Rice hulls or husks are the hard protecting coverings of grains of

rice Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...

. In addition to protecting rice during the growing season, rice hulls can be put to use as

building material Building material is material used for construction. Many naturally occurring substances, such as clay, rocks, sand, wood, and even twigs and leaves, have been used to construct buildings and other structures, like bridges. Apart from natur ...

fertilizer A fertilizer or fertiliser is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from liming materials or other non-nutrient soil amendments. Man ...

, insulation material, or

fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work (physics), work. The concept was originally applied solely to those materials capable of releasing chem ...

. Rice hulls are part of the

chaff Chaff (; ) is dry, scale-like plant material such as the protective seed casings of cereal grains, the scale-like parts of flowers, or finely chopped straw. Chaff cannot be digested by humans, but it may be fed to livestock, ploughed into soil ...

of the rice.

Production

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 Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidit ...

. 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 Food security is the state of having reliable access to a sufficient quantity of affordable, healthy food. The availability of food for people of any class, gender, ethnicity, or religion is another element of food protection. Similarly, househo ...

. 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 Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...

Winnowing Winnowing is a process by which chaff is separated from grain. It can also be used to remove pests from stored grain. Winnowing usually follows threshing in grain preparation. In its simplest form, it involves throwing the mixture into the ...

, 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 A rice pounder is an agricultural tool, a simple machine that is commonly used in Southeast Asia to dehull rice or to turn rice into rice flour. The device has similar functionality to a mortar and pestle, but with more mechanical advantage to ...

were developed to remove hulls. In 1885 the modern rice hulling machine was invented in Brazil. During the

milling Milling may refer to: * Milling (minting), forming narrow ridges around the edge of a coin * Milling (grinding), breaking solid materials into smaller pieces by grinding, crushing, or cutting in a mill * Milling (machining), a process of using ro ...

processes, the hulls are removed from the raw grain to reveal whole

brown rice Brown rice is a whole grain rice with only the inedible outer hull removed. This kind of rice sheds its outer hull or husk but the bran and germ layer remain on, constituting the brown or tan colour of rice. White rice is the same grain without ...

, which is then usually milled further to remove the

bran Bran, also known as miller's bran, is the component of a Cereal, cereal grain consisting of the hard layersthe combined aleurone and Fruit anatomy#Pericarp layers, pericarpsurrounding the endosperm. Maize, Corn (maize) bran also includes the p ...

layer, resulting in

white rice White rice is milled rice that has had the husk, bran, and germ removed. This alters the flavor, texture and appearance of the seed; helps prevent spoilage (extends its storage life); and makes it easier to digest. After brown rice is milled ...

Uses

Ash

Combustion of rice hulls affords rice husk ash (acronym RHA). This ash is a potential source of amorphous reactive

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

, which has a variety of applications in

materials science Materials science is an interdisciplinary field of researching and discovering materials. Materials engineering is an engineering field of finding uses for materials in other fields and industries. The intellectual origins of materials sci ...

. Most of the ash is used in the production of

Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar (masonry), mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in th ...

. 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. Being about 95% silica, it is an easy way of introducing the necessary silica into the glaze, and the small particle size helps with early melting of the glaze. 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 In chemistry, a catalyst support or carrier is a material, usually a solid with a high surface area, to which a catalyst is affixed. The activity of heterogeneous catalysts is mainly promoted by atoms present at the accessible surface of the ma ...

. 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 Whiskers, also known as vibrissae (; vibrissa; ) are a type of stiff, functional hair used by most therian mammals to sense their environment. These hairs are finely specialised for this purpose, whereas other types of hair are coarser as ta ...

" can be manufactured. The SiC whiskers are then used to reinforce ceramic cutting tools, increasing their strength tenfold.

Tooth powder

In southern

India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...

, charred rice hull, known as '' umikkari'' in

Malayalam Malayalam (; , ) is a Dravidian languages, Dravidian language spoken in the Indian state of Kerala and the union territories of Lakshadweep and Puducherry (union territory), Puducherry (Mahé district) by the Malayali people. It is one of ...

and

Tamil Tamil may refer to: People, culture and language * Tamils, an ethno-linguistic group native to India, Sri Lanka, and some other parts of Asia **Sri Lankan Tamils, Tamil people native to Sri Lanka ** Myanmar or Burmese Tamils, Tamil people of Ind ...

, was traditionally used for cleaning teeth before the advent of

toothpaste Toothpaste is a paste or gel dentifrice that is used with a toothbrush to clean and maintain the aesthetics of Human tooth, teeth. Toothpaste is used to promote oral hygiene: it is an abrasive that aids in removing dental plaque and food from th ...

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 Brewing is the production of beer by steeping a starch source (commonly cereal grains, the most popular of which is barley) in water and #Fermenting, fermenting the resulting sweet liquid with Yeast#Beer, yeast. It may be done in a brewery ...

beer Beer is an alcoholic beverage produced by the brewing and fermentation of starches from cereal grain—most commonly malted barley, although wheat, maize (corn), rice, and oats are also used. The grain is mashed to convert starch in the ...

to increase the

lautering Lautering () is the beer brewing process that separates the mash into clear liquid wort and residual grain. Lautering usually consists of three steps: mashout, recirculation, and sparging. Mashout Mashout is the term for raising the temperature ...

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 composted, but their high

content can make this a slow process. Sometimes

earthworm An earthworm is a soil-dwelling terrestrial invertebrate that belongs to the phylum Annelida. The term is the common name for the largest members of the class (or subclass, depending on the author) Oligochaeta. In classical systems, they we ...

s are used to accelerate the process. Using

vermicompost Vermicompost (vermi-compost) is the product of the decomposition process using various species of worms, usually red wigglers, white worms, and other earthworms, to create a mixture of decomposing vegetable or food waste, bedding materials, and ...

ing 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

hydroculture Hydroponics is a type of horticulture and a subset of hydroculture which involves growing plants, usually crops or medicinal plants, without soil, by using water-based mineral nutrient solutions in an artificial environment. Terrestrial or ...

s. The hulls decay over time. Rice hulls allow drainage, and retain less water than growstones. It has been shown that rice hulls do not affect plant growth regulation.

Fireworks

Rice hulls are coated with fine-grained

gunpowder Gunpowder, also commonly known as black powder to distinguish it from modern smokeless powder, is the earliest known chemical explosive. It consists of a mixture of sulfur, charcoal (which is mostly carbon), and potassium nitrate, potassium ni ...

and used as the main bursting charge in aerial fireworks shells.

Fuel

With proper techniques, rice hulls can be burned and used to power

steam engine A steam engine is a heat engine that performs Work (physics), mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a Cylinder (locomotive), cyl ...

s. 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 Gasification is a process that converts biomass- or fossil fuel-based carbonaceous materials into gases, including as the largest fractions: nitrogen (N2), carbon monoxide (CO), hydrogen (H2), and carbon dioxide (). This is achieved by reacting ...

. 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 Biochar is a form of charcoal, sometimes modified, that is intended for organic use, as in soil. It is the lightweight black remnants remaining after the pyrolysis of biomass, consisting of carbon and ashes. Despite its name, biochar is steril ...

makes a good soil amendment. Traditional brickmaking

kilns A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes. Kilns have been used for millennia to turn objects made from clay into ...

Mekong Delta The Mekong Delta ( or simply ), also known as the Western Region () or South-western region (), is the list of regions of Vietnam, region in southwestern Vietnam where the Mekong, Mekong River River delta, approaches and empties into the sea th ...

are using rice hulls as fuel.

Juice extraction

Rice hulls are used as a "press aid" to improve extraction efficiency of

apple An apple is a round, edible fruit produced by an apple tree (''Malus'' spp.). Fruit trees of the orchard or domestic apple (''Malus domestica''), the most widely grown in the genus, are agriculture, cultivated worldwide. The tree originated ...

pressing.

Pet food fiber

Rice hulls are an inexpensive

byproduct A by-product or byproduct is a secondary product derived from a production process, manufacturing process or chemical reaction; it is not the primary product or service being produced. A by-product can be useful and marketable or it can be cons ...

of human food processing, serving as a source of

fiber Fiber (spelled fibre in British English; from ) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often inco ...

that is considered a filler

ingredient In a general sense, an ingredient is a substance which forms part of a mixture. In cooking, recipes specify which ingredients are used to prepare a dish, and the term may also refer to a specific food item in relation to its use in different re ...

pet food Pet food is animal feed 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. Most meat used for animals is a byproduct of the human food indus ...

Pillow stuffing

Rice hulls are used as

pillow A pillow is a support of the body at rest for comfort, therapy, or decoration. Pillows are used in different variations by many species, including humans. Some types of pillows include throw pillows, body pillows, decorative pillows, and man ...

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 A mold () or mould () is one of the structures that certain fungus, fungi can form. The dust-like, colored appearance of molds is due to the formation of Spore#Fungi, spores containing Secondary metabolite#Fungal secondary metabolites, fungal ...

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

. It is also used as roofing after mixing it with mud and water.

Particle boards and cardboard

Rice hulls are used to make

particle board Particle board, also known as particleboard or chipboard, is an engineered wood product, belonging to the wood-based panels, manufactured from wood chips and a synthetic, mostly formaldehyde-based resin or other suitable binder, which is presse ...

s and

cardboard Cardboard is a generic term for heavy paper-based products. Their construction can range from a thick paper known as paperboard to corrugated fiberboard, made of multiple plies of material. Natural cardboards can range from grey to light brown ...

. The silica in rice husk make the particle boards less attractive to

termite Termites are a group of detritivore, detritophagous Eusociality, eusocial cockroaches which consume a variety of Detritus, decaying plant material, generally in the form of wood, Plant litter, leaf litter, and Humus, soil humus. They are dist ...

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. However, the introduction of large quantities of this additive may result in poor workability of the concrete mixture if strong water-reducing additives are not used.

References

Bibliography

* * *

External links

The Rice Hull House
where rice hulls are used for insulation
Uses for rice husk ash, or RHA
{{Authority control Rice products Materials Building materials Cement Concrete Pet foods