During curing, the thin outer layers of the bulb are dried to form one or more complete dry skins, which act as a barrier to water loss and microbial infection. Ideally, the dirty outer skins can be removed after storage to show a clean, intact, inner dry skin before retail sale. Even for fresh-market onions, at least one complete skin should be present.
Initial curing (surface and neck drying) may take between a few hours and several days under forced, heated air ventilation, depending on the temperature and RH of the ventilating air and the stage of maturation of the bulbs. Curing is complete when the necks have dried out and are tightly closed and the skins rustle and have developed an attractive colour. An index of cure was developed for sweet onions in Georgia, USA, graded 1 (neck not dry) to 5 (neck will easily bend and flatten to bulb) (Maw et al, 1997b).
Traditional field-curing was done by 'wind-rowing': detached bulbs shaded by their tops were laid on their sides to dry for 1 or 2 weeks (Thompson, 1996). In hot climates, the bulbs are sometimes covered with straw or leaves while curing. In wet weather, drying takes longer, the bulbs risk becoming watermarked and are prone to rot in subsequent storage; roots may also regrow. Good practice in temperate regions is to move onions into storage straight after lifting and dry them by passing air at 30-32°C through bulk stores or bins - the 'direct harvest' method (Tatham, 1982; Maude et al, 1984). After 3-5 days at higher temperature, the heat is gradually lowered to the safer level of 27°C and 70-75% RH for the completion of drying over about 20 days; this continues gradually until the outer skins reach the 'rustling dry' stage on top of the stack. The temperature can then be lowered to near 0°C for long-term storage.
Biochemical changes during skin curing are being investigated in Japan (Hirota et al., 1999; Takahama and Hirota, 2000). The flavonoid constituents of the drying outer skins of onion bulbs are oxidized and anti-fungal compounds are formed (Takahama and Hirota, 2000).
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