Carbohydrate metabolism in ripening banana fruit

A comprehensive picture of changes in carbohydrates, carbohydrate hydrolases, cell structure and texture in banana fruit during ripening is describe. The insoluble carbohydrates were separated into seven fractions based on differential solubility. Their profiles and composition were followed as hydrolysis products. The starch and pectic fractions decreased considerably in ripe banana, pulp. The significant decrease in glucose concentration in the hydrolysates of cold and hot water-soluble polysaccharides and that of xylose in the hydrolysates of hemicellulosic fractions (Hem A and B), indicated a pronounced glucan and xylan degradation during ripening. Some loss of mannose from the Hem "B" fraction at the ripe stage was also noticed. The various carbohydrate hydrolases, viz., polygalacturonase, pectin methyl esterase, xylanase, laminarinase, alpha-mannosidase, beta-galactosidase, amylase, cellulase and hemicellulase registered a general increase in their activities. Low levels of endo-beta-mannanase and galactanase activities could be detected only at the climacteric stage of ripening. More than 80% of the radio-activity of [C-14] starch was incorporated into soluble sugars, viz.., glucose, fructose and sucrose, indicating active sugar interconversions. The total content of these soluble sugars increased from 1.8 to 19%, with a concomitant decrease in starch content during ripening. Microscopically, loss of cell wall integrity, cell wall thinning, increased intercellular spaces, loosening of cells and disappearance of starch granules were evident. In banana, it appears that pectinase may play a more dominant role in softening than cellulase. Amylase, xylanase and laminarinase may also contribute to loosening of cellular structures. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.