Change in chemical composition of onion ( Allium cepa L. cv . Sunpower) during post‐storage under ambient conditions

Post‐storage deterioration of onion under ambient conditions (temperature 20–25 °C and relative humidity 60%–80%), subsequent to a cold storage period of 8 months, was investigated with an emphasis on changes in chemical composition of flavonols, sugars and phenylalanine. A simultaneous determination of total phenolics, total flavonoids and antioxidant activity was also done. At the 4th week of post‐storage, the total quercetin content was three times (2.725 ± 0.097 µ mol/g fresh weight [FW]) its initial post‐storage value (0.877 ± 0.085 µ mol/g FW) and visible signs of sprouting and decay appeared. The highest content of total quercetin in the onion bulb (3.209 ± 0.350 µ mol/g FW) was found at the 8th week, and by the 10th week, the onion bulbs were severely decayed and were reduced to roughly half of the initial size. The fructose and glucose content decreased continuously during post‐storage from 0.174 ± 0.007 to 0.024 ± 0.004 mmol/g FW and 0.368 ± 0.04 to 0.014 ± 0.006 mmol/g FW, respectively, whereas the sucrose content was almost constant. Total phenolics, total flavonoids and antioxidant activity increased and correlated well with total quercetin content during the post‐storage period.