http://www.fao.org/inpho/vlibrary/t0567e/T0567E0g.htm

Antinutrition factors in the rice grain are concentrated in the bran
fraction (embryo and aleurone layer). They include phytin (phytate), trypsin
inhibitor, oryzacystatin and haemagglutinin-lectin. All except oryzacystatin
have been previously reviewed (Juliano, 1985b).

All the antinutrition factors are proteins and all except phytin (phytate)
are subject to heat denaturation. Phytin is located in 1 - to 3-µm globoids
in the aleurone and embryo protein bodies as the potassium magnesium salt.
Its phosphate groups can readily complex with cations such as calcium, zinc
and iron and with protein. It is heat stable and is responsible for the
observed poorer mineral balance of subjects fed brown rice diets in
comparison to that of subjects fed milled rice diets (Miyoshi et al., 1987a,
1987b).

Trypsin inhibitor has also been isolated from rice bran and characterized
(Juliano, 1985b). The partially purified inhibitor is stable at acidic and
neutral pH and retained more than 50 percent of its activity after 30
minutes of incubation at 90°C at pH 2 and 7. Steaming rice bran for 6
minutes at 100°C inactivates the trypsin inhibitor, but dry heating at 100°C
for up to 30 minutes is not as effective. The inhibitor distribution is 85
to 95 percent in the embryo, 5 to 10 percent in germ-free bran and none in
milled rice.

Haemagglutinins (lectins) are globulins that agglutinate mammalian red blood
cells and precipitate glycoconjugates or polysaccharides. The toxicity of
lectins stems from their ability to bind specific carbohydrate receptor
sites on the intestinal mucosal cells and to interfere with the absorption
of nutrients across the intestinal wall. Rice-bran lectin binds specifically
to 2-acetamido-2-deoxy-Dglucose (Poole, 1989). It is stable for 2 hours at
75°C but sharply loses activity after 30 minutes at 80°C or 2 minutes at
100°C (Ory, Bog-Hansen and Mod, 1981). Rice lectin agglutinates human A, B
and O group erythrocytes. It is located in the embryo but has receptors in
both rice embryo and endosperm (Miao and Tang, 1986).

Oryzacystatin is a proteinaceous (globulin) cysteine proteinase inhibitor
(cystatin) from rice seed and is probably the first well-defined cystatin
superfamily member of plant origin (Kondo, Abe and Arai, 1989). Incubation
at pH 7 for 30 minutes at 100°C had no effect on its activity but inhibition
decreased IS percent at 110°C and 45 percent at 120°C. Oryzacystatin
effectively inhibited cysteine proteinases such as papain, ficin,
chymopapain and cathepsin C and had no effect on serine proteinases
(trypsin, chymotrypsin and subtilisin) or carboxyl proteinase (pepsin).

An allergenic protein in rice grain, causing rice-associated atopic
dermatitis in Japan, is an a-globulin and shows stable immunoreactivity (60
percent) even on heating for 60 minutes at 100°C (Matsuda et al., 1988). It
is present mainly in milled rice rather than in the bran. Hypoallergenic
rice grains may be prepared by incubating milled rice in actinase to
hydrolyse globulins in the presence of a surfactant at an alkaline pH
(Watanabe et al., 1990a) and washing. The color of the processed grain is
improved by treatment with 0.5-N hydrochloric acid and washing with water
(Watanabe et al., 1990b).