Limiting moisture uptake at the grain surface to prevent mite infestation

Abstract

Published details of seasonal changes in moisture content at the surface of grain bulks give an idea of maximum moisture contents (m.c.) of 17% likely to be achieved during the winter. In boxes with closed bases, which were effectively unaerated, barley rose from 12.2% m.c. to 16.3% m.c. in October while in the boxes with a closed base, the increase was greaterm illustrating the difference between purely surface absorption and the increase that can occur as a result of temperature gradients. In a 24 h period in August the ambient relative humidity (r.h.) rose from about 40% between midday and 1700, to over 90% at about 0600 in the morning. In contrast, the surface m.c. rose slowly from about 10.5% to nearly 13%, reaching a peak at about 1000, lagging about 4h behind the ambient r.h., giving some indication of diurnal changes in m.c. The moisture content of rapeseed at the surface of a sealed airtight bin of rapeseed varied between about 7.5 and 9%, in winter and summer, compared with 5% and 12% in the unaerated bin. As a result, mite numbers were severely limited.

Observations made between autumn and spring, 1998-9 at the storage facility at CSL, York, showed differences between the m.c.s achieved at the surface of flat and bins stores within the same building. In the bin stores, surface m.c.s were between 16.2% and 17.7%, in November and February, but in the flat store, the surface range was only 14.7-16.2%.

Bins containing 25 kg of wheat were held at constant conditions of 15C and 80% r.h. (17.6% m.c.) and sealed with shrouds of nylon woven tarpaulin, plastic sheeting (nylon polythene laminate) or butyl rubber or left uncovered as a control. The controls steadily absorbed moisture and had median surface m.c.s of 16.3% after 18 weeks. The shrouded replicated also steadily absorbed moisture but here uptake was reduced, with differences from the controls of 0.5 - 1% depending upon the material. The best reduction in mite population was in the region of x10 for Acarus. The best of the materials was the plastic sheeting by virtue of the lowest m.c. and mite infestation after 18 weeks.