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Hydrocolloids 


Hydrocolloids are hydrophilic polymers, of vegetable, animal, microbial or synthetic origin, that generally contain many hydroxyl groups and may be polyelectrolytes. They are naturally present or added to control the functional properties of aqueous foodstuffs. Most important amongst these properties are viscosity (including thickening and gelling) and water binding but also significant are many others including emulsion stabilization, prevention of ice recrystallization and organoleptic properties. Foodstuffs are very complex materials and this together with the multifactorial functionality of the hydrocolloids have resulted in several different hydrocolloids being required; the most important of which are listed below.

Special Effects With Gums

Alginate 

Arabinoxylan 

Carrageenan 

Carboxymethylcellulose 

Cellulose

Gelatin. 

b-Glucan 

Guar gum 

Gum arabic

Locust bean gum

Tara gum 

Pectin 

Starch 

Xanthan gum 

Tara(caesalpinia Spinosa) Tara Gum Helados

Each of these hydrocolloids consists of mixtures of similar, but not identical, molecules and different sources, methods of preparation, thermal processing and foodstuff environment (e.g. salt content, pH and temperature) all affect the physical properties they exhibit. Descriptions of hydrocolloids often present idealized structures but it should be remembered that they are natural products (or derivatives) with structures determined by stochastic enzymic action, not laid down exactly by the genetic code. They are made up of mixtures of molecules with different molecular weights and no one molecule is likely to be conformationally identical or even structurally identical (cellulose excepted) to any other.

Mixtures of hydrocolloids show such a complexity of non-additive properties that it is only recently that these can be interpreted as a science rather than an art. There is enormous potential in combining the structure-function knowledge of polysaccharides with that of the structuring of water. The particular parameters of each application must be examined carefully, noting the effects required (e.g. texture, flow, bite, water content, stability, stickiness, cohesiveness, resilience, springiness, extensibility, processing time, process tolerance) and taking due regard of the type, source, grade and structural heterogeneity of the hydrocolloid(s).

All hydrocolloids interact with water, reducing its diffusion and stabilizing its presence. Generally neutral hydrocolloids are less soluble whereas polyelectrolytes are more soluble. Such water may be held specifically through direct hydrogen-bonding or the structuring of water or within extensive but contained inter- and intra-molecular voids. Interactions between hydrocolloids and water depend on hydrogen-bonding and therefore on temperature and pressure in the same way as water cluster formation. Similarly, there is a reversible balance between entropy loss and enthalpy gain but the process may be kinetically limited and optimum networks may never be achieved. Hydrocolloids may exhibit a wide range of conformations in solution as the links along the polymeric chains can rotate relatively freely within valleys in the potential energy landscapes. Large, conformationally stiff hydrocolloids present essentially static surfaces encouraging extensive structuring in the surrounding water. Water binding affects texture and processing characteristics, prevents syneresis and may have substantial economical benefit. In particular, hydrocolloids can provide water for increasing the flexibility (plasticizing) of other food components. They can also effect ice crystal formation and growth so exerting a particular influence on the texture of frozen foods.

As hydrocolloids can dramatically affect the flow behavior of many times their own weight of water, most hydrocolloids are used to increase viscosity (see rheology page), which is used to stabilize foodstuffs by preventing settling, phase separation, foam collapse and crystallization. Viscosity generally changes with concentration, temperature and shear strain rate in a complex manner dependent on the hydrocolloid(s) and other materials present. These changes may be fitted to equations such as:

[eta = a x exp(C*)]

 

[eta = A x exp(Ea/RT)]

[graph of viscosity versus concentration]

[graph of log(viscosity) against reciprocal temperature]

where h is the viscosity, a is pre-exponential factor and C* is the concentration in units specific for the circumstances; often about 1% wt/vol.

where h is the viscosity, A is pre-exponential factor and Ea is a constant (known as 'activation energy' from the similarity with the Arrhenius equation), R is the gas constant and T is the absolute temperature.

Unfortunately these equations cannot be combined as the specific concentration units change with temperature. Their non-linear nature means that extreme care should be taken when investigating possible synergistic relationships.

Many hydrocolloids also gel, so controlling many textural properties. Gels are liquid-water-containing networks showing solid-like behavior with characteristic strength, dependent on their concentration, and hardness and brittleness dependent on the structure of the hydrocolloid(s) present. Hydrocolloids display both elastic and viscous behavior where the elasticity occurs when the entangled polymers are unable to disentangle in time to allow flow. Mixtures of hydrocolloids may act synergistically, associating to precipitate, gel or form incompatible biphasic systems; such phase confinement affecting both viscosity and elasticity. Hydrocolloids are extremely versatile and they are used for many other purposes including (a) production of pseudoplasticity (i.e. fluidity under shear) at high temperatures to ease mixing and processing followed by thickening on cooling, (b) liquefaction on heating followed by gelling on cooling,  (c) gelling on heating to hold the structure together (thermogelling), (d) production and stabilization of multiphase systems including films.

These properties of hydrocolloids must be due, both singly and in concert, to their structural characteristics and the way they interact with water. For example:

Hydrocolloids, together with other dietary fiber, are increasingly being seen as contributing to a healthy diet, having a number of positive health benefits. Although this site concentrates on food aspects, hydrocolloids also have many other major economic uses such as in the chemicals, oil and cosmetic industries.

a An introduction to rheology and an introduction to polysaccharides are given on other pages 

b Hydrophilic solutes (i.e. solutes or structures possessing hydrophilicity) interact with water with greater or comparable strength to water-water interactions whereas hydrophobic solutes  (i.e. solutes or structures possessing hydrophobicity) only weakly interact with water with strength far less than water-water interactions. 


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