Technical profile: Solid rice management by chilling
January 01, 1995
by Teresa Acklin
Contributed by suppliers, technical profiles feature new technology, products, specific applications or proprietary concepts. This material was prepared by Goldsaat Fritz Doring GmbH, Pruem, Germany.
Cooling, or the combination of drying and cooling procedures, provides economic advantages as far as handling and processing of sensitive rice are concerned.
Rice is harvested with mostly high and variable moisture contents, depending on different strains and cultivation areas. Moisture content between 16% and 30% is average. Sun drying is the least expensive and most common drying method used today. However, high quality demands and the large quantities harvested, particularly in industrial or cooperative installations, have limited the sun drying method. Losses from birds and rodents, contamination with pathogenic germs, foreign materials, insects, soot and lead from car exhaust fumes further limit sun drying.
Rising rice quantities have left water to be extracted by continuous mixed flow dryers. The rice grain reacts to the quick water extraction by shrinking and cracking, damaging quality and affecting appearance. Drying in batch dryers and continuously operating mixed flow dryers requires several drying passes.
The maximum moisture reduction per pass is between 4% and 2%, depending on initial moisture levels and/or prevailing free water.
The long drying process requires a lot of storage room for buffer storing. During temporary storage, the product is generally moist and warm. Bulk rice is cooled by ambient aeration to guard against spoilage. However, rice cultivating areas are often prey to consistent day and night temperatures. Ambient aeration only lowers temperatures to within a few degrees of the minimum ambient temperature. Processing costs usually rise because the inlet air is often warmer than the product.
To obtain a safe storage level, rice must be overdried to a moisture content of less than 12%. Unfortunately, this causes not only high drying costs, it also reduces the quality of the product. Reloadings and handling can further cause damaged and broken kernels.
Users of grain chillers have found that chilling the grain reduces costs and improves quality in the post-harvest treatment of rice. Economic advantages are abundant as well.
Freshly harvested rice with different moisture contents up to 30% can be temporarily stored for approximately four weeks before starting a drying process. By chilling during immediate storage, the moisture content of bulk rice drops to about 6%.
Pre-dried rice with a moisture content of up to 22% can be stored for approximately five months without being damaged.
Freshly harvested rice with up to 22% moisture content needs only be dried once, to approximately 18%. Then, chilling to 15° or 13°C will protect the product until safe storage conditions can be reached.
By combining drying and chilling, dryer plants are reduced to approximately 40% of their initial capacity.
By chilling, a temperature and moisture balance takes place in the whole bulk. Homogeneous products are easier to handle in further processes at reduced losses.
In many rice cultivating regions, even low-temperature drying can be utilized successfully. A low-temperature drying process must be employed inside small final storage areas. The reduction of varying initial moisture contents to a pre-chilling content of 18% can be performed in one step.
Chilling product down to 15° to 13°C with a final moisture content of 14% is quick and efficient with grain chillers. Chilled rice retains its quality with reduced losses for long periods of time.