Harvesting and post harvest technology of Rice

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(a) Harvesting: Harvesting is to be done at optimum time in the tropics, otherwise, there is loss of grain shedding, scattering, lodging and also damage by birds, over maturity and lodging. Timely harvesting ensures good grain quality, a high market value and improved consumer preference/acceptance.

In India, harvesting between 27 and 39 days after flowering gave maximum head rice recovery. The moisture content at the time of harvest is 18–20%. Taking the average duration of crops as an indication, drain the water from the field 7–10 days before the expected harvest as the drainage hastens the maturity and improves harvesting conditions.

When 80% of the panicles turn straw colour (or) most of the grains at base of the panicle in the selected tillers are in hard dough stage, the crop is ready for harvest. Maturity may be hastened by 3–4 days by spraying 20% NaCl a week before harvest to escape monsoon rains.

(b) Method of harvest: Rice straw is usually cut with a sickle at 15–25 cm above the ground. In Indonesia and Philippines, only panicles are removed. Now, self propelled harvesters, reapers etc. are used for harvesting and combined harvester is available for harvesting, threshing, winnowing and cleaning the seeds.

(c) Post harvest technology: Post harvest technology encompasses an array of handling and processing system from the stage of maturation till consumption of the produce and includes threshing, cleaning, grading, drying, parboiling, curing, milling, preservation, storage, processing, packing, transportation, marketing and consumption system:

1. Threshing: The methods are generally classified as manual, animal or mechanical. The common method of separating grains from panicle is hand beating (hand threshing or using mechanical thresher (small or big thresher). A loss under manual threshing is 8%. IRRI designed a portable thresher. Most of the farmers are using mechanical thresher in the areas where labour availability is a problem.

2. Drying: It is the process that removes moisture from the grain mass for safe storage and preservation of quality, viability and nutritive value. Drying should begin within 12 hours but not later than 24 hours after harvesting. Rice is normally harvested at moisture content of 20% or more. If the moisture content is not reduced to below 14% shortly after threshing, the grain quality is deteriorated because of microbial activities and insect damage. The grains should be dried to 12–14% moisture level. In general, 4–5 days of seed drying are required.

3. Winnowing and cleaning: Presence of impurities like foreign seeds and trashes is more likely to deteriorate in storage and reduce milling recovery rate. Cleaning is mostly done by hand winnower, which takes advantage of wind for removing impurities. Now mechanical winnower is available. Combine harvester is a multipurpose one, which is useful for harvesting, threshing, winnowing and cleaning in one operation. It is highly profitable and economical.

4. Grading: The grains are graded for uniformity in size, shape and colour. Seed cleaner cum graders are also available for effective cleaning and grading.

5. Storage: Low temperature and low moisture are necessary for long-term storage of rice for seed. Rice seed of 10–14% moisture content can be stored in good condition at 18°C for more than 2 years.

(d) Rice processing:

1. Parboiling: In this process, rough rice is soaked, steamed and redried before milling. The advantages of parboiling are: 1. Easy dehusking, 2. low incidence of pests and diseases 3. by milling of raw rice, 80% of fat and18% of crude protein are lost, but starch increases by 5%.

2. Curing: The new rice has low swelling capacity and has the tendency to yield a thick viscous gruel during cooking. To overcome the above defect in newly harvested paddy, methods have been developed to hasten the ageing in fresh rice and such process is called as curing. Steaming for 15–20 minutes is sufficient to bring satisfactory curing effect.

3. Milling: Rice milling involves the removal of husks and bran from rough rice to produce polished rice. Time of harvest and season may affect the milling yield of rice.

4. Polishing: Removal of very fine bran (often called whitening) 2–3 times.

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