30G-1 |
Soluble and cell-wall associated forms of beta-galactosidase in apple fruits |
Y. D. HANG and E. E. Woodams. Dept. of Food Science & Technology, Cornell Univ., W. North St., NYSAES, Geneva, NY 14456 Loss of firmness in fruits is presumably caused by the action of hydrolytic enzymes on the polysaccharides of fruit cell walls. Recently there has been increased interest in beta-galactosidase as the enzyme most likely to be responsible for apple ripening and the loss of apple firmness. For example, beta-galactosidase has been reported to be capable of hydrolyzing the cell-wall polysaccharides of Granny Smith apples. A review of scientific literature reveals little information on the distribution of beta-galactosidase in apple fruits. The objective of this study was (1) to examine the distribution of soluble and cell-wall associated forms of beta-galactosidase in apple fruits, and (2) to determine the relationship between the firmness of apple fruits and the activities of both enzyme forms. Four cultivars of apple fruits (McIntosh, Jonamac, Empire and Crispin) were used. Firnmess was measured using a penetrometer with an 11 mm probe. Soluble beta-galactosidase was extracted by homogenizing apples in 0.025 M acetate buffer (pH 4.5) containing 2% PVP and 0.1% mercaptoethanol. The cell-wall associated enzyme was released from the particulate fraction with the same buffer containing 200 mM NaCl. The enzyme activity was determined by incubating the enzyme extract with 0.2% PNPG in acetate buffer (pH 5.0) at 30 degree C. Both soluble and cell-wall associated forms of beta-galactosidase were found to occur in apple fruits. Of the four cultivars of apples examined, the cell-wall associated enzyme form represented approximately 94-99% of the total beta-galactosidase activity. The coefficient of correlation between the firmness of apples and the activity of cell-wall associated enzyme form was determined to be 0.966. The coefficient of correlation between the fruit firmness and the soluble enzyme form, however, was only 0.838. The results of the present investigation clearly indicate that the cell-wall associated form of beta-galactosidase has a stronger inverse relationship with apple firmness than the soluble enzyme form.
Session 30G, Fruit & Vegetable Product: Processed Fruits and Vegetables
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