Quality problems in milling
There are numerous factors that determine the final quality of rice produced by the rice mill. These factors relate to what extent the rice is milled (i.e. low milling degree), the level of grain breakage, level of impurities, color, and presence of off-type kernels such as chalky or damaged kernels. Some of these problems can be solved at the rice mill, whereas others are related to the way rice is grown and handled after harvest.
Under-milled rice is under-polished rice, or rice with bran streaks left in it. The bran contains much of the desired nutritious elements. Under milling will also yield higher milling recoveries. However, under-milled rice does not store well because of the high oil content of the bran. In addition, rice consumers almost universally desire well-milled rice because of its better appearance. Related to milling degree is the occurrence of red-streaked kernels; rice with part of the bran layer still sticking to the surface.
Grain breakage is a result of fissuring, or the formation of cracks in the endosperm prior to milling. To a certain extent, fissuring in rice grain occurs naturally in the field due daily changes in temperature and relative humidity. More importantly, fissuring can be caused by improper management of grain at all post-harvest operations from harvesting through to milling. Improper drying techniques often lead to fissuring in grain, or rewetting of stored paddy, and \improper milling techniques. Finally, not all fissured grain will break during milling, and medium or long grain varieties are more prone to breakage than short grain varieties.
Although whiteness is a varietal characteristic, the natural color of white rice can be affected by post-harvest discoloration. A general discoloration of the entire rice kernel occurs if wet paddy is left undried for extended periods (i.e. days). The wet grain will heat up, causing the grains to turn yellow or tan.
If part of the milled rice kernel is opaque rather than translucent, it is characterized as chalky. Chalkiness disappears upon cooking and has no effect on taste or aroma, however it downgrades milled rice. Chalky grains are more brittle than non chalky grains and can break more easily during milling. Cause of chalkiness is interruption during the final stages of grain filling.
Damaged rice kernels are those which are fully or partially darkened as a result of insect , mold, water, or heat damage. The presence of even a few damaged grain kernels can severely downgrade rice.
Impurities in milled rice are a sign of improperly cleaned paddy (picture) prior to milling, or contamination of rice during milling.