Pectins

Pectins are mixtures of polysaccharides that originate from plants, contain pectinic acids as major components, are water soluble, and whose solutions gel under suitable conditions (2,4-7). Pectinic acids are galacturonoglycans [poly(a-D-galactopyranosyluronic acids)] with various, but greater than negligible, contents of methyl ester groups. Pectinic acids may have varying degrees of neutralization. Salts of pectinic acids are pectinates. Pectic acids are galacturonoglycans without, or with only a negligible content of, methyl ester groups. Pectic acids may have varying degrees of neutralization. Salts of pectic acids are pectates. The principal and key feature of all these molecules is a linear chain of a-D-galactopyranosyluronic acid units. In all commercial pectins, some of the carboxyl groups are in the methyl ester form; some or all of the remaining car-boxylic acid groups may be in a carboxylate salt form (Fig. 5).

Pectins are subdivided according to their degree of es-terification (DE), a designation of the percentage of carboxyl groups esterified with methanol. Pectins with DE values >50% are high-methoxyl (HM) pectins; those with DE values <50% are low-methoxyl (LM) pectins. The DE strongly influences the solubility, gel-forming ability, con-

Figure 5. Monomer units of pectins.

ditions required for gelation, gelling temperature, and gel properties of the preparation. In some LM pectins, termed amidated pectins, some carboxyl groups have been converted into carboxamide groups. The degree of amidation (DA) indicates the percentage of carboxyl groups in the amide form.

Pectins are soluble in hot water. The importance of pectin is predominately the result of its unique ability to form spreadable gels when a hot solution is cooled. HM-pectin gels are formed by the addition of sugar (at least 55%, but normally —65%, soluble solids) to a hot, acidic (pH ~3) solution of pectin in a fruit juice. LM pectins will gel only in the presence of calcium ions and do not require soluble solids to gel. Increasing the concentration of calcium ions increases the gelling temperature and gel strength.

Any system containing pectin at potential gelling conditions (ie, necessary concentration of an appropriate pectin, pH, concentration of cosolutes, and concentration of divalent cations) must be prepared at a temperature above the gelling temperature. The temperature at which structure is formed upon cooling is the gelling temperature. LM pectins, particularly amidated pectins, are sensitive to divalent cations, such as calcium ions, and require use of a sequestering agent for dissolution if they are present.

The primary use of pectins is in the preparation of spreadable gels (jams, jellies, marmalades, and preserves, including the preserves in fruit yogurt and fillings for chocolates) from fruit juices or whole fruits, with or without added sugar. Pectins are also used in preparation of chew-able fruit candies, fruit roll ups, canned fruit juices, cheese spreads, and icings and frostings.

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