Effect of temperature on soluble invertase activity, and glucose, fructose and sucrose status of onion bulbs (Allium cepa) in store

Abstract

The activity of soluble invertase, and the variation in glucose, fructose and sucrose contents in onion bulbs (Allium cepa) during long-term storage at 10°C and 20°C were investigated. Invertase activity increased progressively after 8 weeks to 0.084 and 0.092 nkat/g fresh weight (FW), then sharply to 0.29 and 0.35 nkat/g FW at 20°C and 10°C, respectively, and remained high during 5 weeks. Then, activity decreased abruptly to 0.039 and 0.041 nkat/g, and remained low during the last 8 weeks and close to that observed initially. Glucose increased to 17.73 and 14.62 mg/g FW after 4 weeks at 20°C and 10°C, respectively, then decreased sharply between week 5 and week 7 to 4.13 and 4.91 mg/g FW, respectively, and remained rather stable ranging from 9 and 10 mg/g FW at both temperatures. Fructose showed a similar pattern and was 14.8 and 21.68 mg/g FW at 20°C and 10°C, respectively. Between week 10 and week 24, fructose ranged from 5 and 6 mg/g FW, and from 6 and 7 mg/g FW at 20°C and 10°C, respectively. Sucrose increased to 19.63 and 14.43 mg/g FW at 20°C and 10°C, respectively, decreased during 3 weeks, and then increased randomly from 5.69 to 9.42 mg/g FW at 20°C, but remained in a steady state at 10°C ranging 5.03±0.78 mg/g FW. During the last 6 weeks, the sucrose content was higher at 20°C than at 10°C. The fructose–glucose ratio varied during the first 8 weeks but remained at a steady level during the last 16 weeks. The (glucose+fructose)/sucrose ratio increased randomly at 10°C, whereas at 20°C the ratio increased during 10 weeks then decreased progressively during the final 14 weeks.