Rice bran antioxidant content: Effect of degree of milling, and kernel milling quality and maturation

Authors: Chen, Ming Hsuan; Bergman, Christine

Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/14/2001
Publication Date: 6/1/2002
Citation: Chen, M.H., Bergman, C.J. 2002. Rice bran antioxidant content: Effect of degree of milling, and kernel milling quality and maturation. In: Proceedings of the 29th Rice Technical Working Group Meetings. P. 113.

Interpretive Summary: Rice bran contains high levels of tocols and the gamma-oryzanol (GA) fraction. Studies indicate these fractions may have positive effects on human health and may have industrial applications. Varietal development efforts focused on enhancing the levels of these antioxidants have the potential to increase the value of rice. The objectives of this research were to examine the effects of sample preparation on the levels of these compounds. The results from this work will lay a foundation for the development of methods for evaluating the genetic variation in antioxidants levels across the world's germplasm. Two medium-grain and two long-grain rice varieties were size-fractionated by kernel thickness. Thicker and thinner kernels were milled separately to different degrees, as determined by the value of surface lipid content (SLC). Tocols and GA were extracted using methanol, separated using reverse-phase HPLC, and their contents quantified by fluorescence and UV absorbance. The starch concentration in the bran samples ranged from 3.2 to 14.6% (w/w), and was negatively correlated to the milling quality of the rice varieties. Milling degree affected the bran antioxidant levels. However, milling the rice kernels to within the narrow SLC range of 0.25 to 0.5% (w/w) gave similar rankings among the cultivars for the bran antioxidant levels. In general, the various antioxidant contents in the mature, thinner kernels were comparable to those of the thicker kernels milled to the same degree. Compared to the mature kernels, the immature (green) kernels had significantly lower antioxidant levels. In conclusion, immature kernels should be removed prior to milling, samples should be well milled and corrections for starch content made when comparing antioxidant levels between rice genotypes that vary in milling quality.

Technical Abstract: Rice bran contains high levels of tocols and the gamma-oryzanol (GA) fraction. Studies indicate these fractions may have positive effects on human health and may have industrial applications. Varietal development efforts focused on enhancing the levels of these antioxidants have the potential to increase the value of rice. The objectives of this research were to examine the effects of sample preparation on the levels of these compounds. The results from this work will lay a foundation for the development of methods for evaluating the genetic variation in antioxidants levels across the world's germplasm. Two medium-grain and two long-grain rice varieties were size-fractionated by kernel thickness. Thicker and thinner kernels were milled separately to different degrees, as determined by the value of surface lipid content (SLC). Tocols and GA were extracted using methanol, separated using reverse-phase HPLC, and their contents quantified by fluorescence and UV absorbance. The starch concentration in the bran samples ranged from 3.2 to 14.6% (w/w), and was negatively correlated to the milling quality of the rice varieties. Milling degree affected the bran antioxidant levels. However, milling the rice kernels to within the narrow SLC range of 0.25 to 0.5% (w/w) gave similar rankings among the cultivars for the bran antioxidant levels. In general, the various antioxidant contents in the mature, thinner kernels were comparable to those of the thicker kernels milled to the same degree. Compared to the mature kernels, the immature (green) kernels had significantly lower antioxidant levels. In conclusion, immature kernels should be removed prior to milling, samples should be well milled and corrections for starch content made when comparing antioxidant levels between rice genotypes that vary in milling quality.