High methoxyl pectins

Pectins are generally classed according to their ester content as high methoxyl pectins (>50% of the carboxyl groups esterified) or low methoxyl pectins (<50% of carboxyl groups esterified) (pectic acid, methyl ester [9049-34-1]). Low methoxyl pectins, like algins, require calcium for gelation. 

Pectins are subdivided according to their degree of esterification (DE), a designation of the percent of carboxyl groups esterified with methanol. Pectins with DE >50% are high methoxyl pectins (HM pectins) [65546-99-8]-, those with DE <50% are low methoxyl pectins (LM pectins) [9049-34-7]. 

T. P. Kravtchenko, Studies on the structure of industrial high methoxyl pectins, Ph.D. Thesis, Wageningen Agricultural University, The Netherlands, 1992. 

The commercial samples of pectins mainly used as food additives represent modified forms of the natural polymers due to the conditions of extraction. Nevertheless, it is usually recognized two categories of pectins the high methoxyl pectins (HM) with a degree of methylation DM>50% forming gels at low pH in presence of saccharose to reduce the water activity and the low methoxyl pectins (LM with DM<50%) forming gel in presence of calcium [4]. 

On the opposite, with copper, gels were obtained more easily for high methoxyl pectins. In general the extension of the gel phase was larger with calcium than with copper. 

The degradation of pectin is initiated by the action of PME, an enzyme removing methylester groups from highly methoxylated pectins. Production of incompletely deesterified pectin, probably due to end-product inhibition of PME (19, 20), may explain that an affinity of PL for both pectate and pectin is required for full pathogenicity of the bacteria. 

The Ridgelimeter is the most commonly used device to standardize high methoxyl pectins for commercial use (Cox and Higby, 1944). This empirical Sag-test is a one-point, nondestructive measurement. 

This treatment has the advantage of avoiding the acid extraction of the pectins it could be also used to obtain extruded products which could be used in various preparations traditionnally requiring high methoxyl pectins as gelling agents. The final products could be therefore enriched in dietary fibres and devoid of additives. 

Marsh grapefhiit (MGF) pulp was homogenized in 5 volumes of extraction buffer at 4 C and maintained at pH 8.0 (28). The homogenate was stirred for one hour, centrifuged and the supernatant used as the PE extract. Activity was measured by titration with a Brinkman (Westbury, NY) pH stat titrator at pH 7.5 and 30°C in 25 mL of 1 % high methoxyl pectin (Citrus Colloids Limited, Hereford, UK) with O.IM NaCl. PE units are expressed as the microequivalents of ester hydrolysed per minute. Uronic acid analyses were conducted based on the m-phenyl phenol (4) as modified for microplate reading (30). 

Gelling agents under appropriate conditions self-associate to produce a three-dimensional structure. Some gelling, as with gelatine, is thermo-reversible while others, such as with high methoxyl pectin, is irreversible. Apart from the effects on the texture of the product an irreversible gelling agent is more of a problem in the factory since it cannot readily be recycled. 

Pectin is used in foods in two forms, high methoxyl pectin and low methoxyl pectin. High methoxyl pectin is the form normally found in fruit while low methoxyl pectin is a chemically modified pectin. Pectins are acidic polysaccharides that occur in the cell walls of fruit. The commercial source of pectin is either citrus peel or apple pomace. The citrus peel is the residue from the production of citrus juices while apple pomace is the residue of cider production. Thus pectin is a by-product of either cider or fruit juice production. 

Most countries restrict the maximum degree of amidation to a 25% maximum. High methoxyl pectins are naturally present in fruit and escape restrictions on use for that reason. Low methoxyl pectins are treated as additives and have restrictive acceptable daily intakes (ADIs). 

The gel properties as well as the gelling conditions are radically different for the two types of pectin. High methoxyl pectins produce a gel that does not remelt, while some low methoxyl pectin gels are thermoreversible. 

Another use of pectin is when making a fruit flan or an open tart. These products are often coated with a pectin jelly based on a high methoxyl pectin. The pectin is dispersed and heated to dissolve it. As high methoxyl pectin requires an acid pH to set, just before use acid is added and the pectin mixture is poured over the flan. These fruit products are expected to be acid as part of the fruit flavour, so an acid gel is acceptable. 

If a product with a neutral pH is being made a high methoxyl pectin would not set, therefore the only option is to use a low methoxyl pectin. Examples of products with a neutral pH are mint flavoured jellies and Turkish delight. 

One problem with high methoxyl pectin is that of using rework. As pectin gels do not remelt this is much harder than it would be with gelatine. This point is important because not only is some of an expensive ingredient lost but simply disposing of the waste produces another cost. In practice high methoxyl pectin rework can be used if it is first comminuted, but the proportion used must not exceed 5%. 

Deesterification of natural polysaccharides accomplishes the same result as chemical derivatization, to wit, the critical DE range necessary for high-methoxyl pectin gelation. Robinson et al. (1988) discovered that the normally gelling, branched polysaccharides studied did not gel when side chains were removed, because the unsubstituted main chain did not subsequently adopt an ordered conformation. Their explanation was that the polydispersed... 

Different grades of high-methoxyl pectin (Hercules, Inc.). d Different grades of amidated pectin (Hercules, Inc.). 

Physical hydrosols and hydrogels are theoretically interconvertible, with the possible exception of high-methoxyl-pectin hydrogels that do not normally revert to a sol by reheating (Walter and Sherman, 1986) the pectin contained therein was recovered by dialysis and was comparable (by T) measurements) to the pectin before jelly formation. 

Specifically, D-glucose < maltose < maltotriose < amylose < starch < amylo-pectin < cellulose (Greenwood, 1967). Trends indicated are that thermochemical stability increases with the DP, branching, and 1,4-fi bonding. Chemical bonds other than 1,4-a and 1,4-(3 introduce heat and acid instability. Either of these two bonds is less easily depolymerized when the sixth pyranose carbon is oxidized to the carboxyl group rather than esterified for this reason, low-methoxyl pectin is more stable than high-methoxyl pectin. 

Using the density equation of dispersed polysaccharides at concentrations obeying Raoult s law, , may be obtained from Vt vs ct (Walter and Matias, 1989). When V vs ci is not linear, < >, must be stated at a given c,. Subsequent to 4>, determinations, configurational AS [Eq. (3.22)] were calculated for low-and high-methoxyl pectin. Low-methoxyl pectins were discovered to be less inclined to order themselves in an aqueous medium than high-methoxyl pectins (Walter, 1991). 

Property High methoxyl pectin Low methoxyl pectin Kappa carrageenan Iota carrageenan Lambda carrageenan... 

Michel, F., Thibault, J.-F., and Doublier, J.-L. (1984). Viscometric and potentiometric study of high-methoxyl pectins in the presence of sucrose. Carbohydr. Polym. 4 283-297. 

Oakenfull, D. G. (1991). The chemistry of high-methoxyl pectins. In The Chemistry and Technology of Pectin, Walter, R. H. (Ed.), pp. 87-105. Academic Press, New York. 

Walter, R. H., and Sherman, R. M. (1986). Rheology of high-methoxyl pectin jelly sols prepared above and below the gelation temperature. Lebensm. Wiss. Technol. 19 95-100. 

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