Moisture movement in grain

• Department of Agriculture, Thailand
• Chiang Mai University, Thailand
• National Postharvest Institute for Research and Extension, Philippines

Under Australian conditions, the initial temperature in bulk grain has proved the single most important factor in determining the rate of moisture migration from warm to cool regions in the grain, a cause of serious spoilage. If insects infest the commodity, the heat that their metabolism liberates will lead to even steeper temperature gradients and thus accelerate spoilage. Storages in the hot humid tropics provide almost optimal conditions for insect development, and moisture migration in stored commodities compounds the already severe difficulties.
Accordingly, scientists in this project will: conduct extensive experiments on heat and moisture transfer by natural convection in porus hygroscopic media; quantify the experiments by means of an analogy between heat transfer alone and the combined effects of heat and mass transfer; determine the effects, if any, of fumigants on the degree of natural convection; and devise and test improved methods of storing commodities in bags and bulk. They will also carry out full-scale field trials of promising improved storage methods and determine the efficacy with which these maintain commodity quality and an environment hostile to insects.
Initial experimental investigations will use a specially constructed apparatus designed to promote heat and moisture transfer in porous media. This consists of two 1.75-metre-diameter parallel disks, separated by a distance adjustable up to 1 m. Steam, regulated to the required pressure, creates a uniform temperature (typically 40C) on the bottom plate, while water maintains the other plate at a lower temperature. The scientists can fill the space between plates with granular commodities or small bags and use well-established dimensional laws to scale the results. By changing the variables within the apparatus, they can promote and study heat and mass transfer in the commodities.
Concurrently, they will carry out a mathematical analysis of the phenomena. This has been done in outline for heat and moisture transfer by natural convection in non-hygroscopic porous media, but will need modification to account for sorption in food commodities.
Using results from experimental and mathematical studies, the team will develop improved grain storage methods and test these first on a laboratory scale and later in full-scale field experiments in close collaboration with the team in ACIAR project 8307. Those scientists are studying long-term storage of grain and other foodstuffs under plastic covers - a potentially very useful and cost-effective method of maintaining grain and other foodstuffs in good condition. However, high ambient temperatures and humidities in the tropics call for precise definition of storage conditions, which the present project can provide. It should also lead to the design of storage systems that minimise losses due to moisture problems. In addition, the information will be equally relevant to short-term storage of high-moisture grain, either in bags or in bulk.