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

Abstract

The sorption behaviour of isolated outer dry scales of onion bulbs showed a higher equilibrium moisture content (EMC) at 25°C than at 40°C, when the relative humidity (RH) was < 80%, while at higher RH a tendency to an increase in EMC at 40°C was seen. Large differences between the adsorption and desorption EMC were noted during holding at room temperature (25–32°C) in the 35 and 80% RH range, and the least at 40°C. At 80% RH, the EMC was ∼ 12%, with the differences between adsorption and desorption EMC being lower than at other RH. In comparison to isolated dry scales, the EMC of intact scales was higher at 50% RH and below, both at 25 and 40°C, 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°C than either at a constant temperature of 35°C or at ambient conditions, as the vapour pressure deficit at 25°C was lower. Further, storage of onion bulbs in ventilated low-density polyethylene (LDPE) bags, at 21 ± 1°C and 60–65% RH showed a highly significant reduction in microbial spoilage along with marginal differences in physiological loss in weight (PLW) and sprouting compared with those stored at ambient conditions (22–32°C and 50–80% RH).