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Gum arabicHydrocolloids and gums

Hydrocolloids are gums that are added to foodstuffs in order to control their functional properties, such as thickening or gelling.

 

link; Introduction to rheology

link; Introduction to polysaccharides

link; Dietary fiber and health

V Hydrocolloid polymers

V Colloids

V Polyelectrolytes

V Mixtures of hydrocolloids
V Effect on viscosity
V Hydrocolloid action
V Hydrogels

V The DLVO theory

Colloids

Colloids are polymer solutions, hydrophilic networks, emulsions, micelles, vesicles, liposomes and similar materials. Proteins and nucleic acids are both colloids and membranes can form colloids, with all of these being dealt with on other pages. Colloids are heterogeneous systems with at least two phases with one having dimension(s) between a few nanometers and a few micrometers. Colloids are distinguished by their inability to pass through 'dialysis' membranes. Most colloids get drawn into the gas-liquid interface [2679] and are, therefore somewhat surface active. Proteins, therefore, are liable to denaturation if shaken to a foam.

water binding at 20 °C [1815] go for more information concerning water activity Hydrocolloid polymers and gums

Hydrocolloids [227], often called gums, a are hydrophilic b 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 solubility [2560], 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. 

 

 Agar (Jmol)

 Alginate (Jmol)

 Arabinoxylan (Jmol)

 Carrageenan (Jmol)

 Carboxymethylcellulose (Jmol)

 Cellulose (Jmol)

 Curdlan (Jmol)

 Gelatin (Jmol)

 Gellan

 β-Glucan (Jmol)

 Guar gum (Jmol)

 Gum arabic

 Locust bean gum (Jmol)

 Pectin (Jmol)

 Starch (Jmol)

 Xanthan gum (Jmol)

 

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 relative molecular mass (molecular weight)s and no one molecule is likely to be conformationally identical or even structurally identical (cellulose excepted) to any other. [Back to Top to top of page]

Properties of polyelectrolytes

Osmotic pressure variation with concentration, see ref 1493Many hydrocolloids are polyelectrolytes (for example alginate, carrageenan, carboxymethyl-cellulose, 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 is 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 stiffen. 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 to top of page]

Mixtures of hydrocolloids

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).

 

differing consequences of concentrating mixed hydrocolloids

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 carboxymethyl-cellulose 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 c and may have substantial economic benefit. In particular, hydrocolloids can provide water for increasing the flexibility (plasticizing, making more rubbery [2245]) of other food components. They can also affect 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 to top of page]

Effect on viscosity

As hydrocolloids can dramatically affect the flow behavior of many times their own weight of water, most hydrocolloids (gums) are used to increase the 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:

 

eta = a x exp(C*)

graph of viscosity versus concentration

 

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

eta = A x exp(Ea/RT) graph of log(viscosity) against reciprocal temperature

 

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.

line

 

hydrocolloid relative viscosities

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 mass distribution and other structural properties, dependent on their source and preparatory methodology, but are approximate 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 pseudo-plasticity (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, thermo-gelling 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. It gives useful results so long as care is taken that the structures do not change during preparation [1242]. [Back to Top to top of page]

Hydrocolloid action

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 to top of page]

Hydrogels

Diapers contain superabsorbent hydrogelHydrogels are water-based hydrophilic cross-linked polymers [2893], such as poly-N,N-dimethylacrylamide (PDMAA) that are capable of swelling to absorb and hold vast amounts of water (10 - 1000 times its own weight) within their three-dimensional network [2455]. They are particularly known for their use in sanitary products and nappies but also have usages over a broad spectrum including food additives, tissue engineering, and pharmaceuticals. Commonly used materials make use of poly-acrylates, but hydrogels may be made by crosslinking soluble hydrocolloids to make an insoluble but elastic and hydrophilic polymer. The water is necessarily held in pools of bound water that have little tendency to drain out. As the hydrogel hydrates, the long-range network of hydrophilic groups expands to hold more water. At the same time, the hydrophobic short-range structures are shortened due to repulsive interactions with the surrounding water molecules [2852].

 

There are problems associated with determining the hydration structure of hydrogels as the structuring encompasses nano-, micro-, and macro-scales with the hydrogel function dependent on the various aqueous pools sizes and their distribution. Cryo-scanning electron microscopy (cryo-SEM) is a popular method for structural elucidation but the hydrated hydrogel must be 'fixed', to withstand the required high vacuum, and the results depend on these 'fixed' structures being representative of the native structures. Many methods appear to give artifactual structures damaged by ice crystal formation. Also, the addition of cryoprotectants can also disturb the native structuring. It has been proposed that the native state is retained by freezing of thin (~ 15 µm) samples using a - 186 °C propane jet [2946].

 

Tough adhesives that bond to wet surfaces may be formed using hydrogels an integrated backing for adhesive surfaces [2980]. Their manufacture was inspired by a slug's defensive mucus that strongly sticks to wet surfaces and prevents the slug's detachment from that surface by predators. The hydrogel backing dissipates any deforming forces.

[Back to Top to top of page]

The DLVO theory

The DLVO theory concerns colloidal dispersions and is named after Derjaguin, Landau, Verwey, and Overbeek [2217]. It assumes the particle stability depends on long range interactions and describes the balance between two forces, electrostatic double layer repulsion and long range van der Waals attraction (involving atom-based electron fluctuations) which gives an energetically favored separation distance (see 'minimum' energy position below). The interaction between charged colloidal particles in a concentrated suspension has been described [3021].

 

As the liquid between interacting bodies is assumed to have bulk properties up to molecular distances, the DLVO theory fails at short distances from hydrophilic surfaces and ions where short range interactions dominate [3012].

 

Energy diagram for DLVO theory

Outline of the DLVO theory

The van der Waals attraction is rather insensitive to the concentration of the electrolytes and dominates when the separation between the surfaces is small, whereas the electrostatic double layer repulsion is quite sensitive to the concentration of electrolytes and is strong at larger separations.The DLVO theory is generally used to qualitatively explain colloid behavior but may not hold at short distances (< ~5 nm) in liquid water [3030]. This is due to the hydrated surfaces having long range secondary hydration forces [1726] and kosmotropic and chaotropic ions behaving differently within the hydrated surface layer.

 

The DLVO theory does not hold for nanobubbles where the van der Waals effect is negligible. [Back to Top to top of page]


Footnotes

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]

 

c Where interactions between hydrophilic polymers increase slowly with time, they eliminate water and syneresis may occur (as in some jam and jelly). Syneresis is also linked to the osmotic pressure difference between the polymer matrix and the surrounding solution [2914]. [Back]

 

 

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This page was established in 2001 and last updated by Martin Chaplin on 11 September, 2017


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