Abstract:

To investigate the degradation characteristics of cell wall polysaccharides and effects of related pectin degrading enzymes on the softening process of kiwifruit fruits at different storage temperatures. Fruit firmness, cell wall polysaccharide content and pectin degradation related enzyme activities of Xuxiang, Jinli and Wanlv fruits during storage at 25 ℃ and 4 ℃ were determined, and their correlation was analyzed. The firmness of Wanlv decreased the fastest and the storage time was the shortest, while the firmness of Xuxiang changed the slowest and the storage time was the longest. With fruit softening, contents of hemicellulose, cellulose and covalent soluble pectin (CSP) in kiwifruit of these three cultivars showed a decreasing trend, content of water soluble pectin (WSP) showed an increasing trend, while content of ionic soluble pectin (ISP) was relatively stable. The degradation rates of cell wall components of different cultivars were different at the two storage temperatures. Change rate of polysaccharide components in Wanlv kiwifruit was the fastest, followed by Jinli and Xuxiang. Storage at 4 ℃ delayed the degradation of cell wall polysaccharides. The results of correlation analysis showed that there was a significant (P<0.05) negative correlation between fruit firmness and WSP content, and a significant positive correlation between fruit firmness and CSP, hemicellulose and cellulose content in three varieties of kiwifruit. The results of pectin degradation enzyme activity determination showed that PG and β-Gal activities of Wanlv were significantly higher than those of the other two cultivars at 25 ℃, and PG and β-Gal activities of Xuxiang kiwifruit were significantly lower than those of the other two cultivars. In the early stage of storage at 4 ℃, activities of PL and α-AF in Xuxiang were significantly lower than those of the other two cultivars. At 25 ℃, PG and β-Gal were the pectin degrading enzymes that was significantly related to the softening of Xuxiang fruit. Under 4 ℃ storage condition, the enzymes significantly related to fruit softening of the three kiwifruit cultivars were different. The pectinase that was significantly related to the softening of Xuxiang fruit was α-AF and PME. The pectinases significantly associated with the softening of Jinli and Wanlv fruits were α-AF and PL, respectively. Wanlv kiwifruit softened the fastest, and its cell wall polysaccharide content changed the fastest. The softening rate of Xuxiang kiwifruit was the slowest, and the change of polysaccharide content in cell wall was also the slowest. In the early stage of storage at 25 ℃, the activities of PG, β-Gal and α-AF in Wanlv were the highest, while the activities of PME, PG, PL, β-Gal and α-AF in Xuxiang were the lowest. In the early stage of storage at 4 ℃, PL and α-AF activities were the highest in Wanlv, and the lowest in Xuxiang. These differences in pectinase activity may be one of the reasons for the rapid softening of Wanlv fruit and the slow softening of Xuxiang fruit.

Key words: kiwifruit, fruit softening, firmness, cell wall polysaccharide, pectinase activity