Rice hulls or 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.
Production
Rice
husk 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
Biogenic silica and
lignin. The hull is hard to eat or swallow (unless finely ground) and mostly indigestible to humans because of its
Vegetable fiber components. However, during
famine in ancient China, a common daily meal was a pastry made from rice husks,
wild vegetables, and
Soybean meal. 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, consists of putting the whole rice into a pan and throwing it into the air while the wind blows. The light hulls are blown away while the heavy rice falls back into the pan. Later, pestles and a simple machine called a rice pounder were developed to remove hulls. In 1885 the modern rice huller was invented in Brazil. During the miller 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.
Uses
Ash
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. Silica is the basic component of sand, which is used with cement for plastering and concreting. This fine silica will provide a very compact concrete. The ash also is a very good thermal insulation material. The fineness of the ash also makes it a very good candidate for sealing fine cracks in civil structures, where it can penetrate deeper than the conventional cement sand mixture.
Rice husk ash has long been used in ceramic glazes in rice-growing regions in the Far East, such as China and Japan.
Possible uses for RHA include absorbents for oils and chemicals, soil ameliorants, a source of silicon, insulation powder in steel mills, repellents in the form of "vinegar-tar" release agent in the ceramics industry, and as an insulation material.
More specialized applications include use as a catalyst support.[J. Chumee et al. "Characterization of platinum–iron catalysts supported on MCM-41 synthesized with rice husk silica and their performance for phenol hydroxylation" Sci. Technol. Adv. Mater. 9 (2008) 015006 free download]
Goodyear announced plans to use rice husk ash as a source for tire additive.
Rice hulls are a low-cost material from which silicon carbide "whiskers" can be manufactured. The SiC whiskers are then used to reinforce ceramic cutting tools, increasing their strength tenfold.
Tooth powder
In southern
India, charred rice hull, known as
umikkari in
Malayalam and
Tamil language, was traditionally used for cleaning teeth before the advent of
toothpaste.
Rice bran oil
Rice bran oil is the oil extracted from the hard outer brown layer of rice called chaff (rice husk). It is popular as a cooking oil in the Indian subcontinent and East Asian countries, including India, Nepal, Bangladesh, Indonesia, Japan, Southern China, and Malaysia.
Brewing
Rice hulls can be used in
brewing beer to increase the
lautering ability of a mash. Rice husk is also used in one step of traditional preparation processes of
kaoliang (sorghum) liquid. After fermentation, rice husk can be added into the wine tank to increase the void, which is advantageous for distillation.
Fertilizer and substrate
Rice hulls can be
Composting, but their high
lignin content can make this a slow process. Sometimes
are used to accelerate the process. Using
techniques, hulls can be converted to fertilizer in about four months.
Rice hulls that are parboiled (PBH) are used as a substrate or medium for gardening, including certain . The hulls decay over time. Rice hulls allow drainage,
Fireworks
Rice hulls are coated with fine-grained
gunpowder and used as the main bursting charge in aerial fireworks shells.
Fuel
With proper techniques, rice hulls can be burned and used to power
. Some rice mills originally disposed of hulls in this way.
The direct combustion of rice hulls produces large quantities of smoke. An alternative is gasification. Rice hulls are easily gasified in top-lit updraft gasifiers. The combustion of this rice hull gas produces a blue flame, and rice hull biochar makes a good soil amendment.
Traditional brickmaking kilns in Mekong Delta are using rice hulls as fuel.
Juice extraction
Rice hulls are used as a "press aid" to improve extraction efficiency of
apple Fruit press.
Pet food fiber
Rice hulls are an inexpensive
byproduct of human food processing, serving as a source of
fiber that is considered a filler
ingredient in
.
Pillow stuffing
Rice hulls are used as
pillow stuffing. The pillows are loosely stuffed and considered therapeutic as they retain the shape of the head.
Insulating material
Rice hulls themselves are a class A thermal insulating material because they are difficult to burn and less likely to allow moisture to propagate mold or
fungus.
It is also used as roofing after mixing it with mud and water.
Particle boards and cardboard
Rice hulls are used to make
and
cardboard. The silica in rice husk make the particle boards less attractive to
.
Geopolymers
Due to high amorphous silica content, the RHA (rice husk ash) can be used as a precursor material for
geopolymer concrete.
Rice concrete
To achieve the best
pozzolanic properties the combustion of the husks has to be carefully controlled by keeping the temperature below and to create conditions to minimize carbon formation by feeding sufficient air.
At a given water-cement ratio, the addition of small amounts of rice hull ash (2 to 3% of cement mass) may be useful to improve the workability of concrete mixtures by reducing the cement milk separation and segregation and increasing the strength and durability of concrete.
See also
Bibliography
External links
