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Grain losses in rice processing

Losses from Drying the Paddy. Sun drying is the traditional method of drying the harvested rice crop. The crop is either left in wind rows in the field to dry (after reaping but before threshing), or spread out on mats or on pavements after threshing. During the wet season, if there is no artificial drying capacity, it is not uncommon for the grain to sprout and rot before it can be dried. If there is any delay in drying the wet grain the result is darker colouration, compared to the summer crop that can be sun dried immediately after harvesting and has a whiter and brighter luster. There has been a proliferation of mechanical drying machines to cope with the new varieties. Now the losses in drying occur because of poor technical performance or improper use of the technology, resulting in fissured grain. Fissured grain results in significantly lower milling yields. The rice drying process has been thoroughly studied. It has been established that thermal stresses, high rates of moisture desorption, or moisture reabsorption by dried grains causes the rice kernel to fissure. The typical HYV medium to long grain Indica variety has 20% hull (or husk), and 10% bran layers. The theoretical milling yield of polished grain should be 70%. In commercial milling, not all the paddy grains are whole grains. Depending on the percentage of perfect grains, state of the art commercial mills properly adjusted and working with "good" quality paddy can yield 67% milled rice, with head rice (¾ to whole grains) above 70%. Poor quality paddy that is badly fissured can lower total milling yields as low as 60%. Much of the grain endosperm is reduced to rice flour that goes with the bran, or as brewer's rice that is separated from the commercial milled rice output by sifters. The bran and rice flour, and small broken grains are used as animal feed. Much is already known about the causes of fissuring, unfortunately the basic principles of proper drying are not yet widely disseminated in the industry.

Losses in Milling the Paddy. Further losses in the milling process are caused by poor technical performance of milling machinery, or operator ineptitude, resulting in poor milling yields. An example of a machine that has been eliminated in some countries is the Engleberg type single pass mill one step process, notorious for breaking the grain in the milling process and yielding as only 53% milled rice. In 1985, about 50% of the milling capacity in the Philippines was processed by Engleberg mills. In Bangladesh today, the Engleberg machine is still the predominant mill. India is reported to have outlawed the Engleberg machines in favour of more efficient mills. No Engleberg machines are found in Thailand. Reporting losses in milling should distinguish between those caused by the drying process and those due to the milling process itself; one should be careful to avoid double counting. If the derived potential loss in drying is 7%, milling the same in an Engleberg will result in an additional 7% loss or a total of 14% loss in the processing plant. In the Philippines, which produces about 10 million tons of paddy, if 50% goes through their antiquated mills, 700 thousand tons of grains will be lost to the consumer.

There has been dissenting opinion on the suggestion that Engleberg mills and the one-pass coffee grinders used to mill rice in rural areas, about to be phased out. Martin Gummert notes that in Sumatra they have a law that forbids micro-mills of the Engleberg design, but the traders continue to import them from China as coffee grinders for milling rice.

Losses in storage. Pest infestation due to insects, rodents, and birds is a real threat when paddy is stored over three months and when milled rice is stored over a month. There are measures to control infestation, such as the fumigation of the stock with phosphine gas. A 200 ton stock of infested paddy was covered with tarpaulin taped to the floor to seal it and phostoxin tablets worth US$80.00 was inserted inside. After ten days, the tarpaulin was opened and all the pests died, including rats, birds, and a cat unwittingly caught inside. The added cost of the lethal gas tablets was acceptable. The only problem was how to fumigate paddy in bags in an open warehouse. There must be willingness to store the bags so that they can be covered with the tarpaulin without having to spend money to re-stock them.

Local private millers do not normally take any pest control measures. They accept the infestation loss. When the paddy is milled the insects are aspirated out, and the damaged grain is screened out. Infested milled rice is remilled (passed through the whitening section); 10% of the rice by weight is lost.

Government marketing authorities that manage buffer stocks, are very concerned with storage pest infestation. Teguh Muhammad Sardjono and Dr Hariyadi Halid of Badan Urusan Logistiv, Indonesia explained that BULOG purchases milled rice instead of paddy for their buffer stocks for greater quality control. However milled rice is more susceptible to infestation by a far greater range of insect pests than paddy. Their pest control measures include regular inspection, bag stock fumigation using phosphine and carbon-dioxide, application of contact insecticides to the surface of the stacks and storage fabric, and attention to hygiene. One of their concerns is the frequent use of chemical pesticides. Their most challenging problem is to reduce losses with out the harmful effects of any possible pesticide contamination. This problem has sharply increased with their larger volume of stocks and lengthened storage period. We imagine that NFA of the Philippines shares this same concern.

Our entomologist friends from CSIRO, the Department of Primary Industries in Brisbane, their collaborators at MARDI in Malaysia, and NAPHIRE in the Philippines plus the support of ACIAR have been very active in pest control. Not only have they identified the insects, but also they know whether they are resistant to the insecticides in their arsenal. They even have mixtures of different insecticides to be sure they achieve a 100% kill. As they seem to be way ahead of immediate needs, we hope someday that they can give us some practical procedures for the level of storage technology currently being used.

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