Pectins, /3-eliminative depolymerization

Processing of vegetables at 100 °C to 120 C results in a major decrease of firmness. Several studies suggest that the major cleavage reaction leading to sofi ning is the 6-eliminative depolymerization of intercellular pectin. Since this chemical breakdown of pectins only occurs at methylated smooth pectic regions, the... 

At pH values of 5-6 pectin solutions are stable only at room temperature. As the temperature 1s raised, pectin chains cleave by a beta-elimination reaction (25-39) (Figure 1), a reaction which is stimulated by organic anions (40). Deesterification of pectin proceeds simultaneously with the beta-elimination depolymerization reaction, which occurs only at monosaccharide units that are esterlfied. At pH values above 6, deesterlfIcatlon eind depolymerization are rapid reactions even at room temperature, the rate of each reaction increasing with Increasing pH. 

Milieu conditions in gastrointestinal tract can influence the pectin structure and properties. Under the acid conditions of the stomach (pH 2-4) extraction of pectin from plant cell walls and hydrolysis of side chains can occur. In small intestine (pH 5-6) -elimination of main chains or de-esterification seems to be possible. In caecum and colon (pH 6-8) a strong fermentation of pectin takes place causing depolymerization to oligomers and leading to formation of short chain fatty acids and gases. The presence of OligoGalA is not yet clarified. 

All catalyze depolymerization by a beta-elimination reaction like that which occurs during base-catalyzed depolyraerization. Pectin lyases, all of which are endo-enzymes, catalyze beta-eliminations at esterified D-galacturonlc acid units (See Figure 1) (68). Pectate lyases catalyze beta-eliminations at nonesterifled D-galacturonic acid units. Both exo- and endo-pectate lyases are T[Pg.10]

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