Solubilization and Distribution of Neutral Sugar Residues Derived from Polyuronides during the Softening in Apple Fruit

Abstract

Arabinose and galactose are lost from the cell walls of apple fruit (Malus domestica Borkh.) during softening. However, the amounts of galacturonic acid, rhamnose, xylose, mannose, and glucose fluctuate very little.
The polyuronides were separated with DEAE-Sepharose and the changes in the sugar residue compositions of the polyuronides during softening were examined. A large amount of galacturonic acid and a small amount of rhamnose increased in the water-soluble polyuronide fraction and decreased in the EDTA- and. HC1-soluble polyuronide frac- tions during softening. The amounts of arabinose and galactose decreased in water-, the EDTA- and HCI-soluble polyuronide fractions. The amounts of xylose, mannose and glu- cose fluctuated little in water-, EDTA- and HC1-soluble polyuronide fractions. KOH-soluble polyuronides were abundant in xylose and glucose, and changed little during softening.
Most of the water-soluble polyuronides which increased during softening exhibited a lower affinity to ConA Sepharose than did the EDTA- and HCl-soluble polyuronides. These water-soluble polyuronides contained a large amount of galacturonic acid and only a small amount of rhamnose and other neutral sugars.
It was suggested that the preferential release of polyuronides poor in neutral sugar residues occurs in apples during softening. Losses of arabinose and galactose residues from the polyuronide molecules may be involved in the solubilization of these polyuronides.