Thickeners and gelling
Thickeners are substances that increase the viscosity of foods that thicken them. Gellants (gellants) are substances that under certain conditions are capable of forming jellies (gels), structured disperse systems. Thickeners and gellants can produce food with the right consistency, improve and preserve the structure of the products, while having a positive effect on their taste perception. Due to the ability to bind water, thickeners and gellants stabilize disperse systems: suspensions, emulsions, foams. They almost always simultaneously perform other technological functions: stabilizers and water-retaining agents. In addition, they are classified as dietary fibers.
A clear distinction between the gelling and thickening is not always possible.
Any substances with varying properties and degree of gelling and thickening agent. Some thickeners can under certain conditions form strong, elastic gels.
Thickening agents and gelling agents in chemical nature are linear or branched polymer chains with hydrophilic groups that enter into a physical interaction with the water present in the product. With the exception of xanthan E415 microbial polysaccharides and E418 gellan gum, as well as gelatin (animal protein), gellants and thickeners are plant-derived carbohydrates (polysaccharides), vegetable hydrocolloids. They are obtained from terrestrial plants or algae. From brown algae, alginic acid Е400 and its salts Е401-404 are obtained. The most popular gelling agents agar (agar-agar) E406 and carrageenan (including furcelleran) Е407 are obtained from red seaweed, and pectin Е440 - most often from apples and citrus. Polysaccharides obtained from plants are subdivided into protective colloids, isolated by the plant in case of damage (exudates, resins), and seed meal (reserve polysaccharides of plants). Resins include arabinogalactan Е409, tragacanth Е413, gum arabic Е414, gum karaya E416, ghatti gum Е419. To reserve polysaccharides - flour (gum) of locust beans Е410, oat gum Е411, guar gum Е412, gum of Е417, gum acacum Е425.
Hydrocolloids chemical structure divided into three groups: acidic polysaccharides uronic acid residues, acidic polysaccharides having sulfuric acid radicals, and neutral polysaccharides. As thickening hydrocolloids used with acidic residues of uronic acid, such as tragacanth (E413) and gum arabic (E414) , and neutral compounds such as locust bean gum (E410) and guar (E412). Acidic polysaccharides having sulfuric acid radicals are used as a gelling agent, for example, agar (E406) and carrageenan (E407).
Thickener molecules are rolled into balls. Once in the water or in a medium containing free water, thickener molecule through solvation coil unwinds, the mobility of water molecules is restricted, and the solution viscosity increases (Table. 6).
6 Table. Viscosity solutions popular thickeners (viscosity of water ~ 1 cps)
The properties of thickeners, especially neutral polysaccharides, can be varied by physical treatment, for example thermal, or by chemical modification, for example by introducing neutral or ionic substituents into the molecule. Esters of cellulose E461-E467 are classified as modified polysaccharides. By chemical or physical modification of starch, it is possible to achieve: a decrease or increase in the temperature of its gelatinization; Lowering or increasing the viscosity of the paste; Increase solubility in cold water; Appearance of emulsifying properties; Resistance to syneresis, acids, high temperatures, cycles of thawing - freezing; Decrease in propensity to retrograde. In this case, different types of modified starches are obtained (E1400-E1405, E1410-E1414, E1420-E1423, E1440, E1442, E1443, E1450, E1451) .
Gels (jellies) are disperse systems, at least two-component systems, consisting of a dispersed phase distributed in a dispersion medium. The dispersion medium is a liquid. In food systems, this is usually water, and the gel is therefore called hydrogel. The disperse phase is a jelly-forming agent whose polymer chains form a cross-linked mesh and do not possess the mobility that is present in the thickener molecules in highly viscous solutions. Water in such a system is physically connected and also loses mobility. A consequence of this is a change in the consistency of the food product. The structure and strength of food gels produced using different gelling agents may vary greatly.
The gel is practically attached form a colloidal solution - sol. To convert the sol into a gel it is necessary that between the molecules distributed in the liquid began to act forces causing intermolecular crosslinking. This can happen in different ways: reducing the amount of solvent by evaporation; decreasing the solubility of the substance distributed by chemical interaction; additive substances and promote the formation of crosslinking bonds; change in temperature and adjusting the pH.
The beginning of gelling is accompanied by a slowing down of the Brownian motion of the particles of the disperse phase (increase in viscosity), their hydration, and the formation of a polymer network. The ability of polymers to form gels depends on the length and number of linearly oriented sections of their molecules, as well as the presence of side chains creating steric hindrance in intermolecular interactions. Gelling mechanisms of gellants can also vary greatly, at present three main mechanisms of gelling are singled out: sugar-acid, egg-packing model and double-helix model (Table 7).
7 table. Gelatione in hydrocolloid solutions
Let us consider in more detail gelation high and low-esterified pectin.
The ability to chew in highly esterified pectins (the degree of esterification from 50 to 75%, molecular weight from 10000 to 300000) is based on the property of linear molecules to form a three-dimensional polymer network in the presence of water, acid and sugar. Intermolecular bonds are represented by hydrogen bridges, free segments of molecules are strongly hydrated. The presence of a certain amount of acid is necessary to inhibit the dissociation of free carboxyl groups. In this case, the total negative charge of the molecules decreases and, thereby, their mutual repulsion is suppressed. A high concentration of neutral sugars, for example sucrose, in turn, reduces the water activity of the system with the simultaneous dehydration of pectin molecules, which leads to an easier convergence of binding zones.
Low esterification pectins (degree of esterification <50%), like other ionic gelling agents, gel in the presence of certain cations, usually calcium. The gelling ability for low esterified pectins is practically independent of the dry matter content and the pH value. So, milk gels have a pH of about 6,5, and gelled fruit and vegetable juices - about 2,5. The binding of polymer chains of low esterified pectins occurs through polyvalent cations (Ca2+). Wherein the concentration of calcium ions is very important for the gel properties, such as their lack of gel is formed and with an excess of a gel is formed, inclined to syneresis, moreover, is precipitated salt - calcium pectinate.