Cell-wall polysaccharide modifications during postharvest ripening of papaya fruit (Carica papaya)

Abstract

Abstract Modifications of cell-wall polysaccharides and flesh firmness, as a measure of texture, were evaluated during postharvest ripening of papaya fruit (Carica papaya). Cell-wall material (CWM) was isolated from pericarp at different ripening stages. Three pectic and one hemicellulosic fractions, as well as a cellulosic residue, were obtained from CWM. A water soluble fraction contained pectins presumably not cell wall associated. Imidazole and sodium carbonate soluble fractions contained pectins with low degree of branching from middle lamella and profusely branched structures with neutral sugar chains arising from primary cell walls, respectively. Solubilization and concomitant depolymerization of both kinds of pectin was observed. Loss of galactose at two phases of ripening was detected, one at the beginning, and the other, with a simultaneous release of rhamnose, at the onset of the postharvest period. Polysaccharide analysis of 4 M sodium hydroxide soluble fraction was compatible with a xyloglucan type and a mannose containing polymer. Depolymerization of this hemicellulosic fraction was not evident during ripening. A continuous depolymerization of water-extracted pectins was evident. For this fraction, a polymer containing galacturonic acid and covalently associated rhamnose units, i.e., a rhamnogalacturonan type structure that lost acidic monomeric units during ripening, appeared beyond the exclusion limit of gel filtration resin. Cellulose residue exhibited decreasing quantities of galacturonic acid and non-glucose monosaccharides during ripening indicating that association between polysaccharides from matrix and microfibrilar phases may be involved in the softening process. Methyl esterification decreased for middle lamella and primary cell wall pectins while it increased steadily for water soluble pectins. Finally, a scheme of suggested temporal events undergone by cell-wall polysaccharides and the possible enzymes involved along with firmness variation during postharvest ripening of papaya fruit is presented.