PECTINESTERASE-CATALYZED FIRMING EFFECTS DURING PRECOOKING OF VEGETABLES

Many vegetables exhibit a firming effect after precooking at a tempera ture between 50 and 70C. This effect has generally been attributed to the action of endogenous pectinesterase which hydrolyzes the methyl es ter linkages in pectin molecules. The resulting free carboxyl groups t hen form Ca-bridges between pectin molecules. We have shown, by using the pectinesterases of pea sprouts, that the enzymes catalyzed not onl y the hydrolysis of the methoxyl groups of pectin molecules, but also a transacylation reaction of the galacturonic acyl groups from methano l to other hydroxyl groups of pectin. The fatter reaction results in t he formation of new ester linkages between pectin molecules, which als o contributes to the firming of the tissue. The pectinesterases have b een separated into four isozymes, PEI, PE2, PE3, and PE5. The isozymes exhibited similar transacylation activity, with the exception of PE5 that did not catalyze transacylation.