Blending with other hydrocolloids

Combinations of more than one hydrocolloid are widely used in foods. In fact, use of blends rather than a single hydrocolloid could be considered standard practice. Non-gelling hydrocolloids are normally used together to obtain optimal rheology. In some cases, xanthan gum guar gum being a good example, the combination is used to obtain a synergistic increase in viscosity. The changes in viscosity that result form blending non-gelling hydrocolloids can be predicted using the so-called log-mean blending law. Viscosity values that differ form those predicted indicate synergistic (greater than expected) or anti-synergistic behaviour. Combinations of gelling and non-gelling hydrocolloids or two or more gelling hydrocolloids are much more complex, and various possible network structures have been proposed. Since not a great deal is known about mixed polysaccharide gelling systems, the concept of synergism applied to these systems is generally in appropriate. Despite this lack of fundamental knowledge, mixed polysaccharide gels are well established commercially. Combinations of k-carrgeenan or agar and locust-bean gum are perhaps the best examples. The inclusion of locust-bean gum provides textural modification and allows reduction of the total polymer concentration required for gel formation. Use of the gelling system xanthan gum locust-bean gum to modify the textures of gels such as those form agar and carrageenan has also been suggested.

The effect of both gelling and non-gelling hydrocolloids on the texture of low-acyl gellan gum gels has been extensively studied. Commonly used thickeners such as guar gum, locust-bean gum, xanthan gum, carboxy-methylcellulose and tamarind gum, when added to gellan gum in progressively increasing amounts while maintaining a constant total gum concentration, cause a progressive reduction in hardness and modulus. Brittleness remains essentially constant, with an accompanying slight increase in elasticity. These effects are shown for low-acyl gellan gum/xanthan gum combinations in Fig.16. For the key textural parameters, hardness, modulus and brittleness, modulus and brittleness, these thickeners function essentially as inert diluents and the texture of the resulting blends is similar to the texture of low-acyl gellan gum alone at a concentration equivalent to that in the blend. It is common practice to include thickeners in gelled systems to reduce syneresis, improve freeze thaw stability and, in some cases, eliminate unfavourable interactions between ingredients. Thus, some products formulated with gellan gum also require the presence of thickener. The textural similarity between gels form low-acyl gellan gum, k-carrgeenan and agar has already been mentioned. Blends of low-acyl gellan gum and agar(0.50% and 0.25% total gum concentration ) provide gels in 4mM Ca2+ that show a decrease in hardness and modulus as the blend becomes richer in the agar gum component, the decrease being more pronounced at the higher gum concentration. Brittleness and elasticity values remain virtually constant around 34% and 14%, respectively. Similar blends of k-carrgeenan and low-acyl gellan gum in 0.16mM K+ show a rapid drop in hardness(4.5 to 2) in going form 0.5% low-acyl gellan gum, k-carrageenan and then rises to around 4 for carrageenan alone at 0.5%. Modulus falls sharply form 4.6 in going form 0.5% low-acyl ggellan gum alone to the 80:20 blend and remains between 1.5 and 2 thereafter. At 0.25% total gum concentration the same trends are apparent but less pronounced. As in the case of the low-acyl / agar blends, the low-acyl / carrgeenan blends have a fairly constant brittleness and elasticity irrespective of blend composition. The values for these latter textural parameters are almost identical to those for the low-acyl / agar blends. These data indicate that the characteristic brittle texture of low-acyl gellan gum gels cannot be substantially changed by progressive substitution with other brittle gelling agents.        

This is not the case when low-acyl gellan gum is used in combination with the xanthan gum / locust-bean gum gelling system. As can be seen in Fig,17, the xanthan gum gels become less brittle as the blend becomes richer in xanthan / locust-bean gum. Figure 18 indicates the other textural changes that take place. Hardness and modulus are reduced, while elasticity is increased. Similar textural changes are induced when locust-bean gum is replaced by other hydrocolloids, such as Cassia gum and konjak mannan , both of which are capable of interacting with xanthan gum to form gels with texture similar to those obtained form xanthan gum and locust-bean gum gels and native or high-acyl gellan gum is evident by comparing Figs18 and 19 . Consequently, it si perhaps not surprising, as indicated in Fig.19, that blends of high- and low-acyl gellan gum provide textural variations similar to those obtained by blending different ratios of low-acyl gellan gum and xanthan gum / locust-bean gum. For labeling purposes, achievement of textureal modification by blending gellan gum alone would clearly be referred. Figure 20 demonstrates the rang of different textures that can be obtained simply by using different proportions of the high-acyl form.  

If starch is excluded, gelatine is the most widely used gelling agent. In contrast to the strong, brittle, non-elastic gels produced by low-acyl gellan gum, gelatine gels have a low modulus or perceived firmness and ggelatine offer another avenue to textural diversity. For example, addition of progressively increasing amounts of 250 Bloom type A gelatine to 0.25% low- acyl gellan gum, a typical in-use concentration, causes a gradual increase in hardness, modulus and elasticity and a gradual reduction in brittleness. Conversely, addition of low levels of low-acyl gellan gum, up to 0.05%, to replace up to around 1% gelatine in a 5% gelatine gel has no marked influence on the characteristic gelatine texture. It is conceivable, however, that the melting / setting temperatures of the gelatine may be advantageously increased by inclusion of low levels of the higher melting and setting low-acyl gellan gum. Low-acyl gellan gum / gelatine combinations may also help prevent toughening of gelatine gels upon refrigerated storage , allow low-grade gelatins to be upgraded in quality or permit lower gum concentrations to be used in certain applications. A recent patent describes combinations of gelatine and different forms of deacylated gellan gum. In the context of gelatine, the excellent flavour release form low-acyl gellan gum gels is worthy of mention. This flavour release is a consequence of gellan gum’s ability to structure water in the gelled state at very low use levels rather than the gels having the ‘melt in the mouth’ characteristics associated with gelatine.

Raw and modified starches are often used to impart a characteristic heavy-bodied consistency and, in some cases, a gel-like structure to foods. Cold-water-dispersible instant starches are available but many starches require cooking to cause gelatinization and generate the desired functional properties. The molecular changes that occur when starch is cooked and cooled are still poorly understood. However, it is generally accepted that a cooked starch paste consists of swollen intact and ruptured granules within a continuous aqueous phase containing the solubilized amylase and amylopectin, the two component polysaccharides of starch. When starch is used, additional hydrocolloids are often required to modify texture, reduce syneresis and improve freeze thaw stability. Although possible mechanisms for the interaction between these hydrocolloids and starch have been suggested, current understanding is again poor. Consequently, starch hydrocolloid combinations are usually selected on an empirical basis. A standard indicator of performance of a starch system is the viscosity changes that occur during heating and cooling as measured on an amylograph. Amylograph data on the influence of low-acyl gellan gum on the modified starch, Col-Flo67 (National Starch and Chemical Corp.), are shown in Fig, 21. The gellan gum produces a more rapid increase in initial build-up of viscosity. Viscosity subsequently remains fairly constant on further cooking and then rises less rapidly than the viscosity of the Col-Flo 67 alone upon cooling. These limited results are difficult to interpret on a fundamental basis but suggest that starch gellan gum combinations are worthy of more detailed study. In practical terms, it has already been shown that in certain pudding and pie fillings the levels of modified starch can be reduced by a half by inclusion of around 0.1% Kelcogel gellan gum. In these products, the structure imparted by the gellan gum results in a fifmer shorter texture.
The compatibility of polysaccharides with proteins depends on a number of factors such as relative concentrations, pH, ionic strength, temperature time and, in case of foods, the nature of the other ingredients present. From a compatibility standpoint, model studies are thus of limited value and, as for starches, incompatibility problems that occur in products are most quickly solved by empiricism. Our experience with the interactions between gellan gum kk and proteins has also been similar. Limited model studies indicated that while low-acyl gellan gum was compatible at neutral pH with milk proteins, soy, egg albumen, whey and sodium caseinate precipitation with all of these proteins occurred around pH 4. In contrast, it has been shown possible to produce a number of direct acidified and cultured dairy products using low-acyl gellan gum in combination with protective colloids such as guar gum and carboxymethylcellulose. Low-acyl gellan gum also shows good compatibility with proteins in non-acidified mile systems. The need to study protein / polysaccharide interactions under specific in-use conditions is emphasized by the fact that. Although low-acyl gellan gum and gelatine combinations are potentially useful as already discussed, precipitation can occur under certain conditions. The observation that sodium caseinate and soy protein can prevent gelation of low-acyl gellan gum without causing precipitation also requires further investigation to define more fully the conditions under which this occurs.