Introduction to polysaccharides
Mixtures of hydrocolloids
Effect on viscosity
Hydrocolloid action
![water binding at 20°C [1815]](images/collaw.gif "water binding at 20°C [1815]")
Hydrocolloids [227], often called gums,a are hydrophilicb 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 (see right [1815]) but also significant are many others including emulsion stabilization, prevention of ice recrystallization and organoleptic properties. The degree with which the hydrocolloid solutions mix with saliva, determined by their degree of chain entanglement, determines flavor perception [1050].
Other more specialist applications include adhesion, suspension, flocculation, foam stabilization and film formation. Foodstuffs are very complex materials and this together with the multifactorial functionality of the hydrocolloids has resulted in several different hydrocolloids being required; the most important of which are listed below.
Each of these hydrocolloids consists of mixtures of similar, but
not identical, molecules and different sources, methods of preparation,
thermal processing and foodstuff environment (for example, 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. [Back to Top 
]
Many hydrocolloids are polyelectrolytes (for example alginate, carrageenan, carboxymethylcellulose, gum arabic, pectin and xanthan gum. The charged groups ensure strong hydration, particularly on a per-molecule basis. The presence of counterions and co-ions (ions with the same charge as the polyelectrolyte) introduce complex behavior that are ion-specific and depends on the concentrations of all ionic solutes [1473]. A proportion of the counterions remain tightly associated with the polyelectrolyte, being trapped in its electrostatic field and so reducing their activity and mobility; with the non-ideality of the solutions increasing on dilution down to very low levels as the associated proportion of the counterions increases. The behavior changes when the concentration of the polyelectrolyte necessitates the overlap of their structures [1493].
The reduced viscosity of such solutions increases on dilution as the polyelectrolyte chains stiiffen. These properties of polyelectrolytes are more noticeable as the linear charge density (number of charged groups per unit length) increases. Properties of these solutions (such as enthalpy of dilution [1473]) are difficult to predict as small changes in the polyelectrolyte concentration, ionic strength or specific ions may result in large changes in the three-dimensional structure of the polyelectrolyte, including the possibility of the formation of aqueous biphasic systems. [Back to Top ]
Mixtures of hydrocolloids (gums) 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 (for example, 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).
Mixed hydrocolloids, at low concentrations (a), influence large volumes of water within their radius of gyration. As the concentrations increase different scenarios are possible. Individual molecules may fold up to form globular structures (b). This entropy-driven process allows the retention of rotational freedom. Further concentration (c) may cause phase separation (d) due to the different influences on the water structuring.
More commonly, hydrocolloids retain their extended structures. This may give rise to mixed entanglement (f; rarer) or phase-separated entanglement (e, commoner). Mixed entanglement may give rise to gels (g) at higher concentrations. Phase separations may be entropy-driven as they may allow greater freedom of movement due to the similarity in the molecular shapes (h).
Sometimes junction-zones arise, forming gels.
All hydrocolloids interact with water, reducing its diffusion and stabilizing its presence. Generally neutral hydrocolloids are less soluble whereas polyelectrolytes are more soluble but hydration kinetics depends on many factors; xanthan, guar and carboxymethylcellulose are soluble in cold water but carrageenan, locust bean gum and many alginates require hot water for complete hydration. 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. Some hydrocolloids, such
as locust bean gum and xanthan
gum, may form stronger gels on freeze-thaw due to kinetically
irreversible changes consequent upon forced association as water
is removed (as ice) on freezing [468]. [Back to Top ]
As hydrocolloids can dramatically affect the flow behavior of many times their own weight of water, most hydrocolloids (gums) 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 [236] and shear strain rate in a complex manner dependent on the hydrocolloid(s) and other materials present; mixtures of hydrocolloids may act synergically to increase viscosity or antagonistically to reduce it. These changes may generally be fitted to equations such as:
where η 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 η 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.
The relative viscosities of the more important hydrocolloids are influenced by their molecular weight distribution and other structural properties, dependent on their source and preparatory methodology, but are approximately as shown opposite;
A carboxymethylcellulose, B Guar, C locust bean gum, D κ-carrageenan, E λ-carrageenan, F ι-carrageenan, G xanthan, H gum arabic, I pectin, J microcrystalline cellulose, K methylcellulose, L konjac mannan.
The viscous (η') and elastic (η'') components of the (complex) viscosity (η*) of the gums, at 0.05% w/v and 0.5% w/v, are compared at 4 °C and 50 s-1 [using data from 694] where η* = η' + iη''.
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 (that is, 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 (for example, cold set gels like agarose, gellan and carrageenan), (c) gelling on heating to hold the structure together (for example, thermogelling heat set gels such as curdlan and methyl cellulose), (d) production and stabilization of multiphase systems including films.
Atomic force microscopy has been used to clarify the network structures on gelling, and gives useful results so long as care is taken that the structures do not change during preparation [1242]. [Back to Top ]
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. [Back to Top ]
a Some hydrocolloids are not gums and some gums are not hydrocolloids. Notably amongst the latter are 'chewing gums' and many gum adhesives, which are water-insoluble rubbery materials. [Back]
b Hydrophilic solutes (that is, solutes or structures possessing hydrophilicity) interact with water with greater or comparable strength to water-water interactions whereas hydrophobic solutes (that is, solutes or structures possessing hydrophobicity) only weakly interact with water with strength far less than water-water interactions. [Back, 2]
Home | Site Index | Hydrocolloid rheology | Hydrocolloids and health | Polysaccharide hydration | Hydrogen bonding | Hofmeister Series | Biphasic systems | LSBU | Top
This page was last updated by Martin Chaplin on 9 May, 2012
