Every rice grain (or seed) consists of the rice enclosed in a hull. The rice itself mainly comprises the embryo (or germ) and the starchy endosperm, but it also has a surface formed of several thin layers of differentiated tissues, collectively known as the bran layer, which forms 5% to 8% of grain weight. The initial milling process of dehusking, to remove the hull (about 20% of grain weight), results in rice with its bran layer intact. This is ‘brown’ or ‘unpolished’ rice, which can be directly prepared for consumption, or further milled or polished to remove the bran layer to create ‘white’ or ‘polished’ rice. All varieties of rice can be processed post-harvest to obtain either white/polished or brown/unpolished rice.
Brown rice is considered healthier than white rice as many vitamins and minerals as well as fiber are contained in the bran layer, which is removed when brown rice is polished to reveal the white grain. Polishing removes roughly 80% of the thiamine from brown rice. Brown rice of 17,587 cultivars in the IRRI germplasm collection averages 9.5% protein content, ranging from 4.3% to 18.2%.
Brown rice has a different taste and texture, being generally more nutty and chewy than white rice. But brown rice is not as popular because (1) it requires more fuel for cooking (it takes longer to cook), (2) it may cause digestive disturbances, and (3) oil in the bran layer tends to turn rancid during storage even at moderate temperatures. The taste of white rice is also preferred in many rice-consuming countries.
Black, purple and red rice
Many varieties of black, purple and red rice are grown in countries around the world, especially China and countries in South Asia and Southeast Asia. Black and purple rices are mainly glutinous rices, while red rices mainly belong to the indica group. As these types of rices have their bran layer intact, they are generally more nutritious, in the same way that brown rice is more nutritious than white rice. In particular, red rices are known to be rich in iron and zinc, while black and purple rices are especially high in protein, fat and crude fiber.
The color of black, purple and red rice is derived from anthocyanin pigments in the bran layer. Anthocyanins are members of the flavonoid group of phytochemicals (or bioflavonoid phytochemicals). Anthoncyanin pigments are known to have free-radical-scavenging and antioxidant capacities, but findings of a wide range of studies suggest that they also have many other health benefits, usually operating in interaction with other phytochemicals.
Another colored rice that is in development, but not yet available to farmers or consumers, is Golden Rice. Golden Rice is a golden or yellow color because it contains beta carotene – a source of vitamin A – to improve its nutritional content. Unlike other colored rice, Golden Rice is colorful all the way through so still appears golden even after it is polished.