CHANGES IN THE PECTIC SUBSTANCES OF APPLES DURING DEVELOPMENT AND POSTHARVEST RIPENING .2. ANALYSIS OF THE PECTIC FRACTIONS

Pectic material was extracted from the alcohol-insoluble residues of apples during growth on the tree and at different stages of ripening during postharvest storage. Three pectic fractions, soluble in the chelating agent trans-1,2-diaminocyclo-hexane-N,N,N',N'-tetraacetic acid (CDTA) in cold dilute sodium carbonate and in sodium carbonate at room temperature, were isolated. The galacturonic acid, neutral sugar and starch content of the fractions were determined as well as the degrees of methyl esterification and acetylation of the CDTA-soluble fraction. In addition, the molecular weight distribution was examined by gel filtration chromatography. The high galacturonic acid content of the CDTA-soluble fraction was consistent with the polysaccharide being essentially linear and originating in the middle lamella. The sodium carbonate-soluble pectin had a higher rhamnose to galacturonic acid ratio, indicating that it was more highly branched. Galactose and arabinose residues were lost in the polymers from all three fractions during ripening, with the greatest loss occurring from the sodium carbonate-soluble fractions. At the same time, the pectins became less polydisperse and shifted towards higher molecular weights. It seems that during development there is a dynamic turnover with new pectic polymers, which contain fewer neutral sugars, but are of high molecular weight, appearing in the middle lamella and primary cell wall.