose
O
“Wattle blossom” structure
= carbohydrate
blocks
= polypeptide
chain (less than
10% protein in
molecule
O
Solution structure
Hydrodynamic radius determinations
suggest a very compact structure
„ This explains the abnormally low
viscosity of arabic dispersions
„ Polypeptide parts must be on the
periphery of the molecule and probably
account for gum arabic’s true
emulsifying capacity
„
6
O
Solution properties
Dissolves easily in water
„ Newtonian flow up to 40% concentration
„ Concentrations up to 50% can be made
„ Highest viscosity is produced at pH 6
„ Solubility is minimized below pH 3
„
O
Unique features of gum arabic
„
Unique because of
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High water solubility
Newtonian rheology up to 40%
concentration
Flavor encapsulation
O
Gum arabic uses
„
Uses
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Confectionary products
„ Highly
compatible with sugar and interferes with
sugar crystallization
„ Also emulsifies fats
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Ice cream
Flavor fixation
„ Prevents
oxidation, volatilization, and
degradation
7
O
Labeling
Gum arabic
„ Arabic gum
„ Gum acacia
„ Acacia gum
„
O
Non-food uses
Medicinal applications
„ Sizing fabrics and paper
„ Printing
„
O
Gum karaya
An acetylated polysaccharide exudate of
Sterculia urens
„ Grows in central and northern India
„ Annual production 5,000-6,000 tons
„ 80% is shipped to US
„ Labeling
„
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–
Gum karaya
Karaya gum
8
O
Gum ghatti
An exudate of Anogeisusus latifolia
„ Grows in India and Sri Lanka
„ Annual production is about 1,200 tons
„ Labeling
„
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Gum ghatti
Ghatti gum
O
Gum tragacanth
„
Source
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„
Astragalus genus shrub. First described
several centuries B.C. by Theophrastus
Grows in Iran, Syria, Turkey. Hand
collected, as is arabic.
Structure
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D-galacturonic acid, L-fucose, D-galactose,
D-xylose, L-arabinose
MW about 840,000
O
Usage
At one time US requirements were
almost 1,000 tons per year
„ Price increase caused other
polysaccharides to be developed and used
in the place of tragacanth
„ Cost of production remains high and
little tragacanth is used
„
9
O
Gum tragacanth
From “tragos” (goat) and “akantha” (horn)
O
Gum tragacanth
„
Tragacanth fractionation
Tragacanth
Tragacanthin demethylation Bassorin
30-40% of gum
60-70% of gum
water insoluble
water soluble
O
Gum tragacanth
„
Viscosity
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High at low concentration (0.5%)
pH independent
Molecular dimensions (19 x 4500 Å) account
for high viscosity
10
O
Gum tragacanth
„
Uses
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Salad dressings and sauces (acid stability)
Pickle relish (acid stability)
Bakery products
Ice creams, ices, sherbets
Frozen pie fillings
O
Labeling
Gum tragacanth
„ Tragacanth gum
„
11