Thorpe, Graham (2005) Recircaeration - a New Low Energy Method of Preserving Cereal Grains. In: 43rd Annual ANZSES Conference, 28th-30th November, 2005, New Zealand.

Abstract

Each year about 2,000,000,000 tonnes of cereal and other food grains are harvested, and their farm gate value is on the order of $500,000,000,000. As a result, the global grains industry is similar in value to that of the oil industry. About 5% of all grains harvested are destroyed during storage by insects, mites and fungi, and in some countries the spoilage exceeds 25%. Grains can be stored for prolonged periods of time by manipulating the stored grains ecosystem by changing the temperature, grain moisture content or the composition of the intergranular atmosphere so that insect pests cannot survive. An effective method of preserving grains is to cool them by blowing through them ambient or refrigerated air. However, because grains are hygroscopic the temperature to which grains can be cooled is a function of both the temperature and the humidity of the air used to aerate them. Ceteris paribus the dryer the air, the lower the temperature to which the grains can be cooled. Warm air cannot be used for aeration and cool ambient air generally has a high relative humidity that limits the degree of cooling that can be achieved. Grains are often stored with moisture contents such that the relative humidity of the intergranular air is less than 60%. In this work it is hypothesised that air leaving the upper surface of an aerated bulk of grain can be cooled and re-admitted to the grain at a thermodynamic state that is more favourable for cooling than ambient air. It is further hypothesised that solar energy incident on the roofs of grain stores passively dries the upper surface of stored grains thus further enhancing the behaviour of grains as a desiccant. A mathematical analysis suggests that grains can indeed be cooled by up to 5ºC more using recirculated air, and this has a profoundly beneficial effect on the preservation of grains. The power consumption of the system is estimated to be about 1W per tonne of grain stored, and the energy consumption is estimated to be 250Wh per tonne per month of storage. The performance of a recircaeration system in a climate that is representative of a low altitude region in Southern Queensland in summer has been investigated. It is estimated that a population of the rice weevil, S. oryzae, would increase by a factor of about 6,400 in unaerated wheat that has a moisture content of 11% (wet basis) and a temperature of 30°C, whereas in grain that is aerated with ambient air or using the proposed recircaeration system the increases are 1500-fold and 350-fold respectively. Similar reductions in populations of the lesser grain borer, Rhyzopertha dominica are obtained. Somewhat lower relative reductions in the decay of chemical pesticides are predicted over a period of 100 days, but the benefits of aeration and recircaeration manifest themselves over greater periods of time. The marginal capital cost of the system is negligible. Solar energy passively dries the upper surface of a bulk of grains by a process dubbed desiccaeration, but the benefit of this appears to be offset by the higher overall heat load on the system. Nonetheless, this aspect of the process is worthy of further research.

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