Hydrocolloids are widely used in many food formulations to improve quality attributes and shelf-life. The two main uses are as thickening and gelling agents. As thickening agents, they find uses in soups, gravies, salad dressings, sauces and toppings while as gelling agents, they are extensively used in products like jam, jelly, marmalade, restructured foods and low sugar/calorie gels. The role of specific hydrocolloids for thickening and for gel formation is reviewed pinpointing specific applications in food formulations and for product development.

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Food gels are viscoelastic substances and several gelled products are manufactured throughout the world. The gelling agents in foods are usually polysaccharides and proteins. In food gels, the polymer molecules are not cross-linked by covalent bonds with the exception of disulphide bonds in some protein gels. Instead, the molecules are held together by a combination of weak inter-molecular forces like hydrogen bonds, electrostatic forces, Van der Waals forces, and hydrophobic interactions. Polysaccharides including hydrocolloids are strongly hydrated in aqueous medium but they tend to have less ordered structures. The mechanism of gelation depends on the nature of the gelling agent(s) and on the conditions of gel formation like the temperature, the presence of ions, the pH, and the concentration of gelling agents, etc. Characterization of gels can be performed in several ways of which rheological measurements are frequently practiced. Multi-component or mixed gel system is an important area of interest in which two or more gelling components are simultaneously used to achieve certain specific structural and functional characteristics. We here discuss about the different gels and gelling agents, the characterization of gels, and the mechanism of gelation with an emphasis on mixed or multi-component gels that would have significant commercial applications.

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The effect of extrusion barrel temperature (80–120 °C) and screw speed (200–400 rpm) on the pasting and gelatinization properties of extruded rice products were studied. A twin‐screw extruder was used without using a die. A rapid viscoanalyser (RVA) was used for the pasting study; the parameters determined were the initial viscosity (IV), peak viscosity (PV), hot‐paste viscosity (HPV) and cold‐paste viscosity (CPV). The PV, HPV and CPV generally decreased with increasing barrel temperature and screw speed. The IV was high, 30–43 rapid viscoanalyser units (RVU), for extrudates compared with raw rice, 3–4 RVU. The extent of gelatinization (GE) of the extruded rice samples ranged between 93.8% and 99.0%, and an increase in screw speed enhanced the GE values. Regression equations (r > 0.991, P < 0.01) that relate pasting and gelatinization behaviour to temperature and screw speeds are reported.

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