Effects of postharvest storage conditions and varietal differences on the chemical properties of onion bulbs were demonstrated. Fresh red and white onion varieties were stored for two months under three different storage conditions: ambient temperature (28-30ºC), refrigeration (5-7ºC) and control cupboard temperature storage (45-50ºC). Main and interaction effects of the two factors (storage and variety) on proximate compositions, flavonoids, vitamin C and mineral contents of the onions were determined weekly using Two-way ANOVA. Storage techniques were found significant (P<0.05) with respect to all the chemical characteristics of both onion cultivars. The rate of moisture uptake in onion stored under refrigerated temperature increased slightly in the first three weeks and remained relatively constant the rest of storage period, whereas ambient and warm temperature caused about 22.5 and 27% moisture loss, respectively at the end storage period. The reverse was true in the case of carbohydrate. Generally, red onion cultivar had higher protein, lipid, flavonoid and ascorbic acid contents irrespective of the storage conditions. Flavonoid content declined as storage time progressed under refrigeration, and highest loss of ascorbic acid; 73 and 69% were experienced in red and white bulbs respectively, during cold storage. There were slight variations in the mineral contents of onions during storage. Generally, ambient and warm temperature conditions retained some of the postharvest quality properties of onions better than refrigeration.
Key words: Fresh onions, storage, cultivars, chemical properties.
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