39B-9

Y. D. HANG and E. E. Woodams. Department of Food Science and Technology, Cornell University, Geneva, NY 14456

Justification

The loss of fruit firmness is presumably caused by the action of hydrolytic enzymes (cellulase and pectinase) on the polysaccharides of fruit cell walls. Recently beta-galactosidase (E.C. 3.2.1.23) has been reported to play an important role in the softening of tomatoes and apples. There is no information in the literature regarding the beta-galactosidase activity of grapes.

Objective

The objective of this study was to examine the relationship between the activity of beta-galactosidase and the firmness of mature grapes.

Methods

Four cultivars of table grapes (Remaily Seedless, Canadice, Suffolk Red, and Einset Seedless) grown in Geneva, New York, were harvested at full maturity and stored in air at 2 degree C until needed. Firmness expressed as Newton (N) was measured with a Loyd Strength of Materials Testing Machine, Model M5K (Lloyd Instruments, Southhampton, UK). Twenty berries/cultivar were used in firmness determination. The beta-galactosidase activity was determined with ONPG (o-nitrophenyl B-D galactopyranoside) as substrate in 0.1 M sodium acetate buffer (pH 5.0) and 30 degree C. Protein was determined by the dye-binding method of Bradford.

Results

The firmness of four grape cultivars ranged from less than 1 to nearly 1.8 N and the beta-galactosidase activity varied from less than 70 to more than 180 units per mg protein. Remaily Seedless grapes, for example, had a high firmness value (1.8 N), but exhibited lower beta-galactosidase activity (64 units per mg protein). In contrast, Suffolk Red grapes contained higher beta-galactosidase activity (186 units per mg protein), but had a much lower firmness value (less than 1 N). The correlation coefficient (r) between the activity of beta-galactosidase and the firmness of mature grapes were 0.97.

Significance

The results of this study indicate that there is an inverse relationship between the activity of beta-galactosidase and the firmness of mature grapes.