Gelling agents are used in the food industry in a wide range of products both traditional and novel, and this use is increasing rapidly with the increase of con- venience foods. An ideal gelling agent should not interfere with the odor, flavor, or taste of the prod- uct to which it is added (Fishman and Jen, 1986). Improvements to existing and development of new ones require basic understanding of the processes
192 Part II: Products Manufacturing of gelatin and the properties of gels at the molec-
ular level (Doublier and Thibault, 1984; May and Stainsby, 1986).
Gels are a form of matter intermediate be- tween a solid and a liquid. They consist of poly- meric molecules cross-linked to form a tangled, interconnected molecular network immersed in a liquid medium (Flory, 1953). The polymer net- work holds the water, preventing it from flowing away (Oakenfull, 1987; Meyer, 1960). In gels, the molecules are held together by a combination of weak intermolecular forces such as hydrogen bonds, elec- trostatic forces, Vanderwaals forces, and hydropho- bic interactions. The cross-linkages are not point in- teractions but involve extensive segments from two or more polymer molecules, usually in well-defined structures called junction zones (Rees, 1969). The gelation process is essentially the formation of these junction zones (Fig. 11.1).
The physical characterizations of gel are the consequence of the formation of a continuous three-dimensional network of cross-linked polymer molecules on a molecular level; an aqueous gel con- sists of three elements (Jarvis, 1984):
1. Junction zones where polymer molecules are joined together.
2. Interjunction segments of polymers those are rel- atively mobile.
3. Water entrapped in the polymer network. Gels are always formed in an aqueous envi- ronment. Thus, the interactions of protein and
polysaccharides with water are by themselves im- portant factors in the gelation process. Both types of polymers are strongly hydrated in aqueous solution, so that some water molecules are so tightly bound that they fail to freeze even at temperatures as low as −60◦C (Eagland, 1975). Although the formation of a stable intermolecular junction is a critical require- ment for gelation, some limitation on the interchain association is also necessary to give a hydrated net- work rather than an insoluble precipitate (Axelos and Thibault, 1991).
It is important to know the condition for the onset of gelation in technological processes involving gelling food products. Several methods are used to char- acterize this change in consistency (Doesburg and Grevers, 1960; Walter and Sherman, 1981; Beveridge and Timber, 1989; Dhame, 1992; Rao, 1992; Rao and Cooley, 1993). Physically, the critical stage of gela- tion may be monitored from the loss of fluidity or from the rise of the elastic property of the growing network (Shomer, 1991). Table 11.1 gives different types of jelling agents used in the manufacture of jellies.
Gelatin is a water-soluble protein formed by initial degradation of collagen from animal skin and bones. Gelatin jellies have a rather soft or rubbery texture. For these, it is normal to use an additional gelling agent such as thin boiling starch. This involves the texture incidentally. Gelatin gels forms reversibly on cooling a gelatin solution. It is now well established that the protein molecules are cross-linked to form a network by junction zones, where the protein chain
Table 11.1. Different Types of Jelling Agents Used in the Manufacture of Jellies
Type of Gelling
Agent Origin Use
Gelatin A protein of animal origin extracted from bones and purified
Generally, must not be boiled. To be added to warm syrup for setting on cooling
Agar/Alginates Extracted from various sea weeds Various products such as neutral jellies, weakened by boiling in acid solution
Gum Arabic/Acacia Exudates from trees Used to produce hard gums, and as an extender and thickener in products, e.g., Marshmallow Starch/Modified starch Seeds and various roots These have been completely and
partly replaced by other jelling agents in gums—Turkish delight Glazer
Pectin Fruit residues particularly citrus and apple pomace
Used largely in acid fruit jellies but with low melting point is used in neutral jellies
have partly refolded in the collagen triple helix struc- ture (Veis, 1964).
Agar/alginates are the major structural polysaccha- rides of algae. Agar jellies have a very soft texture. Straight agar jellies have a characteristic “shortness” that may be modified by the addition of gelatin, gum Arabic, pectin, starch, etc. Alginates with a high ra- tio of poly--d-mannuronic acid (M) and poly-␣-l- guluronic acid (G) form weak, forbid gels, whereas low M/G alginates give transparent, stiff, brittle gels, and the gel strength depends on the nature of the di- valent cation (Smidsred, 1974).
Gum Arabic is the most water-soluble of the natu- ral gums (up to 50%) and their solutions are of rela- tively low viscosity. Other advantages of gum Arabic are its absence of odor, color, and taste. Hard jellies can be produced with gum Arabic.
Unmodified starches, produced by wet milling of field corn, supply the major amount of thickening ma- terial. Modified starch is starch that has undergone one of the varieties of treatment to alter its physi- cal property and/or functionality (Mauro et al., 1991; Furcsik and Mauro, 1991). They are used to extend the bodying or gelling effect of normal starches, to modify gelling tendencies, and to improve texture. Starch is an essentially linear polymer of␣-(1 → 4) linked d-glucose (Wolfram and EI Khadem, 1965). Starch gels consequently have a composite structure of open, porous amylopectin molecules threaded by an amylose matrix. Thus, actual gel-forming poly- mer in starch is amylose. The molecular weight dis- tribution of amylose depends on the plant source and molecular weights of several millions with broad dis- tribution have been reported (Rao et al., 1993).
Pectin is the most frequently used hydrocolloids in processed fruits. Jams and jellies are the major food type using larger amounts of pectin. Pectin is a class of complex hetero polysaccharides found in the cell walls of higher plants, where they function as a hydrating agent and cementing material for the cellulosic network (Muralikrishna and Taranathan, 1994).
When pectin-rich plant materials are heated with acidified water, the protopectin is liberated and is hydrolyzed into pectin that is readily soluble in water. It happens in plant tissues during ripening of fruits with the aid of an enzyme protopectinase. As the ripening of fruit proceeds, more and more of the in- soluble protopectin is converted into soluble pectin (Woodmansee et al., 1959). Their composition varies with the source and conditions of extraction, location, and other environment factors (Chang et al., 1994). Pectic substances in the primary cell wall have a
relatively higher proportion of oligosaccharides chain on their backbone, and the side chains are much longer than those of the pectin of the middle lamella (Sakai et al., 1993).
Pectins are primarily a polymer of d-galacturonic acid (homopolymer of [1→ 4] ␣-d-galacto pyra- nosyluronic acid units with varying degrees of carboxyl groups methylated estrified) and rhamno- galacturonan (hetero polymer of repeating [1→ 2] ␣-l-rhamnosyl [1-l] ␣-d-galactosyluronic acid dis- accharide units), making it an␣-d-galacturonan (Lau et al., 1985). The molecule is formed by l-1,4- glycosidic linkages between the pyranose rings of d-galacturonic acid units. As both hydroxyl groups of d-galacturonic acid at carbon atoms 1 and 4 are on the axial position, the polymer formed is 1,4- polysaccharide (Sakai et al., 1993; Oakenfull, 1991). The chemical structure of galacturonic acid and its methyl ester are shown in Figure 11.3 and the link- ages between different galacturonic acids and their methyl esters in pectic and pectinic acid are shown in Figure 11.4 (Swaminathan, 1987).
Pectic acid is composed mostly of colloidal poly- galacturonic acid molecules, and is essentially free from methyl ester groups. The salts of pectic acids are either normal or acid pectates, whereas pectinic acid ones are colloidal polygalacturonic acids con- taining more than a negligible portion of methyl es- ter groups. Pectinic acid under suitable conditions is capable of forming gels with sugar and acid or, if suit- ably low in methoxyl content, with certain metallic ions. The salts of pectinic acids are either normal or acid pectinates.
Studies on esterified residue in pectin claimed that they are randomly distributed (Garnier et al., 1993; De vries et al., 1986) or non-randomly distributed (Mort et al., 1993a,b). However, ion exchange chro- matography showed a random distribution of change in citrus pectin that had undergone acid catalyzed deesterification (Garnier et al., 1993). Such disparate findings may, impart, be due to the length of galactur- onate residues being examined or due to differences in pectin source (Baker et al., 1996).
Polygalactoronic acid could be considered as a rod in solution, whereas pectins are segmented rods with flexibility at the rhamnose tees (Fry, 1986). The size, charge density, charge distribution, and degree of sub- stitution of pectin molecules can be changed biolog- ically or chemically (Kerstez, 1951; Kratz, 1993).
The most unique and outstanding property of pectin is their ability to form gel in the presence of Ca2+ ions in sugar and acid solution (Gordon et al., 2000; Halliday and Bailey, 1954). Degree of