Rice hulls

Rice hulls (or rice husks) are the hard protecting coverings of grains of rice. In addition to protecting rice during the growing season, rice hulls can be put to use as building material, fertilizer, insulation material, or fuel. Rice hulls are part of the chaff of the rice. Rice hulls are part of the rice seed. The hull protects the grain during the growing season from pests. The hull is formed from hard materials, including opaline silica and lignin. The hull is hard to eat or swallow (unless finely ground) and mostly indigestible to humans because of its enriched fibre components. However, during times of food scarcity in ancient China, a common daily meal was a pastry made from rice husks, wild vegetables, and soybean powder. This led to the idiom "meals of cereal, hulls, and vegetables for half a year," indicating poverty and food insecurity. Testing and commercialization of human grade anti-caking agents were done in the early 2000s. The material is approved for use in USDA Certified Organic products to replace silicon dioxide. Winnowing, used to separate the rice from hulls, is to put the whole rice into a pan and throw it into the air while the wind blows. The light hulls are blown away while the heavy rice fall back into the pan. Later pestles and a simple machine called a rice pounder were developed to remove hulls. In 1885 the modern rice hulling machine was invented in Brazil. During the milling processes, the hulls are removed from the raw grain to reveal whole brown rice, which is then usually milled further to remove the bran layer, resulting in white rice. Combustion of rice hulls affords rice husk ash (acronym RHA). This ash is a potential source of amorphous reactive silica, which has a variety of applications in materials science. Most of the ash is used in the production of Portland cement. When burnt completely, the ash can have a Blaine number of as much as 3,600 compared to the Blaine number of cement (between 2,800 and 3,000), meaning it is finer than cement.