Changes in intergranular air properties and fatty acid profile of wheat bulk in a large warehouse with air-conditioned oversummering

Abstract

Effect of air-condition on wheat's oversummering in a large flat warehouse urgently need to study. In this study, the air-condition was set at 25 °C in the second and third summer of wheat storage in No.19 warehouse of Pingyuan Grain Storage Depot, Shandong province, China. The intergranular air properties of grain bulk were calculated by our developed software. Compared with the control warehouse (No.17), the surface, second, third, and fourth layers of wheat bulk in No.19 warehouse during the running of air-conditioners decreased by 3.4 °C, 2 °C, 1.5 °C and 1.35 °C, respectively, the humidity ratio, relative humidity and wet-bulb temperature in the surface layer and whole bulk respectively decreased by 3.85 and 1.5 g/kg, 3.55% and 3.85%, 3.65 °C and 2.2 °C. Air-conditioning significantly decreased the bulk surface temperature. Free fatty acid (FFA) values in each layer of grain bulk increased continuously since warehousing, and the average FFA value in No.17 warehouse managed by conventional technology was below 40 mg KOH/100g. Twenty-seven species of fatty acids were identified in wheat. Contents of the total and unsaturated fatty acids increased respectively from 11703 to 9348 μg/g at 124-day storage to 14758 and 11834 μg/g at 841-day storage for wheat bulk in No.17 warehouse, but increased respectively from 11674 to 9321 μg/g to 16235 and 13027 μg/g for grain bulk in No.19 warehouse. Infrared spectroscopy confirmed that during 841-day storage, the absorption values of C-H stretching vibration (2919.7 cm⁻¹) of saturated carbon and C-H stretching vibration (3304.9 cm⁻¹) of unsaturated carbon in No.19 warehouse were significantly higher than those of No.17 warehouse. Scanning electron microscopy shows that air-conditioned warehouse effectively maintain the internal distinct microstructure of wheat grains. These results suggest that air-conditioning can reduce the bulk surface temperature and FAA value of stored wheat and keep the content of unsaturated fatty acids, and FFA value of properly stored wheat can be assumed as ≤50 mg KOH/100g.