Technical profile: Cold storage of paddy rice

by Teresa Acklin
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   Contributed by suppliers, technical profiles feature new technology, products, specific applications or proprietary concepts. This material was prepared by Sulzer Escher Wyss, Lindau, Germany.

   Rice is one of the oldest crops grown by mankind and is one of the most important staple foods in the world.

   Normally, rice is harvested once a year, but two to three crops are possible in areas with especially favorable climatic conditions. Paddy rice is mostly harvested under very damp conditions and must be dried for storage and processing.

   It is of utmost importance to dry the paddy with great care. The heat and moisture inside the kernel must be allowed to dissipate to the surface very slowly (vapor diffusion).

   If the paddy is dried too quickly, the surface generally will dry and the dissipation of the humidity contained in the core of the kernel will be too slow, resulting in fissures and cracks. It is still common practice to sun-dry the paddy on the ground, where it is turned quite often.

   Mechanization in agriculture brought tremendous changes. Nowadays, harvesting and threshing machines generally are used, and the harvested grains are stored in bulk.

   Sun drying is no longer the most efficient way of drying, and fuel or gas operated dryers are most commonly used. Normally, the paddy rice is dried from its harvest moisture content to approximately 14% storage moisture content in four to five drying steps. It is quite common to dry even to 12% to 13% moisture content for safety reasons (see Figure 1).

   Not even extremely gentle drying will avoid the increase in broken kernels during each drying process. It is also common knowledge that the energy consumption for drying depends not only on the difference of the moisture content, but also on the humidity range in which the product is dried.

   Fuel consumption is generally calculated at 1 to 1.5 liters per percentage point per tonne. This value is correct in the 20% to 16% drying range. The value is, however, much higher in the lower ranges (14%), where it reaches approximately 2 liters per percent of humidity reduction (see Figure 2).

   Ventilation is not a good solution for tropical countries. All that can be done with ambient air is to avoid an excessive temperature increase of the grain.

   Decades ago it was realized in Europe that cold storage of grain could help overcome bottlenecks and the resulting losses. It already had been known for a long time that grain cooling with cold, dry air during the night assists grain conservation and drying.

   The advantages of cold storage for paddy rice were recognized very early in Spain and Italy. The experience acquired in Spain encouraged silo operators in tropical countries such as Malaysia, Thailand, Vietnam and China to introduce paddy cold storage.

   The chilled, dry air generated by the Granifrigor system also leads to a certain drying effect. The combination of drying and cooling enables a reduction in the number of drying steps to two to three (see Figure 3 on page 24). This is of special advantage in the 18% to 12% drying range, since in this range it is of utmost importance to dry very carefully and at lower temperatures. The combination of drying and cooling makes it possible to increase the intake capacity of the dryers by up to 50% for the same drying temperatures.

   The yield of head rice is between 50% and 60% with conventional storage. Figure 4 on page 24 shows how the yield of head rice increases with the combination of drying and cooling.

   Considerable fuel savings are achieved by the combination of drying and cooling. Energy costs for cold storage depend on the ambient temperatures. In tropical climates they are approximately 8 to 12 kilowatt hours per tonne of grain.

   Paddy rice has excellent insulation properties so that a single cooling step to approximately 15°C to 18°C is sufficient for a long storage period, without the necessity to recool. Insulation of the storage facility is never required.

   Insects do rather little damage to whole paddy rice. But in the case of milled rice storage, it is easy for the insects, e.g. the rice weevil, to enter the soft endosperm of the kernel where they can cause a lot of damage. High storage temperatures offer excellent conditions for insect activity, while cold storage temperatures stop it; chemical treatments are no longer required.

   The required cold storage temperature is set on the grain chiller and kept absolutely constant even under fluctuating ambient temperatures. The units also include devices to maintain a constant chilled air humidity (Hygrotherm device). This technique ensures that the required air humidity is maintained even under damp ambient conditions — humidification of the grain is excluded.

   Cost calculations also have shown that the amortization time for grain cooling equipment is short.