are the hard protecting coverings of grains of rice
Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...
. In addition to protecting rice during the growing season, rice hulls can be put to use as building material
Building material is any material which is used for a construction purpose. Many naturally occurring substances, such as clay, sand, wood and rocks, even twigs and leaves have been used to construct buildings. Apart from naturally occurring materials, many man-made products are in use, some more...
Fertilizer is any organic or inorganic material of natural or synthetic origin that is added to a soil to supply one or more plant nutrients essential to the growth of plants. A recent assessment found that about 40 to 60% of crop yields are attributable to commercial fertilizer use...
, insulation material, or fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
Rice hulls are the coating for the seeds, or grains, of the rice plant. To protect the seed during the growing season, the hull is made of hard materials, including opaline silica and lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
. The hull is mostly indigestible to humans.
One practice, started in the seventeenth century, to separate the rice from hulls, it to put the whole rice into a pan and throw it into the air while the wind blows. The hulls are blown away while the rice fell back into the pan. This happens because the hull isn't nearly as dense as the rice. These steps are known as winnowing. During the milling
A miller usually refers to a person who operates a mill, a machine to grind a cereal crop to make flour. Milling is among the oldest of human occupations. "Miller", "Milne" and other variants are common surnames, as are their equivalents in other languages around the world...
process, the hulls are removed from the grain to create brown rice
Brown rice is unmilled or partly milled rice, a kind of whole, natural grain. It has a mild nutty flavor, is chewier and more nutritious than white rice, but goes rancid more quickly because the germ—which is removed to make white rice—contains fats that can spoil...
, which is then milled further to remove the bran layer, creating white rice.
A number of rice-producing countries, (e.g. Thailand), are currently conducting research on industrial uses of rice hulls. Some of the current and potential applications are listed below.
Rice hulls can be used to produce mesoporous molecular sieve
A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids.Molecules small enough to pass through the pores are adsorbed while larger molecules are not. It is different from a common filter in that it operates on a...
s (e.g., MCM
Mobil Composition of Matter is the initial name given for a series of mesoporous materials that were first synthesized by Mobil's researchers in 1992. MCM-41 and MCM-48 Mobil Composition of Matter (MCM) is the initial name given for a series of mesoporous materials that were first synthesized by...
), which are applied as catalysts for various chemical reactions, as a support for drug delivery system and as adsorbent in waste water treatment.
Pet food fiber
Rice hulls are the outermost covering of the rice and come as organic rice hulls and natural rice hulls. Rice hulls are an inexpensive byproduct of human food processing, serving as a source of fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....
that is considered a filler
In processed animal foods, a filler is an ingredient added to provide dietary fiber, bulk or some other non-nutritive purpose.Products like corn and corncobs, feathers, soy, cottonseed hulls, peanut hulls, citrus pulp, screening, weeds, straw, and cereal by-products are often included as...
An ingredient is a substance that forms part of a mixture . For example, in cooking, recipes specify which ingredients are used to prepare a specific dish. Many commercial products contain a secret ingredient that is purported to make them better than competing products...
in cheap pet food
Pet food is plant or animal material intended for consumption by pets. Typically sold in pet stores and supermarkets, it is usually specific to the type of animal, such as dog food or cat food...
Rice hulls are a class A insulating material because they are difficult to burn and less likely to allow moisture to propagate mold
Molds are fungi that grow in the form of multicellular filaments called hyphae. Molds are not considered to be microbes but microscopic fungi that grow as single cells called yeasts...
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
. It has been found out that when burned, rice hull produces significant amounts of silica. For these reasons it provides excellent thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
Rice hulls are used as pillow
A pillow is a large cushion support for the head, usually used while sleeping in a bed, or for the body as used on a couch or chair. There are also throw pillows , which are pillows that are purely decorative and not designed for support or comfort...
stuffing. The pillows are loosely stuffed and considered therapeutic as they retain the shape of the head.
Rice hulls are organic material and can be composted. However, their high lignin content can make this a slow process. Sometimes earthworm
Earthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female...
s are used to accelerate the process. Using vermicompost
Vermicompost is the product or process of composting utilizing various species of worms, usually red wigglers, white worms, and earthworms to create a heterogeneous mixture of decomposing vegetable or food waste, bedding materials, and vermicast...
ing techniques, the hulls can be converted to fertilizer in about four months.
Rice hulls are a low-cost material from which silicon carbide "whiskers
Metal whiskering is a crystalline metallurgical phenomenon involving the spontaneous growth of tiny, filiform hairs from a metallic surface. The effect is primarily seen on elemental metals but also occurs with alloys....
" can be manufactured. The SiC whiskers are then used to reinforce ceramic cutting tools, increasing their strength tenfold.
With proper techniques, rice hulls can be burned and used to power steam engines. Some rice mills originally disposed of hulls in this way.
However the direct combustion of rice hulls tends to produce a lot of smoke. A far better alternative is to gasify rice hulls. Rice hulls are easily gasified in top-lit updraft gasifiers. The combustion of this rice hull gas produces a beautiful blue flame, and rice hull biochar makes a wonderful soil amendment.
Rice hulls can be used in brewing beer to increase the lautering
Lautering is a process in brewing beer in which the mash is separated into the clear liquid wort and the residual grain. Lautering usually consists of 3 steps: mashout, recirculation, and sparging.-Mashout:...
ability of a mash.
Rice hulls are used as a "press aid" to improve extraction efficiency of apple pressing.
Rice husk ash
The ash produced after the husks have been burned, (abbreviated to RHA), is high in silica. A number of possible uses are being investigated for this. These uses include
- aggregates and fillers for concrete and board production
- economical substitute for microsilica / silica fumes
- absorbents for oils and chemicals
- soil ameliorants
- as a source of silicon
- as insulation powder in steel mills
- as repellents in the form of "vinegar-tar"
- as a release agent in the ceramics industry
- as an insulation material for homes and refrigerants
- in Kerala, India- Rice husks (Umikari- in malayalam)was universally used for over centuries in cleaning teeth - before toothpaste replaced it.