Free sugar contents in onion bulbs on different cultivars and different production areas, and their changes by storage and gamma-irradiation

Changes in the contents of free sugar in onion bulbs of different cultivars and different production areas were investigated during storage and processing by using gas-chromatography. No significant difference was observed in the total free sugar contents among the onions of different cultivars and different production areas except early season's cultivars which showed a slightly lower content of sugar. The free sugar was mainly composed with glucose (1.7 - 3.2%), fructose (1.1 - 2.5%) and sucrose (0.7 - 2.6%), and the composition was somewhat different among the onions of different cultivars and different production areas. During the storage at 4 - 50C, the total free suger contents gradually decreased during the first few months but more markedly to about 30% after sprouting. The middle part of onion bulbs contained more amount of total free sugar than the other parts (middle part, 5.6 → 3.5%, inner part, 5.2 → 3.2%, outer part, 4.8 → 1.5%), and the total free sugar contents in the outer part decreased markedly during the storage. The changes of the sugar contents after the freeze-drying were almost negligible, but showed significant decrease after the hot-air drying, especially for sucrose. The gamma irradiation with 5 - 50 krad gave no appreciable effects on the total free sugar contents though it seemed to make a slight increase in the sucrose content. (author)