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 a pples during growth on the tree and at different stages of ripening du ring postharvest storage. Three pectic fractions, soluble in the chela ting agent rans-1,2-diaminocyclo-hexane-N,N,N',N'-tetraacetic acid (CD TA) in cold dilute sodium carbonate and in sodium carbonate at room te mperature, were isolated. The galacturonic acid, neutral sugar and sta rch content of the fractions were determined as well as the degrees of methyl esterification and acetylation of the CDTA-soluble fraction. I n addition, the molecular weight distribution was examined by gel filt ration chromatography. The high galacturonic acid content of the CDTA- soluble fraction was consistent with the polysaccharide being essentia lly linear and originating in the middle lamella. The sodium carbonate -soluble pectin had a higher rhamnose to galacturonic acid ratio, indi cating that it was more highly branched. Galactose and arabinose resid ues were lost in the polymers from all three fractions during ripening , with the greatest loss occurring from the sodium carbonate-soluble f ractions. At the same time, the pectins became less polydisperse and s hifted towards higher molecular weights. It seems that during developm ent there is a dynamic turnover with new pectic polymers, which contai n fewer neutral sugars, but are of high molecular weight, appearing in the middle lamella and primary cell wall.