Common cultivated rice can be divided into two subspecies: indica rice and japonica rice. Indica rice has a slender grain shape, and its length is more than three times its width. Flat, short and sparse, generally without awning, even if there is awning. Rice husk is thin, with white abdomen and few hard grains. Rice is easy to be broken during processing, and the rice yield is low. Rice is swollen and weak in viscosity. Japonica rice has short grain shape, length 1.4-2.9 times wide, long and dense villi, long awn, thick rice husk, white or not white belly, and many hard grains, which are not fragile during processing, high rice yield, small rice swelling and strong viscosity.
Indica rice and japonica rice can be divided into glutinous rice and glutinous rice according to the different starch properties of rice. Glutinous rice starch contains amylose 10-30%, and the rest is amylopectin, which has low viscosity and large swelling. The viscosity of japonica rice is greater than that of indica rice. Glutinous rice starch is almost all amylopectin, with small swelling and high viscosity, among which japonica rice has the largest viscosity. In addition, the colors of glutinous rice and glutinous rice are different. The endosperm of glutinous rice is translucent (except the belly white), the rice is hard and brittle, the waxy endosperm of glutinous rice is opaque (but translucent), and the rice is loose.
Indica or japonica rice can be divided into early rice, middle rice and late rice according to different growth periods. The growth period of early rice is 90- 125 days, middle rice is 125- 150 days and late rice is 150- 180 days. Due to the different growth period and climatic conditions, the quality of the same type of rice also shows some differences: early rice is generally white, with less hard grains, loose rice quality and poor quality, while late rice has better quality.
In addition, according to the different soil moisture in planting areas, it can be divided into rice and upland rice (early rice). Rice is planted in paddy fields, which requires a lot of water, has high yield and good quality. Upland rice planted in dry land has strong drought tolerance, early maturity, low yield, thick husk and husk layer, loose grain structure, low hardness, low rice yield and poor color and taste.
In the national standard of grain and oil quality in China, rice is divided into three categories according to its grain shape and grain quality:
The first category: indica rice, that is, non-waxy rice of indica rice. According to grain quality and harvest season, it can be divided into early indica rice and late indica rice.
The second category: japonica rice, that is, japonica non-waxy rice. According to grain quality and harvest season, it can be divided into early japonica rice valley and late japonica rice valley.
The third category: waxy rice, which is divided into two categories according to its grain shape and grain quality: indica glutinous rice and japonica glutinous rice.
Brown rice is obtained by husking rice with a husker.
Brown rice is caryopsis with smooth and shiny surface. In brown rice, we call the embryo-containing side white and the embryo-free side back. There are five longitudinal grooves on the surface of brown rice grains, one on the back is called back groove, and two on both sides are called rice grooves. It is difficult to completely remove the cortex at the brown rice ditch during rice milling. For many kinds of rice, the more skin left in furrow, the lower the processing accuracy, so the processing accuracy of rice is often expressed by the degree of skin left in grain surface and back furrow. Some brown rice has opaque white spots in the abdomen or the center of rice grains, which are white in the abdomen or white in the heart. White belly and white heart are caused by the inadaptability of climate, rainfall and fertilizer during rice growth.
Structure of brown rice: Brown rice consists of pericarp, seed coat, exoendosperm, endosperm and embryo. Pericarp includes exocarp, mesocarp, row cells and tubular cells, with a total thickness of about 65438 00 microns. The seed coat is very thin, about 2 microns thick, and the structure is not obvious. Some brown rice seed coats contain pigments and are colored. Exoendosperm is a membranous tissue attached to the seed coat, with a thickness of 1 and 2 microns, which is difficult to distinguish from the seed coat. Endosperm is the largest part of rice grain, including aleurone layer and starch cells. Aleurone layer cells are full of tiny aleurone particles, which contain protein, fat, vitamins and so on, but do not contain starch. Starch cells are filled with starch granules. Embryo is located at the base of the ventral surface of rice, which is oval and consists of embryo, embryonic stem, radicle and peltate. The peltate body is connected with the endosperm. When the seed germinates, it secretes enzymes, decomposes substances in the endosperm and provides nutrition for the embryo.
Brown rice is processed and ground to remove the cortex and germ, leaving the endosperm, which is edible rice.