When the rice grain is harvested, it is unusable as human food until the inedible hull is removed. Since the grain is usually consumed as white rice, the bran layer must also be removed (through a process called polishing or milling).
Thus, the normal sequence in the handling of a rice crop after it matures is harvesting, cleaning, drying storage, milling and distribution to the market (or retention for farm family consumption).
Parboiling, if done, occurs sometime before milling.
The chief consideration in harvesting is the degree of maturity of the grain, which is determined by measuring moisture content. The optimum moisture content of the rice grain at harvest time is 21 to 24 percent.Under tropical conditions this point is generally reached 28 to 32 days after flowering. If the crop is allowed to stand in the field after it reaches maturity, large losses occur in the both the field yield of the crop harvested and the percentage recovery of head rice after milling.
Early harvesting produces a higher quality milled rice. When grain is allowed to remain in the field after it is mature “sun checking” (cracking of the grain) occurs and many of the grains break during the grain is wetter and requires more drying before it can be stored. In addition, threshing early harvested paddy is more difficult.
Mechanical threshers remove the drudgery from the process and save time. Although they require a capital investment, the cost of operation is low. Small portable threshers powered by horsepower engines are available and are light enough to be carried readily from field to field.
Postharvest Technology of Rice: Harvesting and Threshing
