Water vapour sorption and transmission by onion (Allium cepa L.) scale under different temperature and humidity conditions.

The sorption behaviour of isolated outer dry scales of onion cv. Nasik Red bulbs showed that they had a higher equilibrium moisture content (EMC) at 25 than at 40C when the RH was <80%, but when RH was >80% the EMC was higher at 40. Large differences between adsorption and desorption EMC were noted during holding at room temperature (25-32) at RH between 35 and 80%. At 80% RH, the EMC was approximately 12% with the differences between adsorption and desorption EMC being lower than at other RH. The EMC of intact scales was higher at _ 50% RH, at both 25 and 40, than the EMC of isolated dry scales, but tended to decrease with an increase in RH from 50 to 80%. Under all RH combinations tested, the water vapour transmitted through the scales was lower at a constant temperature of 25 than either at a constant temperature of 35 or at room temperature, as the vapour pressure deficit at 25 was lower. Storage of onion bulbs in ventilated low-density polyethylene bags, at 21 1 and 60-65% RH showed a highly significant reduction in microbial spoilage (Aspergillus niger) along with marginal differences in physiological loss in weight and sprouting compared with those stored at ambient conditions (22-32 and 50-80% RH).