Desarrollo de organogeles comestibles como alternativa al uso de

Transcription

ABSTRACT
The main component of candelilla wax (CW) is hentriacontane, an n-alkane (C31H64)
with self-assembly capability when dissolved in organic solvents. We have shown
that CW develops organogels using safflower oil as the liquid phase. This research
also has showed that the structural organization of CW -organogels depends on
cooling rate, the supercooling (i.e., the thermodynamic strength for crystallization),
and the annealing process. Additionally, the thermomechanical and microstructural
characteristics of CW-organogels in safflower oil were compared with the ones
showed by dotriacontane (C32H66; C32). Such comparisons were done using a
rheometer with a True-gap™ system and a factorial experiments design where the
variables evaluated were: gelator concentration (1% and 3%), gelation temperature
(Tset, 5ºC and 25ºC), and cooling rate (1ºC/min and 10ºC/min). The results showed
that the True-gap™ system provided a better correlation with the thermal behavior,
the solid phase content (SPC) and the microstructure. The DSC results shown that
independently of the cooling rate, T set, and gelator concentration, C32 had better
self-assembly properties than CW. Additionally, the microphotograph showed that
C32 developed larger needle-like crystals with higher extent of branching while CW
crystallized as platelets. The presence of minor molecular components (i.e.,
triterpenoids, nonacosane, and tritriacontane) in CW had a profound effect in the
crystal size developed by hentriacontane from the CW-organogeles. The results
showed that C32 developed organogels at significantly lower SPC than CW.
Nevertheless, C32 organogels achieved higher G’ profiles than CW, particularly at
1ºC/min and 3% of gelator concentration. The G’ profiles of the CW and C32
organogels also were higher at lower T set (i.e., 5ºC) and lower cooling rate (i.e.,
1ºC/min), indicating a more structured three-dimensional network in the gels.
Furthermore, under specific time-temperature conditions, CW organogels achieved a
better structural order as a function of time (i.e., annealing) through a solid-solid
transition from a rotator phase developed by the n-alkanes of CW. In contrast, C32organogels did not show this transition during the experimental conditions
investigated. In conclusion, this investigation showed that through organogelation it
is possible to structure vegetable oils into functional systems with potential
application in the development of edible trans-free food products.
RESUMEN
El componente principal de la cera de candelilla (CW) es el hentriacontano (C 31H 64),
un hidrocarburo con capacidad de auto-ensamblarse cuando se disuelve en
solventes orgánicos. Nosotros hemos demostrado que CW desarrolla organogeles
usando aceite de cártamo como fase líquida. Esta investigación también ha
demostrado que la organización estructural de los organogeles de CW depende de
la velocidad de enfriamiento, el superenfriamiento (i.e. la fuerza termodinámica para
la cristalización), y el proceso de temperado. Adicionalmente, las características
termo-mecánicas y microestructurales de los organogeles de CW en aceite de
cártamo fueron comparadas con las mismas características mostradas por
dotriacontano (C 32H 66; C32). Estas comparaciones fueron hechas usando un
reómetro con un sistema de True-gap™ y un diseño de experimentos factorial
donde las variables evaluadas fueron: concentración del gelante (1% y 3%),
temperatura de gelación (T set = 5ºC y 25ºC) y velocidad de enfriamiento (1ºC/min y
10ºC/min). Los resultados muestran que el sistema de True-gap™ proporcionó una
mejor correlación con el comportamiento térmico, el contenido de fase sólida (SPC)
y la microestructura. Los resultados de DSC mostraron que independientemente de
la velocidad de enfriamiento, T set y concentración del gelante, C32 tuvo mejores
propiedades de auto-ensamblado que CW. Adicionalmente, las microfotografías
mostraron que C32 desarrollo cristales grandes tipo aguja con alto mayor grado de
ramificacion mientras que CW cristalizó
como placas. La presencia de
componentes moleculares minoritarios (ej., triterpeniodes, nonacosano y
tritriacontano) en CW tuvieron un profundo efecto en al tamaño de cristal
desarrollado por el hentriacontano de los organogeles de CW. Los resultados
mostraron que C32 desarrollo organogeles a SPC significativamente menor que
CW. Sin embargo, los organogeles de C32 alcanzaron perfiles de G’ más altos que
CW, particularmente a 1ºC/min y 3% de concentración del gelante. Los perfiles de
G’ de los organogeles de CW y C32 también fueron más altos a bajas T set (ej., 5ºC)
y bajas velocidades de enfriamiento (ej., 1ºC/min), indicando una estructura
tridimensional más estructurada en los geles. Además, bajo condiciones específicas
de tiempo-temperatura, los organogeles de CW alcanzaron un mejor orden
estructural como una función del tiempo (i.e. anillado) a través de una transición
sólido-sólido desde la fase de rotación desarrollada por n-alcanos de CW. En
contraste, los organogeles de C32 no mostraron esta transición durante las
condiciones experimentales investigadas. En conclusión, esta investigación
demostró que a través de la organogelación es posible estructurar aceites vegetales
en sistemas funcionales con potencial aplicación en el desarrollo de productos
comestibles libres de trans.
INDICE
I.
ABSTRACT
II.
INTRODUCCIÓN
III.
RESUMEN EN EXTENSO Y PRIMER ARTÍCULO
IV.
RESUMEN EN EXTENSO Y SEGUNDO ARTÍCULO
V.
CONCLUSIONES GENERALES
VI.
ANEXO
a. ARTÍCULO EXTRA
INTRODUCCIÓN.
Típicamente, los aceites vegetales, tales como el aceite de olivo, soya, maíz, girasol,
canola, cártamo, entre otros, están formados principalmente por triacilglicéridos (ej., 9498%), los cuales son la esterificación de ácidos grasos saturados e insaturados en una
molécula de glicerol. Los triacilglicéridos de los aceites tienen relativamente un bajo
contenido de ácidos grasos saturados, y la doble ligadura que se encuentra en el ácido
insaturado están en configuración cis. Sin embargo, estos aceites vegetales no poseen de
manera natural las propiedades funcionales que cumplan con las expectativas de textura y
estabilidad que el consumidor demanda en los productos alimenticios que se distribuyen
actualmente en el mercado mundial. Para incrementar dicha funcionalidad (i.e., la
estabilidad oxidativa, el punto de fusión), los aceites son sometidos a procesos de
hidrogenación, el cual consiste en la saturación de los dobles enlaces de los ácidos grasos
insaturados manteniendo como reactivo limitante el hidrógeno (ej., hidrogenación parcial).
Este proceso es particularmente usado para impartir cremosidad, estabilidad, sabor a
muchos productos alimenticios como margarinas, productos para untar, mantecas y grasas
para freído. Sin embargo, durante la hidrogenación parcial, algunas de las dobles ligaduras
son isomerizadas de la configuración original cis a la forma trans. Desafortunadamente, las
investigaciones clínicas y nutricionales de los últimos años indican el que dietas que
incluyen ácidos grasos trans ocasionan, al igual que los ácidos grasos saturados, un
decremento de las lipoproteínas séricas de alta densidad (“colesterol bueno” o HDL) y un
aumento en las lipoproteínas séricas de baja densidad (“colesterol malo” o LDL), resultando
en un incremento en el nivel de colesterol sanguíneo y por lo tanto del riesgo de
enfermedades coronarias. Además, Investigaciones recientes indican que una vez ingeridos
las grasas trans se aumenta el riesgo de desarrollar diabetes tipo 2 en mujeres y en
pacientes obesos.
En consecuencia a toda esta evidencia del impacto en la salud derivada del
consumo de ácidos grasos trans, en julio del 2003 la Administración Reguladora de
Alimentos y Drogas de los Estados Unidos de América (Food and Drug Administration, FDA)
reglamentó que a partir del 1º de Enero del 2006, todos los alimentos producidos en los
Estados Unidos o bien importados por este país, incluyan como información al consumidor
la concentración de ácidos grasos trans en la etiqueta de composición de nutrientes. Esta
reglamentación tuvo un profundo impacto en la industria de aceites comestibles y alimentos
a nivel mundial, y en México no fue la excepción, las industrias tuvieron que buscar nuevas
estrategias de reformulación y procesamiento para disminuir el contenido de ácidos grasos
trans y saturados en alimentos que contienen aceites e incrementar el valor nutrimental.
Para mejorar la calidad en cuanto a la funcionalidad y la estabilidad oxidativa, varias
alternativas para la elaboración de grasas han sido empleadas por la industria. Las
alternativas que actualmente se investigan a nivel mundial son:
1. Desarrollo de procesos de interestificación (química o enzimática) entre aceites y
grasas comestibles, o bien entre fracciones de TAGS de alta y baja temperatura de
fusión.
2. Desarrollo de estructuras tipo gel a base de mezclas de TAGS de baja y alta
temperatura de fusión.
3. Desarrollo de organogeles a través de mezclas de aceites vegetales y agentes autoensamblantes (ej., ácidos grasos, ceras, carbamatos).
El proceso de interestificación de aceites vegetales para modificar las propiedades
funcionales de aceites vegetales es, actualmente, una tecnología que se encuentra limitada
por factores de carácter económico. Por lo anterior, el costo del producto generado (ej.,
fracciones de familias de TAGS con alta temperatura de fusión y grasas “cero” trans) es
elevado. Por otro lado, el desarrollo de organogeles y geles a base de mezclas de TAGS
de baja y alta temperatura de fusión, representan alternativas con alto potencial de
implementación, menor costo de inversión y producción, pero aún escasamente
investigadas en sus aspectos básicos.
El desarrollo de organogeles es una alternativa prometedora que puede ser
utilizada para modificar las propiedades físicas (ej., reología) de aceites vegetales sin el
uso de modificaciones químicas (ej., hidrogenación parcial) que resulten en la producción
de ácidos grasos trans. Los organogeles son materiales visco-elásticos formados por una
molécula gelante y un solvente orgánico. En general, la formación de un organogel se
basa en el auto-ensamblaje de moléculas de bajo peso molecular (< 3000 Da) en una fase
apolar, que deriva en el desarrollo de una red tridimensional formada por estructuras
fibrilares entrelazadas, o bien por estructuras tipo placas. Entre las moléculas gelantes
dentro de esta categoría se encuentran derivados de ácidos grasos, derivados de
carbohidratos, esteres de alcoholes de cadena larga y n-alcanos de cadena larga. Los nalcanos de cadena larga constituyen el componente más importante de aceites y
lubricantes automotrices, además son componentes importantes en parafinas, ceras
animales (ej. cera de abeja) o ceras vegetales, como la cera de la candelilla.
De manera particular, la cera de candelilla, principal producto obtenido de la
candelilla (E. antisyphilitica), se compone de ésteres, ácidos grasos, alcoholes y, de manera
predominante, de n-alcanos de cadena larga (29 a 33 carbonos) en aproximadamente un
40% a 50% en peso. Dada su inocuidad comprobada en diversos estudios clínicotoxicológicos puede utilizarse, sin restricción alguna, como un aditivo alimentario acorde a la
regulación 184.1976 de la Administración Reguladora de Alimentos y Drogas de los Estados
Unidos (Food and Drug Administration, FDA). Así, se emplea como agente aglomerador en
la fabricación de chicles, como desmoldante en productos panaderos y como agente
“lustrador” en diversos productos incluyendo frutas. Debido a la alta cantidad de n-alcanos
que presenta la cera de candelilla así como su utilización en la industria de alimentos puede
emplearse como un fuerte candidato en el desarrollo de organogeles comestibles.
Propiedades Termo-mecánicas de Organogeles de Cera de Candelilla y
Dotriacontano en Aceite de Cártamo
Juan A. Morales-Rueda1, Elena Dibildox-Alvarado2, Miriam A. Charo-Alonso2, Richard G. Weiss3 y
Jorge F. Toro-Vazquez2.
1
Universidad Autónoma de Querétaro, DIPA-PROPAC, México. 2 Universidad Autónoma de San Luis
3
Potosí, Facultad de Ciencias Químicas, México. Georgetown University, Department of Chemistry,
Washington, USA.
RESUMEN
Las propiedades termo-mecánicas de organogeles desarrollados por una mezcla
compleja de n-alcanos presente en la cera de candelilla (CW) fueron investigadas y
comparadas con organogeles desarrollados con dotriacontano (C32), un n-alcano puro.
Ambos organogeles fueron desarrollados usando como fase líquida aceite de cártamo alto
en trioleína (SFO). La cera de candelilla (Multiceras, Monterrey, México) y el dotriacontano
(Humphrey Chemical Co., CT, USA) fueron analizadas por GC-MS. Muestras de CW, C32 y
las correspondientes dispersiones en SFO (desde 0.5% hasta 10% p/v) fueron usadas para
determinar los termográmas de gelificación, a dos velocidades de enfriamiento (1ºC/min y
10ºC/min), y de fusión (5ºC/min) por DSC. De los termográmas de gelificación se obtuvieron
la temperatura de gelificación, T g y la entalpía de gelificación,
Hg. Así mismo, de los
termográmas de fusión se obtuvieron las temperaturas de fusión, T M y la entalpía de fusión
HM. Para ambos casos, CW o C32, las variables estudiadas fueron dos niveles de
concentración del agente gelante (1% y 3%), dos velocidad de enfriamiento (1ºC/min y
10ºC/min) y dos niveles de temperatura establecida T set para la gelificación (Tset; 5ºC y
25ºC). Los tratamientos asignados para cada sistema fueron resultado de un diseño
factorial
completamente
aleatorio.
Para
cada
tratamiento,
fueron
hechas
dos
determinaciones independientes (n=2). Los organogeles desarrollados bajo cada condición
fueron caracterizados por reometría, calorimetría, contenido de fase sólida por resonancia
magnética nuclear y microscopía de luz polarizada.
Los resultados del análisis de la CW mostraron que está compuesta principalmente
de hentriacontano (78.9 ± 0.1%), un n-alcano de 31 carbonos, mientras que los
componentes minoritarios incluyen nonacosano (C29, 4.2 ± 0.1%) y tritriacontano (C31, 8.0
± 0.2%), triterpenos como germanicol, lupeol o moretenol (7.4 ± 0.1%) y compuestos no
identificados (1.6 ± 0.1%). Por otro lado, la pureza del dotriacontano fue de 99.5 ± 0.01%.
Las propiedades térmicas de CW y C32 en estado puro mostraron lo siguiente. El
termograma de gelificación para CW (Fig. 1) mostró una exotérma con una T g de 76.6 ± 0.7
ºC y dos picos a 59ºC y 53ºC, un
Hg de 147.4 ± 1.9 J/g. Así mismo, el termográma de
fusión mostró una endotérma con una temperatura de fusión (T p) de 64.4 ± 0.2 ºC y un HM
de 149.8 ± 1.2 J/g. La posible explicación del comportamiento térmico de CW fue dada por
el desarrollo de una fase de rotación. La fase de rotación es comúnmente observada en nalcanos y es caracterizada por un desorden en la orientación; la orientación espacial de las
moléculas en su eje axial es preservada, sin embargo las moléculas pueden rotar alrededor
de este eje. Entonces, durante el enfriamiento, el hentriacontano pudo desarrollar una fase
de rotación proveniente de la materia fundida hasta el primer pico de 59 ºC (Fig.1), seguida
de una transición sólido-sólido hasta el estado cristalino del alcano a 53ºC. Por otro lado, el
termográma de enfriamiento de C32 mostró dos exotermás con temperaturas de pico de
67.5 ºC y 62.2 ºC, los cuales fueron asociados al desarrollo de una fase de rotación
proveniente del fundido y a la transición de la fase de rotación hacia el estado cristalino,
respectivamente (Fig.1). Los termográmas de fusión para C32 mostraron dos principales
endotérmas con Tp de 66 y 69 ºC (Fig.1). El primer endotérma fue asociado con una
transición sólido-sólido de la forma cristalina a la fase de rotación y el segundo endotérma
fue asociado a la transición de la fase de rotación a la fase líquida.
Las propiedades térmicas de los organogeles de CW y C32 en SFO mostraron
termográmas simples con dos picos principales, en donde el mayor pico de la
endotérma/exotérma fue asociado a la cristalización de los triacilglicéridos. El pico
minoritario fue asociado al proceso de gelificación o fusión de CW o C32 (Fig.2A). Durante
el proceso de gelificación y fusión de los organogeles en SFO no se presento el desarrollo
de fase de rotación. La figura 2B mostro que independientemente de la concentración de
agente gelante y la velocidad de enfriamiento. T g fue mayor para C32 que para CW,
además, mayor concentración de CW que C32 fue requerida para alcanzar la misma T g.
Estos resultados sugieren que las moléculas de C32 tienen mayor capacidad para
autoensamblarse en el aceite que la mezcla de alcanos presente en la CW. Estos
resultados fueron confirmados por el mayor calor de fusión (Fig. 3) y tamaño de cristal de
C32 (Fig. 4A y 4C) que lo presentado por CW (Fig. 4B y 4D). El comportamiento de
HM
(Fig. 3) y las microfotografías (Fig. 4) sugieren que, la presencia de otros alcanos (i.e.,
nonacosano y tritriacontano) y triterpenos pudo provocar el desarrollo de una estructura
tridimensional con baja cristalinidad y cristales más pequeños que lo desarrollado por un
alcano puro como el C32.
Para el contenido de fase sólida (SPC) de CW se observó que no fue afectado por la
velocidad de enfriamiento ni por la temperatura de gelificación establecida, Tset (Fig. 5).
Además, el SPC de los organogeles de CW fue menor cuando se utilizó una T set de 25ºC
que cuando se uso una Tset de 5ºC. Sin embargo, la velocidad de enfriamiento tuvo un
efecto en los organogeles de C32, excepto para la concentración del 1%. La T set no tuvo
efecto en el SPC de organogeles a 1% de concentración, pero si afecto a lo organogeles del
3%. Entonces, el C32 forma organogeles con menor SPC que los organogeles de CW (Fig.
5), esto es independiente de la velocidad de enfriamiento y de la T set. Estos resultados
demostraron la alta solubilidad y capacidad de autoensamble de C32 en SFO.
Los perfiles del módulo de almacenamiento (G’) de ambos sistemas bajo las
diferentes condiciones estudiadas son mostrados en la figura 6. Los perfiles de G’ de los
organogeles de CW son más altos que los presentados por C32. Este comportamiento
puede estar asociado al alto SPC y al menor tamaño de los cristales que componen la red
tridimensional de los organogeles de CW (Fig. 4B y D). Los resultados mostraron que a
mayor contenido de sólidos, mayor es el perfil de G’. Esto fue más evidente con los
organogeles desarrollados a bajas velocidades de enfriamiento (compare las figuras 6A y
6B). El mismo comportamiento mostró los organogeles de C32, donde los geles
desarrollados a 1ºC/min (Fig. 6C) tuvieron mayor perfil reológico que los organogeles
desarrollados a 10ºC/min (Fig. 6D). Este fenómeno fue independiente de Tset. Se puede
notar en los reogramas que hubo un cambio en la estructura de la red de ambos sistemas
en función del tiempo; los valores de G’ decrecieron desde G’ 0 (i.e., el valor de G’ al tiempo
cero) hasta G’f (i.e. valor de G’ después de 180 min). Este fenómeno se presento más
evidente con organogeles de C32 desarrollados a 10ºC/min y una T set de 25ºC. Por otro
lado, cuando los módulos de ambos sistemas, CW y C32, fueron graficados en función de
SPC (Fig. 7), fue evidente que G’0 y G’f incrementaron de manera logarítmica en función de
SPC. Sin embargo, no hubo diferencia significativa entre G’ 0 y G’f a 1ºC/min (Fig. 7A y C).
En contraste, a 10ºC/min (Fig. 7B y D) G’ 0 y G’f fueron menores que a 1ºC/min,
particularmente en organogeles de C32. Estos resultados mostraron que las estructuras de
los organogeles fueron más estables a temperaturas de 5ºC y bajas velocidades de
enfriamiento.
Para entender mejor el comportamiento reológico, es necesario reconocer el
desarrollo de la fase de rotación y su impacto en las zonas de unión de las microplacas que
forman los agregados de la red tridimensional de los organogeles. La formación de la fase
de rotación y la transición sólido-sólido ocurre en función del tiempo y depende de la
temperatura Tset. Por lo tanto, durante el periodo de enfriamiento, la fuerza termodinámica
para la formación del gel incremento más rápido a medida que se incremento la velocidad
de enfriamiento. Como consecuencia, las moléculas del agente gelante tuvieron menos
tiempo para organizarse a 10ºC/min que a 1ºC/min. El resultado general fue el desarrollo de
un paquete molecular menor organizado (i.e. fase de rotación) a 10ºC/min que a 1ºC/min,
particularmente cuando se empleo altas temperaturas (i.e. T set = 25ºC). Con este esquema y
asumiendo que el modelo estructural para organogeles de alcanos propuesto por Abdallah y
col. (Referencia 9) se aplica aquí, la presencia de una fase de rotación, particularmente en
las zonas de unión, puede resultar en una modificación de la microestructura a nivel de las
microplacas durante la medición de G’ (Fig. 6). Basado en lo anterior, se espera que las
zonas de unión sean menor organizadas en organogeles desarrollados a 10ºC/min,
particularmente a Tset = 25ºC (Fig. 6B y D).
Los resultados del calor de fusión mostraron, particularmente en geles de CW, que
HM fue mayor en organogeles desarrollados a 1ºC/min que a 10ºC/min, esto
independientemente de la Tset (Fig.8). Además, los organogeles desarrollados a
temperaturas de 25ºC, el HM incremento en función del tiempo, un proceso probablemente
asociado con el perfeccionamiento del empaquetamiento molecular proveniente del
empaquetamiento alcanzado originalmente (ej., fase de rotación). Este efecto fue más
evidente en organogeles de CW a 3% (Fig. 8B). En contraste, los organogeles de C32,
aunque hubo valores de
HM más grandes a 1ºC/min que a 10ºC/min bajo todas las
condiciones evaluadas, las diferencia no fueron significativas. Además, en todas las
condiciones valoradas, los valores de HM permanecieron contantes en todo el intervalo de
tiempo, indicando la auncencia de una transición sólido-sólido. Esto es explicado debido a
que el superenfriamiento al que fueron sometidos los sistemas de C32 fue suficiente para
desarrollar los cristales en ausencia de fase de rotación. Por lo tanto, el desarrollo de una
fase de rotación pudo ser más probable en organogeles con CW que con organogeles con
C32.
En conclusión, los resultados obtenidos en esta investigación demostraron que es posible
gelificar aceite de cártamo en un proceso de organogelación con CW y sin la presencia de
ácidos grasos trans. Basados en la comparación con organogeles de C32, la presencia de
componentes minoritarios de la cera de candelilla puede tener un profundo efecto en la
formación de cristales de n-alcanos, y por consiguiente en sus propiedades físicas.
Propiedades Reológicas de Organogeles de Cera de Candelilla y
Dotriacontano Medidos con un Sistema True-Gap™.
b
a
a
Juan A. Morales-Rueda , Elena Dibildox-Alvarado , Miriam A. Charó-Alonso , and Jorge F. ToroVazqueza*
a
b
Universidad Autónoma de San Luis Potosí, Facultad de Ciencias Químicas, México; Universidad
Autónoma de Querétaro, DIPA-PROPAC, México
RESUMEN
Mediciones reológicas de organogeles desarrollados con cera de candelilla (CW) y
un n-alcano puro (dotriacontano, C32) fue evaluada con un reómetro equipado con un
sistema de medición True-gap™ y comparadas con los reogramas obtenidos de un equipo
con un sistema de espesor fijo. Los dos sistemas usaron geometrías de cono-plato. En
contraste con el sistema de espesor fijo, el True-gap™ realiza correcciones en el espesor
asociados a la expansión/contracción de la muestra y/o la geometría del equipo cuando
ocurren cambios de temperatura durante la medición. Los organogeles de CW y C32 fueron
preparados usando como fase líquida aceite de cártamo alto en trioleína (SFO) y los
tratamientos estudiados resultaron de la combinación factorial de dos niveles de
concentración de la molécula gelante (1% y 3%), dos temperaturas para el desarrollo de los
geles (Tset; 5ºC y 25ºC) y dos velocidades de enfriamiento (1ºC/min y 10ºC/min). Los
tratamientos fueron distribuidos aleatoriamente en las dispersiones de CW y C32 en aceite
de cártamo. Para cada tratamiento fueron realizadas dos mediciones independientes (n=2).
Los módulos de almacenamiento (G’) y de pérdida (G”) de los organogeles fueron
evaluados con un espectrómetro mecánico (Para Physica MCR 301, Stuttgart, Alemania)
usando una geometría de plato-cono (50 mm de diámetro, 1º, CP50-1/TG, Anton Paar,
Graz-Austria) equipado con un sistema True-gap™. Los módulos G’ y G” fueron
determinados en función del tiempo entre 0 y 180 minutos, siempre dentro de la región
lineal viscoelástica (RLV) de las muestras. Los parámetros calorimétricos (i.e. calor de
fusión; HM), el contenido de fase sólida (SPC) y las microfotografías de los organogeles de
CW y C32 determinados en un trabajo previo fueron usados en esta investigación. Todos
estos parámetros fueron medidos en función del tiempo (0 hasta 180 min) después de que
alcanzaron la temperatura preestablecida a una dada velocidad de enfriamiento.
Los resultados de la composición de CW y C32 ya han sido reportados, pero de
manera rápida tenemos que CW está compuesta principalmente por ≈79% de
hentriacontano (C31H64), mientras que el C32 tuvo una pureza de 99.5%. Las
microfotografías obtenidas por microscopía de luz polarizada (PLM) (Fig.1-4) mostraron que
C32 desarrolló cristales más grandes con alto grado de birrefringencia (Fig. 3 y 4) que los
cristales desarrollados por CW (Fig. 1 y 2). Estos resultados señalaron que C32 tiene mejor
capacidad de auto-ensamblarse en SFO que CW. La presencia de componentes
minoritarios en CW pudo influir en el desarrollo de estructuras de menor cristalinidad, menor
calor de fusión y menor tamaño que las estructuras desarrolladas por C32.
Perfiles de G’ para organogeles de C32 al 1% y CW al 3% obtenidos con las
técnicas de espesor fijo y True-gap™ a 1ºC/min y 10ºC/min son mostrados en la figura 5.
Además, los valores de sólidos presentes en los oganogeles son mostrados,
independientemente de la técnica usada. En general, con el sistema True-gap™ se observó
un patrón constante en los valores de G’ en función del tiempo para los organogeles de C32
en todas las condiciones evaluadas, esto para concentraciones del 1% (Fig. 5A y 5C) y del
3% (datos no mostrados). En contraste, bajo las mismas condiciones de tiempo y
temperaturas evaluadas, el sistema de espesor fijo mostró un continuo descenso en los
valores de G’ en geles al 1% (Fig. 5A y 5C) y geles al 3% (datos no mostrados) hasta
alcanzar una meseta. Resultados similares fueron observados con los organogeles de CW
(Fig. 5B y 5D), Sin embargo, con el sistema de espesor fijo un comportamiento distinto fue
observado a la velocidad de 10ºC/min y una T set de 25ºC. Con estas condiciones, los
organogeles de CW al 3% mostraron un descenso continuo en los valores de G’ a T set de
25ºC (Fig. 5D). Este descenso fue más evidente en organogeles al 1% y también fue
observado a Tset de 5ºC. Sin embargo, con el sistema True-gap™, los organogeles de CW
al 3% desarrollados a la velocidad de enfriamiento de 10ºC/min a ambas T set se observó un
incremento inicial en los valores de G’ hasta alcanzar paulatinamente una meseta (Fig. 5D).
Los organogeles de CW al 1% mostraron un comportamiento similar (datos no mostrados).
Los resultados obtenidos con el sistema de espesor fijo para los perfiles de G’ fueron
explicados a que cuando la rampa de temperatura usada durante las mediciones reológicas,
particularmente a altas velocidades de enfriamiento (ej., 10ºC/min; Fig. 5C y 5D), el ajuste
automático del espesor fue inadecuado debido a que la geometría del equipo y la muestra
no alcanzaron su expansión/compresión final. Por lo tanto, los resultados mostrados en la
fig.5 demostraron que la falta de un apropiado ajuste en el tamaño del espesor afecto
nuestras mediciones, particularmente a velocidades de 10ºC/min. Por otro lado, el sistema
True-gap provee un tamaño de espesor real y contante durante las mediciones reológicas
de los organogeles. En general, los perfiles de G’ obtenidos con este sistema mostraron que
para el mismo tipo de molécula gelante, los valores de G’ fueron mayores a 1ºC/min que a
10ºC/min (i.e. los organogeles desarrollados a 1ºC/min alcanzaron un mayor orden
estructural que los organogeles desarrollados a 10ºC/min). Cuando los valores de G’
después de 180 minutos (G’180) fueron graficados en función del correspondiente contenido
de sólidos (Fig. 6), fue evidente que para la misma concentración de agente gelante y
condiciones de tiempo-temperatura los valores de G’180 fueron más altos con C32 que con
CW, particularmente a 1ºC/min y 3% de concentración. Estos resultados nuevamente
puntualizan que C32 tiene mayor capacidad de auto-ensamblarse en SFO.
Investigaciones han demostrado que la forma estructural de la red tridimensional de
organogeles fibrilares es dependiente de la supersaturación del sistema. Se ha demostrado
que la formación de zonas de unión transitorias (ej., fibras entrelazadas) y permanentes (ej.,
ramificaciones fibrilares) determinan las propiedades reológicas de la red tridimensional del
gel. Por lo tanto, los organogeles formados con una red fibrilar (i.e., alto contenido de zonas
de unión transitorias) proporcionan mayor elasticidad que los organogeles formados con
redes formadas por estructuras esferulíticas (i.e., alto contenido de zonas de unión
permanentes). Las microfotografías obtenidas por PLM demostraron que los organogeles de
C32 desarrollados a 1ºC/min tienen mayor cantidad de estructuras tipo aguja con alto grado
de ramificación (Fig. 3) que los organogeles desarrollados a 10ºC/min. Una explicación
similar puede ser aplicada a los organogeles de CW, sin embargo la magnificación usada en
este estudio no fue suficiente para apreciar las características
microscópicas de los
cristales de CW (Fig. 1 y 2).
De la misma manera, los reogramas obtenidos con el sistema True-gap™ mostraron
que los organogeles de CW fueron alcanzando un orden estructural mayor a medida que
incrementó el tiempo. Este fenómeno fue más evidente a 10ºC/min (Fig. 5D), bebido a que
la fuerza termodinámica impulsora para la formación del gel incremento más rápido a
velocidades de enfriamiento altas. Consecuentemente, las moléculas gelantes tienen menor
tiempo para organizarse a 10ºC/min que a 1ºC/min. Lo que significa que las moléculas de nalcano presentan un menor empaquetamiento molecular a 10ºC/min. Esto fue más evidente
a altas Tset (ej. 25ºC), donde un menor superenfriamiento prevaleció (Fig. 5D). Una vez
alcanzada la Tset los organogeles de CW evolucionaron conforme pasó el tiempo desde un
fase de rotación desarrollada por los n-alcanos a un estado de mayor orden a través de una
transición sólido-sólido. Entonces, los organogeles de CW desarrollados a 1ºC/min
presentaron mayor
HM (alto nivel estructural y por consiguiente mayor G’) que los
organogeles de CW desarrollados a 10ºC/min (Fig. 7A). Adicionalmente, los organogeles
desarrollados a Tset de 25ºC,
HM incremento en función del tiempo, un proceso
probablemente asociado con el desarrollo de una fase mejor organizada que la inicialmente
alcanzada (i.e. fase de rotación) a través de un proceso de anillado.
Por otro lado, los organogeles de C32 desarrollados con el sistema True-gap™
mostraron un perfil continuo en los valores de G’ a todas las condiciones evaluadas. Los
perfiles de G’ para los organogeles de C32 fueron siempre mayores a velocidades de
1ºC/min (Fig. 6). Con esta velocidad de enfriamiento los cristales de C32 fueron más
grandes que los desarrollados a 10ºC/min (Fig. 4), un proceso que resultó en la formación
de organogeles con alto nivel de organización tridimensional (i.e., alta dimensión fractal)
(Fig. 6). Además, los organogeles de C32 no presentaron una diferencia significativa en los
valores de
HM cuando fueron evaluados a las distintas velocidades de enfriamiento.
Adicionalmente,
HM permaneció constante en función del tiempo bajo todas las
condiciones de tiempo y temperatura evaluadas, indicando la ausencia de una transición
sólido-sólido. Esto es debido a que el superenfriamiento aplicado a los organogeles de C32
fue el suficiente para no desarrollar una fase de rotación.
En conclusión, los resultados obtenidos con el sistema True-gap™ concordaron con
el comportamiento de HM (Fig. 7), tamaño de cristal y organización tridimensional de la red
cristalina observada en los organogeles de CW y C32 (Fig. 1-4). Entonces, el empleo de la
técnica de espesor fijo debe hacerse con cuidado especialmente en determinaciones de G’
dependientes del tiempo y mediciones reológicas donde se involucren el uso de rampas de
temperatura de alta velocidad (ej. 10ºC/min).
CONCLUSIONES
Los resultados obtenidos en esta investigación demostraron que es posible gelificar
aceite de cártamo en un proceso de organogelación con CW y n-alcanos de cadena larga
sin la presencia de ácidos grasos trans y ácidos grasos saturados. El proceso de
organogelación se presenta como una estrategia factible para la modificación de las
propiedades físicas de los aceites vegetales sin el empleo de la hidrogenación parcial.
Bajo todas las condiciones evaluadas en esta investigación, la cera de candelilla
demostró ser un excelente agente gelificante de aceite de cártamo alto en trioleína, sin
embargo, es muy probable la cera de candelilla gelifique en la mayoría de los aceites
vegetales. Los estudios han demostrado que los organogeles de CW tienen una estabilidad
a la separación de fases al menos por a un año con una textura con alto potencial en la
industria de los alimentos.
Esta investigación también demostró que la presencia de componentes minoritarios
de la cera de candelilla puede tener un profundo efecto en la formación de cristales de nalcanos, y por consiguiente en sus propiedades físicas. Sin embargo, las microfotografías
obtenidas por PLM para los organogeles de CW demostraron
no tener la suficiente
magnificación para apreciar las características microscópicas de los cristales de CW.
Además, los estudios realizados en este trabajo demostraron que bajo ciertas
condiciones de tiempo y temperatura se pueden desarrollar geles de diferente plasticidad
(ej., fase de rotación de los n-alcanos presente en la cera de candelilla), los cuales puedes
ser manipulados para los fines que la industria de los alimentos requiera.
Finalmente, los resultados obtenidos con el sistema True-gap™ concordaron con el
comportamiento calorimétrico, tamaño de cristal y organización tridimensional de la red
cristalina observada en los organogeles de CW y C32. Entonces, el empleo de reómetros
sin el sistema de True-gap™ debe hacerse con cuidado especialmente en determinaciones
de G’ dependientes del tiempo y mediciones reológicas donde se involucren el uso de
rampas de temperatura de alta velocidad (ej. 10ºC/min).
ANEXOS
J Am Oil Chem Soc
DOI 10.1007/s11746-009-1414-3
ORIGINAL PAPER
Rheological Properties of Candelilla Wax and Dotriacontane
Organogels Measured with a True-Gap System
Juan A. Morales-Rueda Æ Elena Dibildox-Alvarado Æ
Miriam A. Charó-Alonso Æ Jorge F. Toro-Vazquez
Received: 14 April 2009 / Revised: 14 May 2009 / Accepted: 26 May 2009
Ó AOCS 2009
Abstract The rheology of organogels developed by candelilla wax (CW) and a pure n-alkane (dotriacontane, C32)
was evaluated with a rheometer equipped with a true-gap
system and compared with the rheograms obtained with a
fixed-gap system. The two systems used a cone and plate
geometry. In contrast to the fixed-gap system, the true-gap
system makes the corrections in the gap size associated with
the expansion/shrinkage of the sample and/or the rheometer
geometry when changing temperature conditions are used
during measurements. The CW and C32 organogels were
prepared using safflower oil high in triolein (SFO) as the
liquid phase, and the treatments studied resulted from the
factorial combinations of two levels of gelator concentration (1 and 3%) and two gel setting temperatures (Tset; 5 and
25 °C) achieved using a cooling rate of 1 or 10 °C/min. The
use of the true-gap system provided rheological parameters
(i.e., G0 profiles) that agreed with the micro structure and the
calorimetric (i.e., heat of melting, DHM) behavior of both
the CW and the C32 organogels. The use of a fixed-gap
system in the rheological characterization of organogels
must be treated with caution, specially with time dependent
E. Dibildox-Alvarado M. A. Charó-Alonso J. F. Toro-Vazquez
Facultad de Ciencias Quı́micas,
Universidad Autónoma de San Luis Potosı́,
San Luis Potosı́, Mexico
J. A. Morales-Rueda
Universidad Autónoma de Querétaro,
DIPA-PROPAC, Querétaro, Mexico
J. F. Toro-Vazquez (&)
Facultad de Ciencias Quimicas-CIEP,
Zona Universitaria, Av. Dr. Manuel Nava 6,
78210 San Luis Potosı́, Mexico
e-mail: [email protected]
G0 determinations involving the use of fast temperature
ramps (i.e., 10 °C/min).
Keywords Rheology \ Lipid chemistry/Lipid analysis Lipid chemistry/Lipid analysis
Introduction
During recent years, organogels developed with low
molecular weight compounds, such as fatty acids and nalkanes, have received a great deal of attention mainly
because these molecules require only a small concentration
(B2%) to achieve gelation [1–4]. However, very little is
known about their rheological properties as affected by
different time–temperature conditions. Rogers and Marangoni [5, 6] studied the effect of cooling rate and type of
solvent on the non-isothermal nucleation and crystallization
kinetics of organogels made with 12-hydroxystearic acid.
These authors observed that cooling rates lower than
5 °C/min resulted in organogels with few crystal nuclei that
grew large and showed very little branching. In comparison,
the organogels developed at cooling rates higher than
5 °C/min showed higher nucleation and a highly branched
fibrillar network [5]. Since crystal size, branching, and
junction zones among fibers that form the three-dimensional network determine the gel rheological properties [7],
Rogers and Marangoni [5] proposed that the organogel’s
elasticity might be engineered through the control of variables that establish nucleation and its kinetics such as
cooling rate. Unfortunately, Rogers and Marangoni [5, 6]
did not evaluate the cooling rate effect on the rheological
properties of the 12-hydroxystearic acid organogels.
Within this context, the rheological behavior of n-alkane
based organogels was investigated recently using candelilla
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J Am Oil Chem Soc
wax (CW) and dotriacontane (C32) as gelling agents and
high-triolein safflower oil as the liquid phase [8]. C32
(C32H66) was used in its pure form (&99.0%), while CW
was a mixture of n-alkanes with uneven carbon number
(C29H60, &4.2%; C33H68, &8.0%) and alcohols of pentacyclic triterpenoids (&7.4%) with hentriacontane (C31H64)
as its main component (&79%) [8]. The rheological profiles of CW and C32 organogels, developed at two gel
setting temperatures (5 and 25 °C) achieved using a cooling rate of 1 or 10 °C/min, were determined with a
mechanical spectrometer (Paar Physica UDS 200; Stuttgart, Germany) using a cone (50 mm, 1°; MK22) and plate
geometry with a truncated fixed gap of 0.05 mm [8]. Under
some time–temperature conditions G0 in both, the CW and
C32 organogels, showed a steady and significant decrease
as a function of time (i.e., 0–180 min), particularly in the
organogels developed at 10 °C/min and a gel setting temperature of 25 °C [8]. This decrease in G0 was associated
with the presence at the junction zones of less organized
structures (i.e., alkanes in the rotator phase), that would
result in the rupture of the bonds between the microplatelets of n-alkanes during G0 measurements. However, we
could not fully explain this rheological behavior, mainly
because the strain applied during G0 measurement
(180 min) was always within the linear viscoelastic region
of the system.
A possible explanation for the decrease in G0 observed
in that study might be associated with the expansion/
shrinkage suffered by the rheometer geometry and the
organogel due to changing temperature conditions used
during rheological measurements. These process ought to
affect gap size and therefore the rheological measurements.
Additionally, the expansion/shrinkage of the rheometer
frame due to altering laboratory conditions might also
modify the actual gap, particularly in long lasting experiments. Läuger et al. [9] detailed addressed the effect of
these conditions on the viscosity of silicon oil. Within this
framework, in an attempt to understand the rheological
behavior of the CW and C32 organogels previously
observed [8], in this investigation we determine the G0
profiles of these organogels using a mechanical spectrometer (Paar Physica MCR 301, Stuttgart, Germany) equipped
with a true-gap system. This device makes the corrections
in gap size associated with the expansion/shrinkage of the
sample and/or the rheometer geometry due to changing
temperature conditions used during measurements [9].
Materials and Methods
CW and C32 organogels were developed using the same
experimental design, gelator concentrations and time–
temperatures conditions reported by Morales-Rueda et al.
123
[8]. In short, the treatment conditions investigated resulted
from the factorial combination of gelator concentration (1
and 3%, w/w), gel setting temperatures (Tset, 5 and 25 °C),
and cooling rates (1 and 10 °C/min). The treatments were
randomly distributed among aliquots of the CW or C32
dispersions in SFO. For each treatment two independent
measurements were done (n = 2).
The main difference with our previous work [8] is that in
the present investigation the elastic (G0 ) and loss (G00 )
moduli of the organogels were determined with a
mechanical spectrometer (Paar Physica MCR 301, Stuttgart, Germany) using a steel cone-plate geometry (50 mm,
1°; CP50-1/TG, Anton Paar, Graz-Austria) equipped with a
true-gap system. Temperature was controlled by a Peltier
system located in both the base and top of the measurement
geometry through a Peltier-controlled hood (H-PTD 200).
The control of the equipment was made through the software Start Rheoplus US200/32 version 2.65 (Anton Paar,
Graz-Austria). The gel dispersion at room temperature was
applied on the base of the geometry and the cone was set
using the true-gap function of the software. Any excess of
the sample was removed from the borders of the rheometer
geometry with the help of a spatula. After 20 min at 90 °C
the system was cooled at the corresponding cooling rate
until achieving a particular Tset. At a given Tset the G0 and
G00 were determined as a function of time within 0 and
180 min, always within the linear viscoelastic region
(LVR) of the system. For the systems at 1% CW and 1%
C32 concentration the strain applied was between 0.01 and
0.05%. At 3% gelator concentration the strain used was
between 0.05 and 0.1%. A frequency of 1 Hz was used in
all cases.
The same SFO, CW, and C32 used in our previous
investigation [8] were also utilized in the present work.
Therefore, the calorimetric parameters (i.e., heat of melting, DHM), the solid phase content (SPC), and the microphotographs of the CW and C32 organogels determined in
the original work [8] were also used in this investigation.
All these parameters were measured as a function of time
(0–180 min) after achieving a preestablished Tset at a given
cooling rate.
Results and Discussion
The composition of CW, C32, SFO have been previously
reported [8]. In short, CW contained 78.9 ± 0.1% of
hentriacontane (C31H64), while the C32 utilized had a
purity of 99.50% (±0.01%). The microphotographs
obtained by PLM (Figs. 1, 2, 3, 4) showed that, independent of gelator concentration and Tset, C32 developed larger needle-like crystals, highly branched and with a higher
extent of birefringence (Figs. 3, 4) than the crystals
J Am Oil Chem Soc
Fig. 1 Polarized light
microphotographs of CW
organogels developed at 1 °C/
min at the Tset’s of 5 °C (a) and
(c) and 25 °C (b) and (d) at 1%
(a) and (b) and 3% (c) and (d)
gelator concentration
microphotographs of CW
(c) and 25 °C (b) and (d) at 1%
developed under the same conditions by CW (Figs. 1, 2).
In our previous work [8] we showed that, independent of
the gelator concentration and the cooling rate used, the
gelation temperature for C32 was always higher than for
CW. This indicated that for a given Tset higher supercooling was achieved by C32 than by CW, independent of
gelator concentration and cooling rate. Additionally, we
observed that a higher concentration of CW than C32 was
required to achieve the same gelation temperature (see
Fig. 2b in [8]). All these results pointed to the fact that C32
has a higher self-assembly capability in the SFO than CW.
However, it is important to point out that although hentriacontane was the major component in CW, nonacosane,
tritriacontane, and triterpene alcohols were also present in
CW [8]. This mixed composition might result in the
development of mixed self-assembled structures with a
123
J Am Oil Chem Soc
microphotographs of C32
(c) and 25 °C (b) and (d) at 1%
microphotographs of C32
(c) and 25 °C (b) and (d) at 1%
lower extent of three-dimensional molecular structure (i.e.,
lower crystallinity and DHM) and smaller crystals than the
ones achieved by pure C32. In organogels developed with
stearic alcohol and stearic acid, Schaink et al. [10] and
Gandolfo et al. [11] obtained smaller crystals at a 3:7
stearic acid to stearyl alcohol ratio than with the pure
compounds.
123
Figure 5 shows some characteristics G0 profiles for 1%
C32 and 3% CW organogels obtained under isothermal
conditions once a particular Tset (i.e., 5 or 25 °C) was
achieved at a given cooling rate (i.e., 1 or 10 °C/min). The
rheological profiles were obtained with the fixed and the
true-gap systems. The corresponding SPC present in the
organogels was independent of the type of system used
J Am Oil Chem Soc
Fig. 5 G0 profiles for 1% C32
(a) and (c) and 3% CW (b) and
(d) organogels measured with
the fixed and the true-gap
systems. The rheological
profiles were obtained under
isothermal conditions at the Tset
of 5 and 25 °C, achieved using a
cooling rated of 1 °C/min (a)
and (b) or 10 °C/min (c) and (d)
(fixed gap vs. true gap) in the rheological measurements,
and is also shown in Fig. 5. It is important to point out that
for the same gelator type and cooling rate used the SPC in
the organogels remained constant during the 180 min
involved in the rheological measurements (i.e., once Tset
was achieved no additional CW or C32 crystallization
occurred). For a given type of organogel developed at the
same Tset, cooling rate, and gelator concentration different
G0 magnitudes and G0 profiles as a function of time were
obtained with each type of system. Overall, with the truegap system under all time temperature conditions investigated a constant G0 pattern as a function of time was
observed in the C32 organogels developed at 1% (Fig. 5a,
c) and 3% concentration (data not shown). Under the same
time–temperature conditions, the use of the fixed-gap
system showed a steady decrease in G0 at both the 1%
(Fig. 5a, c) and the 3% (data not shown, see [8]) concentration until a plateau was achieved. Similar observations
applied to the rheological measurements with CW (Fig. 5b,
d). However, with the CW we observed a distinctive
behavior at a cooling rate of 10 °C/min, particularly at the
Tset of 25 °C. Under these conditions the use of the fixedgap system with 3% CW organogels, resulted in a concomitant decrease of G0 at the Tset of 25 °C (Fig. 5d). This
decrease was more evident at 1% concentration, and was
also observed at the Tset of 5 °C (data not shown, see [8]).
However, with the true-gap system, the 3% CW organogels
developed at both Tset’s at a cooling rate of 10 °C/min,
123
J Am Oil Chem Soc
followed a G0 profile that initially increased slowly
attaining a plateau (Fig. 5d). Similar behavior was
observed with the 1% CW organogels (data not shown).
In high quality rheometers, like the one used by Morales-Rueda et al. [8] (i.e., Paar Physica UDS 200; Stuttgart,
Germany), the gap position can be set accurately to within
1 lm [9]. However, such models use a compensating gap
adjustment routine (i.e., auto gap control) during temperature dependent rheological measurements that rely on a
constant thermal expansion coefficient (e.g., 1 lm/K). The
software makes the appropriate gap adjustments based on
empirically established temperature position functions
using the expansion coefficient determined once the thermal expansion is completed [9]. It seems that when a
temperature ramp was used during rheological measurements using a fixed-gap system, particularly a high cooling
rate (i.e., 10 °C/min; Fig. 5c, d) the automatic gap adjustment was inaccurate since the equipment geometry and the
organogel had not reached their final expansion/shrinkage.
These results showed that the lack of an appropriate
adjustment in gap size due to the expansion/shrinkage of
both the rheometer geometry and the organogel affected
our previous G0 measurements [8], particularly when a
cooling rate of 10 °C/min was used to achieve the Tset’s.
The true-gap system constantly measures the magnetic
impedance and the voltage between the lower and the
upper plate of the geometry using appropriate electronics.
Both the magnetic impedance and the voltage have specific
relationships with the gap size. Thus, during the rheological measurements the voltage between the lower and the
upper plate of the geometry is measured by the rheometer’s
electronics, and by taking into consideration the particular
relationship between voltage and gap size, an electronic
feedback mechanism constantly adjusts the gap to the
desired constant value through the rheometer’s software
[9]. Thus, in contrast with the fixed-gap system, the true
gap provides a constant and real gap size during the organogels’ rheological measurements.
In general, the G0 profiles obtained with the true-gap
system showed that, for the same type of gelator and
independent of its concentration and the Tset used, higher
G0 profiles were obtained at 1 °C/min than at 10 °C/min
(i.e., at 1 °C/min the organogels achieved a higher level of
structural organization than at 10 °C/min). This was particularly evident with C32 organogels (i.e., Fig. 5a, c).
When the G0 values after 180 min (G0 180) at both cooling
rates were plotted as a function of the corresponding SPC
(Fig. 6), it was evident that for the same gelator concentration and time–temperature conditions higher G0 180 were
obtained with C32 than with CW organogels, particularly
at 1 °C/min and at 3% of gelator concentration. This in
spite of the higher SPC (P \ 0.05) and smaller crystal sizes
developed by CW organogels (Figs. 1, 2) in comparison
123
Fig. 6 G0 values of the CW and C32 organogels after 180 min
(G0 180) as a function of the solid phase content (SPC %). The values
plotted are the means of two independent determinations obtained at
1 °C/min (a) and 10 °C/min (b) at the corresponding Tset
with the ones developed by C32 organogels (Figs. 3, 4).
This agreed with our previous conclusion [8] that indicated
that with the exception of the 1% gelator concentration at
Tset of 25 °C at both cooling rates, C32 developed organogels at significantly lower SPC than CW. This is
independent of the cooling rate and Tset used [8] (i.e., at the
Tset investigated C32 observed higher solubility in SFO
than CW). These results again pointed out that C32 has
higher self-assembly capability in the SFO than CW.
Wang et al. [7] have shown that the topological structure
of a three-dimensional fiber network in an organogel
depends on the supersaturation of the system. Specifically,
these authors showed that the development of transient (i.e.,
entanglement of fibers) and permanent (i.e., branching of
fibers) junction zones in a gel network is supersaturation
dependent [7]. In turn, both the transient and permanent
junction zones determine the rheological properties of the
self-assembled fibrillar network [7]. These authors concluded that organogels with fibrillar network structures with
a high extent of transient junction zones, have higher elasticity than organogels with spherulitic network structures
(i.e., a network with high extent of permanent junction
zones). The microphotographs obtained by PLM showed
that independent of Tset, C32 organogels developed at 1 °C/
J Am Oil Chem Soc
min showed larger needle-like crystals with a higher extent
of branching (Fig. 3) than the organogels developed at
10 °C (Fig. 4). As a result higher G0 180 was observed in the
C32 organogels developed at 1 °C/min than in the ones
obtained at 10 °C/min (Fig. 6). A similar explanation might
be applied to CW organogels. However, the magnification
used in the PLM was not sufficient to appreciate the
microscopic characteristics of CW crystals (Figs. 1, 2).
The rheograms obtained with the true-gap system
showed that CW organogels were achieving a higher
structural order as a function of time (i.e., G0 steadily
increased as a function of time). This phenomenon was
more evident when a cooling rate of 10 °C/min was used to
achieve the Tset’s (Fig. 5d). As discussed in our previous
paper [8], during the cooling stage the thermodynamic
driving force for gel formation (e.g., the difference between
the temperature of the system and the gelation temperature,
Tg) increased faster at the higher cooling rate. Consequently, during the cooling stage the gelator molecules had
less time to organize at 10 °C/min than at 1 °C/min, i.e.,
for a given Tset, a less well organized molecular packing of
the n-alkanes was achieved at 10 °C/min than at 1 °C/min.
This was particularly evident at the higher Tset (i.e., 25 °C)
where the lower supercooling conditions prevailed
(Fig. 5d). Once Tset was achieved, the CW organogels
evolved into a higher state order as a function of time
through a solid ? solid transition from a rotator phase
developed by the n-alkanes. Rotator phases are commonly
observed in n-alkanes and are characterized by a crystalline
lattice of the molecular centers while molecules rotate
about their chain axes (i.e., structural disorder) [12–14].
Thus, as noted in our original work [8], independent of the
Tset used, CW organogels developed at 1 °C/min observed
higher DHM (i.e., higher level of structure and therefore
higher G0 ) than the CW organogels developed at 10 °C/min
(Fig. 7a). Additionally, in organogels developed at Tset of
25 °C the DHM increased as a function of time, a process
probably associated with the development of a more
organized molecular packing from the one originally
achieved (e.g., rotator phase) through an annealing process.
These effects were more evident in 3% CW organogels
than in 1% CW organogels (data not shown, see [8]),
suggesting that a concentration effect is also involved in
the development of a better structured three-dimensional
network. Previously our group described a similar phenomenon also in CW organogels [15], which was observed
later in 12-hydroxystearic acid organogels by Rogers et al.
[16]. As pointed out in our previous reports [8, 15] the
DHM increment observed in CW organogels as a function
of time (Fig. 7a) could not be associated to changes in the
SPC, mainly because the SPC achieved by the organogels
right after attaining Tset remained constant during the
whole period of rheological and DHM measurements.
Fig. 7 Heat of melting (DHM) as a function of time for organogels of
3% CW (a) and 3% C32 (b) developed at the Tset and cooling rate
show in the legend. Values plotted are the mean and standard
deviation of two independent determinations
With the true-gap system, the C32 organogels showed a
steady G0 profile at all time–temperature conditions
investigated. Although SPC was statistically higher in the
C32 organogels developed at 10 °C/min than in the ones
developed at 1 °C/min (P \ 0.05), as for CW, the G0
profiles of C32 organogels were always higher at 1 °C/min
(Fig. 6). As previously mentioned, at this cooling rate
larger C32 crystals with a greater extent of branching were
obtained at all Tset’s (Fig. 3) than at 10 °C/min (Fig. 4), a
process that resulted in organogels with a higher level of
three-dimensional organization (i.e., higher fractal dimension) than the one achieved at 10 °C/min (Fig. 6). With the
C32 organogels although there was a tendency to obtain
higher DHM at 1 °C/min than at 10 °C/min under all conditions investigated, the differences were not significant
(Fig. 7b). Additionally, at all conditions investigated the
DHM of C32 organogels remained constant as a function of
time indicating the absence of a solid ? solid transition.
Given the Tg values previously reported for C32 [8], at the
Tset investigated this gelator system achieved higher thermodynamic driving force for gelation during the isothermal
stage than the CW system, and therefore, C32 organogels
123
J Am Oil Chem Soc
achieved a higher structural order (i.e., higher G0 ) than CW
organogels. Thus, the development of a rotator phase
would be less probable in the C32 organogels than in the
CW organogels (i.e., development of a higher state order
by C32 organogels as a function of time through a
solid ? solid transition from a rotator phase will not
exist). Therefore, under the time–temperatures conditions
investigated both the DHM (Fig. 7b) and the G0 for C32
organogels remained constant during the 180 min of
experimentation (Fig. 5a, c).
In conclusion, contrary to the rheological results
obtained with the fixed-gap system, the G0 results obtained
with the true-gap system agreed with the DHM behavior
(Fig. 7), crystal size, and three-dimensional organization of
the crystal network observed by the C32 and CW organogels (Figs. 1, 2, 3, 4). Thus, we must use a fixed-gap
system with caution specially with time dependent G0
determinations (i.e., creep and recovery measurements),
and rheological measurements involving the use of fast
temperature ramps (i.e., 10 °C/min).
Acknowledgments The investigation was supported by grant #
48273-Z/25706 from CONACYT. The technical support from Concepcion Maza-Moheno and Elizabeth Garcia-Leos is greatly
appreciated.
References
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6. Rogers MA, AG Marangoni (2009) Solvent-modulated nucleation and crystallization of 12-hydroxystearic acid: a nonisothermal approach. Langmuir ARTICLE ASAP DOI:10.1021/
1a8035665
7. Wang R, Liu X, Xiong J, Li J (2006) Real-time observation of
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8. Morales-Rueda JA, Dibildox-Alvarado E, Charó-Alonso M,
Weiss RG, Toro-Vazquez JF (2009) Thermo-mechanical properties of candelilla wax and dotriacontane organogels in safflower
oil. Eur J Lipid Sci Technol 111:207–215
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10. Schaink HM, van Malssen KF, Morgado-Alves S, Kalnin D, Van
der Linden E (2007) Crystal network for edible oil organogels:
possibilities and limitations of the fatty acid and fatty alcohol
systems. Food Res Intern 40:1185–1193
11. Gandolfo FG, Bot A, Flöter E (2004) Structuring of edible oils by
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12. Espeau P, White JW (1997) Thermodynamic properties of nalkanes in porous graphite. J Chem Soc Faraday Trans 93:3197–
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ortho-Positronium in some n-alkanes: influence of temperature
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14. Tozaki K, Inaba H, Hayashi H, Quan C, Nemoto N, Kimura T
(2003) Phase transitions of n-C32H66 measured by means of high
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15. Toro-Vazquez JF, Morales-Rueda A, Dibildox-Alvarado E,
Charo-Alonso M, Alonzo-Macias M, Gonzalez-Chavez MM
(2007) Thermal and textural properties of organogels developed
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Eur. J. Lipid Sci. Technol. 2009, 111, 207–215
207
Research Paper
Thermo-mechanical properties of candelilla wax and
dotriacontane organogels in safflower oil
Juan A. Morales-Rueda1, Elena Dibildox-Alvarado2, Miriam A. Charó-Alonso2, Richard G. Weiss3
and Jorge F. Toro-Vazquez2
1
Universidad Autónoma de Queretaro, DIPA-PROPAC, Mexico
Universidad Autónoma de San Luis Potosí, Facultad de Ciencias Quimicas, Mexico
3
Georgetown University, Department of Chemistry, Washington, USA
2
The thermo-mechanical properties of organogels developed by a complex mixture of n-alkanes present in
candelilla wax (CW) were investigated and compared with the ones of organogels developed by a pure nalkane, dotriacontane (C32). In both cases, the liquid phase used was safflower oil high in triolein (SFO)
and the variables studied were two levels of gelator concentration (1 and 3%), cooling rates of 1 and 10 7C/
min, and two gel setting temperatures, 5 and 25 7C (Tset). Based on comparisons of the organogels made
with C32, the presence of minor molecular components in CW had a profound effect on the crystal habit of
the n-alkanes in CW-based organogels, and therefore on their physical properties. Thus, independent of
the cooling rate and Tset, C32 showed a higher solubility and higher self-assembly capability in the SFO
than CW. Nevertheless, for the same gelator concentration and time-temperature conditions, C32 organogels had lower G’ profiles than CW organogels. Additionally, independent of the type of gelator, more
stable organogel structures were developed at Tset = 5 7C and using the lower cooling rate. The rheological
behavior of the organogels was explained considering the formation of a rotator phase by the n-alkanes, its
solid-solid transition, and their dependence as a function of the cooling rate and Tset. The results here
obtained showed that it is possible to gelate SFO through organogelation with CW and without the use of
trans fats.
Keywords: Organogels / Candelilla wax / Trans-free / Rheology / Hentriacontane
Received: July 10, 2008; accepted: September 19, 2008
DOI 10.1002/ejlt.200810174
1 Introduction
Candelilla wax (CW) is a wax derived from the leaves of a
small shrub native to northern Mexico and the southwestern
USA, Euphorbia cerifera and Euphorbia antisyphilitica, from
the family Euphorbiaceae. CW is a worldwide recognized
food additive approved by the FDA (under regulations
21CFR, 175.105, 175.320, 176.180), used mainly as a glazing
agent and binder for chewing gums. It is also used in the
manufacture of lip balms and lotion bars, and in the paint
industry to make varnishes. Additionally, CW can be used as a
Correspondence: Jorge F. Toro-Vazquez, Facultad de Ciencias Quimicas-CIEP, Av. Dr. Manuel Nava 6, Zona Universitaria, San Luis Potosí,
SLP 78210, Mexico.
E-mail: [email protected]
Fax: 151 444 8262372
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
substitute for carnauba wax and beeswax in different food
systems. Reports on CW composition show the presence of
49–50% n-alkanes with 29–33 carbons, 20–29% esters of acids
and alcohols with even-numbered carbon chains (C28–C34),
12–14% alcohols and sterols, and 7–9% free acids [1, 2].
In a previous investigation, we showed that under several
time-temperature conditions, the n-alkanes present in CW,
particularly the hentriacontane (C31), develops thermoreversible organogels in dispersion with safflower oil [3]. This
investigation and several others [4–6] have shown that organogelation is a promising alternative that might be used to
modify the physical properties of vegetable oils without the
use of chemical modifications that result in the presence of
trans fatty acids. This opens new alternatives to produce transfree margarines, vegetable oil-based spreads, and coatings.
During the last years, organogels developed with low-molecular-weight compounds, such as fatty acids and n-alkanes,
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208
J. A. Morales-Rueda et al.
have received particular attention, mainly because these gelator molecules require only a small concentration (2%) to
achieve gelation [5, 7–9]. However, very little is known about
their rheological and calorimetric properties as affected by
different time-temperature conditions. Within this framework, the objective of this paper is the investigation of the
thermo-mechanical properties of organogels developed by a
complex mixture of n-alkanes, the ones present in CW, in
comparison with the organogels developed by pure n-alkanes,
i.e. dotriacontane (C32). In both cases, the liquid phase used
was safflower oil high in triolein (SFO). The variables investigated include two levels of gelator concentration (1 and 3%),
two different cooling rates (1 and 10 7C/min) and two different gel setting temperatures (5 and 25 7C). An important
characteristic of this experimental setup is that, while the main
components in CW are n-alkanes with an odd number of carbons and with lengths between 29 and 33 carbons [3], dotriacontane is an n-alkane with 32 carbons.
2 Materials and methods
2.1 Vegetable oil, CW, dotriacontane, GC-MS, and
HPLC analysis
SFO extracted from genetically modified seed was obtained
from Coral Internacional (San Luis Potosí, Mexico). Micronized high-purity CW obtained from E. cerifera was supplied
from Multiceras (Monterrey, Mexico) and the dotriacontane
(C32) was of reagent grade (Humphrey Chemical Co., CT,
USA) recrystallized several times from petroleum ether. The
SFO was analyzed for triacylglycerols (TAG) by HPLC [10],
while the CW and the C32 were analyzed by capillary GC-MS
as described previously [3]. The composition is reported as
the mean 6 standard deviation of at least two independent
determinations (n = 2).
heat of melting (DHM) were calculated with the equipment
software (TA Instruments Universal Analysis 2000, v. 4.0)
using the first derivative of the heat flux. Tg is the temperature
where the first derivative of the heat capacity of the sample
initially departed from the baseline. In contrast, Tp is the
temperature where the first derivative of the heat capacity
associated with the melting endotherm crossed the baseline.
The DHg and DHM values correspond to the exotherm and
endotherm areas associated with the gelation and melting
process, respectively. At least two independent determinations
were done and the corresponding mean of the thermal parameters was plotted as a function of the gelator concentration.
2.3 Experiment design
For each system, CW or C32, the treatment conditions investigated resulted from the factorial combination of the different
levels of gelator concentration (1 and 3%), gel setting temperatures (Tset, 5 and 25 7C), and cooling rates (1 and 10 7C/
min). The CW concentrations (i.e. 1 and 3%) and Tset (i.e. 5
and 25 7C) investigated were selected based on the results
previously obtained with CW [3]. At the 1 and 3% CW concentrations a Tset of 5 7C provided a high thermodynamic
drive for gelation since this temperature is 30–40 7C below the
melting temperature (Tp) for CW organogels. In contrast, a
Tset of 25 7C provides a lower thermodynamic drive for gelation since this temperature was just 10–20 7C below the CW
organogels’ melting temperature. The treatments were randomly distributed among aliquots of the CW or C32 dispersions. For each treatment two independent measurements
were done (n = 2). The organogels developed under such
conditions were characterized by rheometry, calorimetry,
solid phase content (SPC), and polarized light microscopy.
2.4 Oscillatory rheometry, heat of melting, and SPC of
the organogels
2.2 Dynamic gelation and melting of organogels
Different concentrations of CW or C32 were dispersed in SFO
to achieve gelator concentrations within 0.5 and 10% (wt/vol).
The CWor the C32 was solubilized in the SFO by heat (90 7C)
and agitation during 20 min. Samples of CW, C32 and the
corresponding dispersions were used to determine the dynamic gelation and melting thermograms by differential
scanning calorimetry (DSC) using a TA Instruments Model
Q1000 (TA Instruments, New Castle, DE, USA). Samples of
the dispersions (5–7 mg) were sealed in aluminum pans,
heated at 90 7C for 20 min and then cooled to –80 7C at a rate
of 1 or 10 7C/min. After 2 min at –80 7C, the system was
heated up to 90 7C at a rate of 5 7C/min. The thermal parameters corresponding to the temperature at the beginning of the
gelation endotherm (Tg), the heat of gelation (DHg), the temperature at the peak of the melting endotherm (Tp), and the
The elastic (G’) and loss (G”) modulus of the organogels
were determined with a mechanical spectrometer (Paar Physica UDS 200; Stuttgart, Germany) equipped with a steel
cone (50 mm, 17) and plate geometry (MK-22) with a truncated gap of 0.05 mm (sample size 0.5 mL). The CW or C32
dispersion was applied inside the geometry and after 20 min at
90 7C the system was cooled at 1 or 10 7C/min until achieving
the corresponding Tset. Temperature was controlled by a Peltier system located in the base of the measurement geometry.
At a given Tset, G’ and G” were determined as a function of
time within 0 and 180 min using a frequency of 1 Hz. On the
other hand, the heat of melting (DHM) and the SPC of the
organogels, as a function of time (0–180 min) after achieving
a pre-established Tset at a given cooling rate, were determined
by DSC and low-resolution NMR (Minispec Bruker model
mq20; Bruker Analytik, Rheinstetten, Germany), respectively.
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2.5 Polarized light microscopy
Polarized light microphotographs of organogels were obtained
using a polarized light microscope (Olympus BX51; Olympus
Optical Co., Ltd., Tokyo, Japan) equipped with a color video
camera (KP-D50; Hitachi Digital, Tokyo, Japan) and a platina
(TP94; Linkam Scientific Instruments, Ltd., Surrey, UK)
connected to a temperature control station (LTS 350; Linkam
Scientific Instruments, Ltd.) and a liquid nitrogen tank. To
guarantee a uniform sample thickness, a drop of the melted
sample was gently smeared over a preheated glass microscope
slide (90 7C) using another glass slide at a 457 angle. The slide
with the sample was placed in the platina and, after 20 min at
90 7C, the system was cooled (at 1 or 10 7C/min) to a given
Tset with the temperature control station (Linksys32 version
1.3.1; Linkam Scientific Instruments, Ltd., Waterfield, UK).
Polarized light microphotographs of the organogels were
obtained as a function of time (0 and 180 min) once Tset was
attained.
3 Results and discussion
3.1 Composition and thermal analysis of CW and C32
The results of the CW analysis showed that the main component of CW was hentriacontane (78.9 6 0.1%), an n-alkane of
31 carbons (C31). Minor components include other alkanes
also with an odd number of carbons, particularly nonacosane
(C29, 4.2 6 0.1%) and tritriacontane (C33, 8.0 6 0.2%), triterpene alcohols with a molecular formula of C30H49OH (i.e.
Rheology and thermal properties of organogels
209
germanicol, lupeol or moretenol; 7.4 6 0.1%), and 1.6%
(6 0.1%) of unidentified compounds. The purity of C32 by
GC-MS was 99.50% (6 0.01%).
The CW cooling thermograms (Fig. 1) showed one major
exotherm with a Tg of 76.58 7C (6 0.68 7C), two temperature
peaks at 59 and 53 7C, and DHg of 147.35 J/g (6 1.91 J/g).
The melting thermograms show one endotherm with a melting temperature (Tp) of 64.42 7C (6 0.23 7C) and a DHM of
149.75 J/g (6 1.20 J/g). The melting temperature reported for
99.5% pure hentriacontane (67.05 7C) [11] is close to the Tp
of CW. However, expressing the DHM for CW per unit mass
of hentriacontane (82.9 kJ/mol), this value was greater than
the DHM for 99.5% hentriacontane (i.e. 73.3 kJ/mol [11]). In
addition to triterpenoids, other n-alkanes (i.e. nonacosane and
tritriacontane) were present as minor components in CW.
Thus, during cooling, the nonacosane and tritriacontane
might develop a mixed molecular packing with hentriacontane. This would explain the two temperature peaks
reported in the CW exotherm and the higher DHM than that of
pure hentriacontane [3]. However, the development of a
rotator phase might be considered as an alternative explanation for the thermal behavior observed by hentriacontane.
Rotator phases are commonly observed in n-alkanes and are
characterized by a crystalline lattice of the molecular centers
while molecules rotate about their chain axes (i.e. structural
disorder) [12–14]. Thus, during cooling, hentriacontane
might develop a rotator phase from the melting at 59 7C
(Fig. 1), followed by the transition to the alkane crystal at
53 7C. To the authors’ knowledge, the rotator phases of hentriacontane have not been investigated. However, with the
exception of n-alkanes with an even number of carbons below
Figure 1. Dynamic cooling (10 7C/min)
and corresponding melting thermograms (5 7C/min) for CW and C32. Tg and
Tp are defined in the text.
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210
22 atoms, rotator phases are present in n-alkanes with carbon
chains from 11 to 40 atoms long [12, 13]. In any case, based
on the CW composition and the thermal parameters discussed
above, the phase transitions observed in the CW thermograms
were associated to the phase behavior of hentriacontane.
Both the cooling and heating thermograms for C32 showed
similar behavior as the one reported by Tozaki et al. using a
high-resolution super-sensitive DSC and low cooling and
heating rates (i.e. 0.4 mK/s) [14]. Thus, the first and second
exotherms with peak temperatures at 67.5 and 62.2 7C were
associated with the development of a rotator phase from the
melt and the transition from the rotator phase to crystal,
respectively (Fig. 1). Despite the different cooling rates used
in our study and the one by Tozaki et al. [14], the two phase
transitions had similar peak temperatures. Nevertheless, additional phase transitions reported by Tozaki et al. [14] (i.e.
rotator phase IV to rotator phase III) were not observed in our
work. This was mainly due to the higher heat flow sensitivity
of the equipment designed and used by these authors [14] in
comparison with the one used in the present work. The heating thermograms showed two major endotherms with Tp of 66
and 69 7C (Fig. 1). The first endotherm was associated with a
solid-solid transition from the crystalline to the rotator phase,
and the second one to the transition from the rotator to the
liquid phase [14]. The minor endotherm with a Tp of 64.5 7C
has been associated with the disorder of the n-alkane structure
near the molecules’ end [14]. As with the cooling thermograms, despite the higher cooling and heating rates used in the
present work, all these endotherms had peak temperatures
similar to the ones reported by Tozaki et al. [14].
3.2 Thermal-mechanical properties of CW and C32
organogels in SFO
The cooling and heating thermograms for the corresponding
CW and C32 dispersions in SFO were simple, showing just two
peaks. The major peak, associated with the crystallization and
melting process of TAG from SFO, was always present at temperatures below 0 7C in both the cooling and heating thermograms (data not shown). The minor peak, found at temperatures above 0 7C (Fig. 2A), was associated with the gelation and
melting process of CW and C32 components in the SFO. The
peaks associated with the development and melting of the
rotator phase were not observed in the CWand C32 dispersions.
Given the dilution of CWand C32 with SFO, lower cooling and
heating rates might be required to achieve the appropriate signal resolution to observe the rotator phase transitions. However, such conditions were not used in the present investigation.
Independent of the gelator concentration and the cooling
rate used, Tg was always higher for C32 than for CW (Fig. 2B).
Additionally, a higher concentration of CW than C32 was
required to achieve the same Tg (Fig. 2B). This indicated that
dotriacontane molecules have higher self-assembly capability
in the SFO than the mixture of alkanes with an odd number
Figure 2. Dynamic cooling (10 7C/min) (A) and corresponding
melting thermograms (5 7C/min) for 3% CW and C32 dispersions in
SFO (B). Tg, Tp, DHg and DHM are defined in the text.
of carbons as the ones present in CW. This was confirmed by
the higher heat of melting (Fig. 3) and a larger crystal size
with a high extent of birefringence of the structures developed by C32 (Fig. 4A and C for 1 and 3%, respectively, both
at a cooling rate of 10 7C/min), in comparison with the ones
developed by CW (Fig. 4B and D for 1 and 3%, respectively,
both at cooling rate of 10 7C/min). It is important to point
out that the previous results are not associated with the critical (i.e. minimum) concentration needed by C32 and CW to
develop a gel. The minimum concentration required by C32
and CW to gel in SFO was not determined in the present
study. The DHM behavior (Fig. 3) and the microphotographs
shown in Fig. 4 suggested that, although hentriacontane was
the major component in CW, the presence of other alkanes
(i.e. nonacosane and tritriacontane) and triterpene alcohols
might result in the development of mixed self-assembled
structures with a lower extent of three-dimensional molecular
structure (i.e. lower crystallinity and DHM) and smaller crystals than the ones achieved by pure C32. Schaink et al. [15]
and Gandolfo et al. [16] showed the strong influence that the
gelator composition has on crystal size, shape, and rheology
of organogels developed in sunflower oil by different stearic
acid-to-stearyl alcohol ratios. In general, smaller crystals with
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211
Figure 3. Heat of melting (DHM) values of dispersions of CW and C32 in SFO after dynamic cooling at
two cooling rates (1 and 10 7C/min).
Figure 4. Polarized light microphotographs of C32 and CW in SFO
at Tset = 5 7C cooled at 10 7C/min.
C32 organogels at 1% (A) and 3%
(C); CW organogels at 1% (B) and
3% (D).
higher rheological profiles were obtained at a stearic acid-tostearyl alcohol ratio of 3 : 7 than with the pure compounds
[15, 16].
Independent of the Tset used to develop the CW organogels, the cooling rate did not affect the SPC (Fig. 5). Additionally, CW organogels developed at Tset = 25 7C had a lower
SPC than at Tset = 5 7C (p ,0.05). However, with C32 organogels a higher SPC was developed at 10 7C/min than at 1 7C/
min (p ,0.05; Fig. 5). This is except for the 1% C32 system at
Tset = 25 7C where both cooling rates provided the same SPC.
On the other hand, with C32, Tset had no effect on the SPC of
1% organogels while with 3% organogels a higher SPC was
obtained at Tset = 25 7C than at Tset = 5 7C (p ,0.05; Fig. 5).
Thus, with the exception of the 1% gelator concentration at
Tset = 25 7C at both cooling rates, C32 forms organogels at
significantly lower SPC than CW (Fig. 5). This is indewww.ejlst.com
212
Figure 5. SPC of organogels developed with 1% (A) and 3% (B)
CW and C32 dispersions in SFO at two cooling rates (1 and 10 7C/
min) and two Tset (5 and 25 7C).
pendent of the cooling rate and the gel setting temperature
used (Fig. 5). These results demonstrate, as a corollary, the
higher solubility and higher self-assembly capability of C32 in
the SFO, in comparison with the n-alkanes with an odd number of carbons present in CW. Note that the SPC values plotted in Fig. 5 are the mean values of SPC determined by NMR
every 10 min during 180 min. This is since, for the same
gelator type and cooling rate used, once Tset was achieved, the
SPC in the organogels remained constant during the 180 min
of the experiment. The standard error for the mean SPC
values was 0.09%.
The G’ profiles in both gelator systems under the different
conditions of organogel formation studied are shown in Fig. 6.
For the same gelator concentration and time-temperature
conditions, lower G’ profiles were observed in the C32 organogels than in the CW organogels. This result was probably
associated with the higher SPC and smaller crystal size of the
network components in the CW organogels (Fig. 4B, D) than
in the C32 organogels (Fig. 4A, C). A more detailed analysis
showed that with CW the higher the SPC, the higher is the
organogels’ G’ profile. This phenomenon was more evident in
the organogels developed at the lower cooling rate (compare
Fig. 6A with 6B). With C32, despite the higher SPC present in
the organogels developed at 10 7C/min, the ones developed at
1 7C/min showed higher G’ profiles (Fig. 6C) than the organogels developed at 10 7C/min (Fig. 6D). This phenomenon
was independent of the Tset utilized. Note that, although the
strain applied during the 180 min of the experiment was
always within the linear viscoelastic region (LVR), some
changes in the gelator network structure of both types of
organogels occurred as a function of time: G’ values decreased
from G’0 (i.e. G’ value at time zero) to G’f (i.e. G’ value after
180 min). This phenomenon occurred mainly in organogels
developed at 10 7C/min (Fig. 6B, D), particularly at Tset =
25 7C and with the C32 system. It is important to point out that
the mechanical spectra (i.e. G’ and G” vs. frequency) of the
organogels developed after 180 min showed that, in all cases
and for the whole frequency interval (0.01–100 Hz), G’ was
higher than G” (data not shown). The profiles of the
mechanical spectra indicated that under the treatment conditions investigated strong and weak gels were developed. These
results will be presented in another paper.
When the moduli for the CW and C32 organogels were
plotted as a function of the SPC (Fig. 7), it was evident that
G’0 and G’f increased in a logarithmic fashion as a function of
SPC in the organogels. However, at 1 7C/min, no significant
difference between G’0 and G’f was observed, particularly at
the higher SPC (Fig. 7A, C). In contrast, at 10 7C/min
(Fig. 7B, D), both G’0 and G’f were lower than at 1 7C/min
(Fig. 7A, C), particularly in C32 organogels, and there was a
significant difference between G’0 and G’f. These results
showed that, independent of the type of gelator, more stable
organogel structures were developed at Tset = 5 7C and using
the lower cooling rate.
To understand this rheological behavior, it is necessary to
recognize the development of the rotator phase and its impact
on the junction zones between the microplatelet units that
form the primary building blocks of the three-dimensional
networks of the organogels. The rotator phase formation and
its solid-solid transition occur over a period of time that
depends on Tset. Thus, during the cooling stage, the thermodynamic driving force for gel formation (e.g. the difference
between the temperature of the system and Tg) increased faster at the higher cooling rate. Consequently, the gelator molecules had less time to organize at 10 7C/min than at 1 7C/min.
Consistent with this, at 10 7C/min a lower temperature (i.e.
lower Tg) was required to achieve the molecular packing for
organogel formation (Fig. 2B) than at 1 7C/min. In turn,
achieving a given Tset at 10 7C/min resulted in a lower thermodynamic drive for gelation during the isothermal stage (i.e.
Tg – Tset) than when the same Tset was achieved at 1 7C/min.
The overall result was the development of a less well organized
molecular packing (i.e. rotator phase) at 10 7C/min than at
1 7C/min, particularly at the higher Tset (i.e. 25 7C). Within
this framework and assuming that the structural model for nalkane organogels advanced previously [17] applies here, the
presence of less organized structures (i.e. rotator phase), particularly at the junction zones, would result in a modification
of the microstructure at the microplatelets level during G’
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213
Figure 6. Elastic modulus (G’) as a function of time of CW and C32 organogels in SFO. The Tset, gelator concentration, and the
mean SPC with corresponding standard deviation are shown in each case. CW organogels developed at 1 7C/min (A) and
10 7C/min (B); C32 organogels developed at 1 7C/min (C) and 10 7C/min (D).
Figure 7. Elastic modulus at time zero (G’0) and after 180 min (G’f ) as a function of SPC of CW and C32
organogels in SFO. CW organogels developed at 1 7C/min (A) and 10 7C/min (B); C32 organogels developed
at 1 7C/min (C) and 10 7C/min (D).
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214
measurements (Fig. 6). This, despite the strain applied, was
always within the LVR of the organogels. Based on the above
discussion we would expect the junction zones to be less
structured in organogels developed at 10 7C/min, particularly
at Tset = 25 7C (Fig. 6B, D).
Should the rotator phase be developed at a Tset = 25 7C,
this phase with “structural disorder” might evolve into a
higher-order state as a function of time through a solid-solid
transition. A transition such as the one proposed has been
observed in n-heneicosane (C21) and n-pentacosane (C25)
[18]. The changes of DHM as a function of time, particularly in
the CW organogels, were consistent with this process. Thus,
independently of the Tset used, organogels developed at 1 7C/
min showed higher DHM (i.e. higher level of molecular structure) than the organogels developed at 10 7C/min (Fig. 8).
Additionally, in organogels developed at Tset = 25 7C, the DHM
increased as a function of time, a process probably associated
with the achievement of a more organized molecular packing
from the one originally achieved (e.g. rotator phase). These
effects were more evident in 3% CWorganogels (Fig. 8B) than
in 1% CW organogels (Fig. 8A). In contrast, in the C32 organogels, although there was a tendency to obtain a higher DHM
at 1 7C/min than at 10 7C/min under all conditions tested, the
differences were not significant (results not shown). Additionally, under all conditions investigated, the DHM in C32
organogels remained constant as a function of time, indicating
the absence of a solid-solid transition. This might be explained
considering that under the time-temperature conditions
investigated, the supercooling (i.e. Tg – Tset) in the C32 system
was high enough to develop crystals with no rotator phase.
Given the Tg values shown in Fig. 2B, at a given Tset, C32
ought to have a higher thermodynamic driving force for gelation during the isothermal stage than the CW system. Thus,
the development of a rotator phase would be more probable in
the CW organogels than in the C32 organogels.
The results here obtained showed that it is possible to gel
SFO through organogelation with CW and without the use of
trans fats. Previous studies showed that 3% organogels of CW
have a phase separation stability at least up to 3 months with a
texture of potential use for the food industry [3]. Based on
comparisons of gels made with C32 as gelator, the presence of
minor molecular components in CW seemed to have a profound effect on the crystal habit of the n-alkanes in CW-based
organogels, and therefore on their physical properties. Ongoing investigation using CW, pure hentriacontane (C31), and
different proportions of C31 with other n-alkanes addresses
this important issue. Additionally, investigations that evaluate
the presence of a rotator phase as a function of Tset and its
effect on organogel rheology will be undertaken.
Acknowledgments
The investigation was supported by grant no. 48273-Z/25706
from CONACYT. We acknowledge and appreciate the fellowship
from CONACYT for J.A.M.-R. during his stay at Georgetown
University, Department of Chemistry. The technical support from
Concepcion Maza-Moheno and Elizabeth Garcia-Leos is greatly
appreciated.
Conflict of interest statement
The authors have declared no conflict of interest.
References
Figure 8. Heat of melting (DHM) as a function of time for CW organogels developed at Tset of 5 or 25 7C and 1 or 10 7C/min. 1% CW
organogels (A), 3% CW organogels (B).
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