Changes in fruit size, fruit dry matter and carbohydrate composition at different stages in developing water chestnut fruit

Water chestnut (Trapa bispinosa Roxb.) is an important aquatic fruit crop in south east Asian countries like India, Bangladesh and Thailand. Apart from its use as fresh fruit, dry nut flour is a chief source of non-cereal carbohydrate diet. Five different water chestnut varieties, which included two green types and three red fruit types were grown under water logged condition and showed characteristic changes in composition of carbohydrates with the fruit age. Fruit size increased up to 14 days after fruiting initiation. The dry matter of fruits increased from 7 days to 14 days but after 21 days dry matter content almost doubled in all the five varieties compared to that at 14 days. Soluble carbohydrate, imparting sweetness to fruit was at its peak level at 7-day stage. By 14 days after fruiting initiation, the soluble carbohydrate was 49% and starch fraction was about 51%. In over-mature fruit, 21 days after fruit initiation, starch concentrations in all five varieties increased to 88 to 94% and soluble carbohydrate level decreased to 6 to 12%. For raw consumption 14-day stage was optimal for sweetness, dry matter and starch content as well as desirable nut size. Delayed hay-vest to 21 days subjected fruit to over-maturity and reductions in sweetness. But as dry matter and starch content after this stage increased considerably, fruits were more suitable for making into flour rather than using as fresh fruit.