Effect of different storage temperature on chemical composition of onion (Allium cepa L.) and its enzymes.

Onion stored at 4, 10, and 25 °C for 9 months were analyzed for changes in quercetin and its glucosidase content, enzymes, pyruvic acid, and sugar content. During storage, concentration of quercetin and its glucosidase showed an irregular variation at all studied temperature but at 4 °C the rate was high as compared to 10 and 25 °C. The enzymatic activity of Q4'G glucosidase and Q4'glucosyltransferase increased progressively until six months at 4, 10 and 25 °C, but later it started to decrease. At 4 and 10 °C, peroxidase activity increased during the first five weeks then decreased, while at 25 °C peroxidase activity decreased progressively after two months storage. Fructose, glucose and sucrose showed a different although more regular pattern by decreasing progressively at 4, 10 °C. At 4 °C fructose and glucose accumulated in the initial 3 to 4 months of storage while sucrose was unchanged. However, at 10 and 25 °C, fructose and glucose concentration continuously decreased, while sucrose increased consistently. Onion pyruvic acid increased at 4 and 10 °C during the first six months, while at 25 °C the fluctuation was observed during the whole storage period. Overall, we conclude that storage at 4 °C maintained the quality of onions best, as evidenced by the positive changes.