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 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. 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 process, the hulls are removed from the grain to create brown rice, the brown rice is then milled further to remove the bran layer to become 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 sieves (e.g., MCM), 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 that is considered a filler ingredient in cheap pet foods.
Rice hulls are a class A insulating material because they are difficult to burn and less likely to allow moisture to propagate mold or fungi. It has been found out that when burned, rice hull produces significant amounts of silica. For these reasons it provides excellent thermal insulation.
Rice hulls are used as pillow 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 earthworms are used to accelerate the process. Using vermicomposting techniques, the hulls can be converted to fertilizer in about four months.
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.
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 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.