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Organisation: International Rice Research Institute, Philippines (IRRI)
Author: Ray Lantin
Edited by AGSI/FAO: Danilo Mejia (Technical), Beverly Lewis (Language&Style), Carolin Bothe (HTML transfer)

CHAPTER X RICE: Post-harvest Operations

3. Overall losses

Postproduction losses of rice can be quantitative or physical which means a reduction in weight or volume of the final usable product from the potential yield or harvestable paddy. The losses can also be qualitative which means a reduction in value of the usable product due to the physical and chemical changes which diminish the grain size, cause poor appearance, taste, aroma, cleanliness due to admixtures or contaminants and chemical residues and other factors which the consumers of the product will otherwise undervalue or the grain standards authorities will degrade resulting in low demand for the product, low price or rejection and declaration as unfit for consumption by human or animal.

Losses are incurred in pre-harvest and post-harvest processing operations consisting of harvesting, threshing, cleaning, drying, storage, transportation and milling. The flow chart of loss components of each operation are shown in Figure 4.0.1, Annex 4.0 F. The theoretical estimated losses of rice incurred in each operation are shown in Table 4.0.1, Annex 4.0 T.

Values of loss percentages for the different post-harvest operations by method in each operation in China, Indonesia, and the Philippines are respectively shown in Table 4.0.2, Table 4.0.3 and Table 4.0.4, Annex 4.0 T. Losses of rice within the post-harvest system in the developing countries of Asia, West Africa and Latin America are shown in Table 4.0.5, Annex 4.0 T.

The percentages of loss components incurred in different methods of harvesting, threshing, and storage in Zheijang, China during 1987-1989 have been documented by Ren-yong et al., 1990 and values are given in Figure 4.0.2, Figure 4.0.3, Figure 4.0.4, and Figure 4.0.5, Annex 4.0 F.

The following summarises the wrong practices in each major field operation and processing activity lead to physical grain losses:

(a) Pre-harvesting

(b) Harvesting

(c) Threshing

Summary notes on field operations The following are some choices of technology which is characterised by the degree by which grain could be lost. Manually beating the panicles on a drum or wood block will shatter grains and needs a wide threshing mat. A portable manual threshing platform enclosed by cloth or plastic sheet or fine net will contain shattering. A pedal-operated thresher increases threshing productivity and minimises shattering losses. An engine-powered axial-flow thresher combines threshing and cleaning in most designs but inefficient designs could lead to non-thorough separating and cleaning functions. An engine-powered stripper harvester combines harvesting and threshing and minimises handling losses but can lead to substantial grain losses if the crop is lodged and the field is wet and has poor trafficability.

(d) Drying

1. Grains shatter from the stalks or spill out of the grain bags during transport and handling;

2. During sundrying, birds and domestic fowls feed on the grains; grains spill outside the drying area;

3. Overdrying the grain, especially when sundrying by traditional method;

4. Delayed drying or no grain aeration which causes stackburning.

(e) Storage

The farmers in Lao PDR store paddy in bags or in bulk on a roofed elevated platform with removable stairs and supported by six posts. The walls are made of woven or pounded bamboo mat or slats. Rodents are kept out by enveloping each post by a galvanised iron sheet cone guard or a circular horizontal wooden plate barrier.

(f) Milling

Summary notes on off-field operations. Sundrying, the most widely used drying method for home-consumed rice by farming families in developing countries, needs concrete pavement facilities. Table 4.0.6, Annex 4.0 T gives values of losses in drying and storage.

Insect pests can not only cause physical losses but also affect the nutritive value of stored rice. Table 4.0.7, Annex 4.0 T gives the estimated losses in samples infested by Sitophilus oryzae.

The theoretical milling recovery is 71-73 percent, depending upon the variety of rice. In a well -operated modern mill, it is possible to obtain a milling recovery of 68-70 percent from a good variety of paddy.

Milling losses can be reduced by adopting small-scale modern rubber roll sheller and introducing parboiling of paddy before milling. The suggested mill for village level is cleaner ---- rubber roll sheller --- horizontal abrasive polisher. This combination is expected to give 66 percent milling recovery.


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