CHANGES IN APPARENT MOLECULAR-MASS OF PECTIN AND HEMICELLULOSE EXTRACTS DURING PEACH SOFTENING

Pectin and hemicellulose were solubilized from cell walls of peach [Pr unus persica (L.) Batsch] fruit differing in firmness by extraction wi th imidazole and sodium carbonate (pectin extracts), followed by a gra ded series of potassium hydroxide (hemicellulose extracts). The extrac ts were subjected to size exclusion chromatography. In imidazole extra cts, as fruit softened, there was an increase in proportion of a large apparent molecular mass peak, with a galacturonosyl to rhamnosyl resi due ratio resembling a rhamnogalacturonan-like polymer. A smaller appa rent molecular mass peak was enriched in galacturonic acid and probabl y represented a broad polydisperse peak derived from more homogalactur onan-like polymers. In sodium carbonate extracts, a homogalacturonan-l ike polymer appeared to elute primarily as a higher apparent molecular mass constituent, which increased in quantity relative to other const ituents as fruit softened. In cold 1 mol.L-1 KOH extracts three peaks predominated. A xyloglucan-like polymer appeared to elute predominantl y in the second peak and fucose was strongly associated with it. In 4 mol.L-1 KOH extracts (tightly bound hemicellulose) the higher apparent molecular mass peak was predominantly acidic and presumably of pectic origin. The smaller apparent molecular mass peaks were composed prima rily of neutral sugars, the second peak became smaller and the third p eak larger as fruit softened. The ability to separate pectin and xylog lucan-like polymer as two separate fractions based on charge suggests that the nature of any pectin-hemicellulose interaction in this fracti on is probably one of physical entrapment of pectin fractions by hemic ellulose and not principally by covalent crosslinking between the two polysaccharide classes in peach. Flesh firmness serves as an important determinant of quality in peaches. Our results indicate that apparent molecular mass of both pectins and hemicelluloses changed as peaches softened, resulting in alteration of cell wall structure and associate d with decreased tissue cohesion.