Rice Quality Training Manual 07

Rice Quality Training Manual
3.0 Quality characteristics of paddy or rough rice
A number of interrelated features determine the quality of paddy. These are:
• Moisture content of paddy,
• Purity degree,
• Varietal purity,
• Cracked grains,
• Immature grains,
• Discolored/fermented grains and damaged grains.
These quality characteristics are determined by the environmental weather conditions during production, crop production practices, soil conditions, harvesting, and post harvest practices
Moisture content Moisture content has a marked influence on all aspects of paddy and rice quality and it is essential that paddy be milled at the proper moisture content to obtain the highest head rice yield. Paddy is at its optimum milling potential at moisture content of 14%. Grains with high moisture content are too soft to withstand hulling pressure without undue breakage and may be pulverized. Grain that is too dry becomes brittle and has greater breakage.
Moisture content and temperature during the drying process is also critical as it determines whether fissures and/or full cracks are introduced into the grain structure.
Degree of purity. Purity is related to the presence of dockage in the grain. Dockage refers to material other than paddy and includes chaff, stones, weed seeds, soil, rice straw, stalks, etc. These impurities generally come from the field or from the drying floor.
Unclean paddy increases the time taken to clean and process the grain. Foreign matter in the grain reduces milling recoveries and the quality of rice and increases the wear and tear on milling machinery.
Variety Purity. A mixture of varieties causes difficulties at milling and usually result in reduced capacity, excessive breakage, lower milled rice recovery and reduced rice Different sizes and shaped grains makes it more difficult to adjust the hullers and polishers to produce whole grains. This results in low initial de-hulling efficiencies, a higher percentage of re-circulated paddy, non-uniform whitening, and lower grade of milled rice.
Grain dimensions. Grain size and shape (length-width ratio) is a very stable varietal property. Long slender grains normally have greater breakage than short, bold grains and consequently have a lower mill rice recovery. The grain dimensions will also dictate to some degree the type of milling equipment needed. As an example, the Japanese designed milling equipment may be better suited to short-bold grains.
Cracked grains. Overexposure of mature paddy to fluctuating temperature and moisture conditions leads to development of fissures and cracks in individual kernel. Cracks in the kernel are the most important factor contributing to rice breakage during milling. This results in reduces milled rice recovery and head rice yields.
Immature grains. The amount of immature paddy grains in a sample has a major affect on head rice yield and quality. The immature rice kernels are very slender and chalky and this results in excessive production of bran, broken grains and brewer’s rice. The optimal stage to harvest grain is at about 20-24% grain moisture or about 30 days after flowering. If the harvest is too late, many grains are lost through shattering or dry out and are cracked during threshing, which causes grain breakage during milling
Damaged grains. Paddy deteriorates through biochemical change in the grain, the development of off-odors and changes in physical appearance. These types of damage are caused from water, insects, and heat exposure.
Yellowing is caused by over-exposure of paddy to wet environmental conditions before it is dried. This results in a combination of microbiological and chemical activity that overheats the grain similar to a milled form of parboiling. These fermented grains frequently possess partly gelatinized starch cells and generally resist the pressures applied during grain milling. While the presence of fermented grain does not affect milling yields it does downgrade the quality of the milled rice because of the unattractive appearance.
The presence of black spots around the germ end of the brown rice kernel is caused by the microorganisms (fungi) and is increased by unfavorable weather conditions. In the process of milling, these black spots are only partly removed which consequently increases the presence of damaged grains.