Distribution of γ-oryzanol in the outer layers of brown rice and its variation among cultivars

Abstract

Rice (Oryza sativa L.) bran accumulates many compounds that are associated with the promotion and maintenance of human health, such as γ-oryzanol (gOZ) and γ-aminobutyric acid (GABA). These beneficial compounds in the bran fraction are mostly removed in polished rice. The present study sought rice cultivars with high concentrations of gOZ, and also of GABA, in polished rice, and examined the distribution of gOZ content in the outer layers of brown rice. The effects of germination treatment were also examined as a means to enrich gOZ and GABA in polished rice. GABA concentration showed no significant variation among 19 cultivars in polished rice, whereas a wide variation was detected in brown rice, for which cultivar Kinuhikari had the highest amount. Germination treatment for 5 h significantly enhanced GABA concentration. gOZ concentrations also showed a wide variation among cultivars in both brown rice and polished rice, with no clear correlation. Hattan, a sake-brewing cultivar, exhibited the highest gOZ concentration in polished rice. This cultivar involved higher gOZ in the outermost layer of polished rice, whereas it had a lower content in the bran fraction. This permeation of gOZ into polished rice might be related to the endosperm properties characteristic to sake-brewing cultivars. In contrast to GABA, gOZ concentration did not increase with germination treatment in brown rice or polished rice. Because the gOZ concentration in polished rice was insufficient even in the Hattan cultivar, breeding and/or other processing techniques are needed to obtain gOZ-enriched polished rice.