Shomer, I., Institute for Technology and Storage of Agricultural Products, Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel

Protein coagulation was studied in a serum of citrus fruit aqueous peel extract with regard to its role in cloud stability. Heat coagulation in enzymatic pectin degraded serum occurred within the temperature range 40-100°C. In the same serum, conversion of soluble proteins into insoluble ones occurred also during heat vacuum concentration. Heat-coagulated proteins were found in the flavedo extract, while they were not detected in the albedo extract. The coagulation intensity was affected by pH, the largest amount of coagulate being obtained around pH 4.5. Enzymatic pectin degradation markedly increased the coagulation due to reduction in the pectic polymer size. Coagulated protein bound pectin and associated neutral sugars were identified, and their amounts were found significantly lower in heat-inactivated extract, probably by inhibition of demethoxylation by pectin methyl esterase. The amounts of the coagulated protein bound neutral sugars (particularly rhamnose, arabinose, and glucose) were in accordance with the amount of the coagulate-bound pectin. © 1991 American Chemical Society.