Analysis of genotypic diversity in lipid hydrolytic stability of rice bran during storage

Authors: Chen, Ming Hsuan; Yan, Wengui

Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 2/5/2008
Publication Date: 2/5/2008
Citation: Chen, M., Yan, W. 2008. Analysis of genotypic diversity in lipid hydrolytic stability of rice bran during storage. Rice Technical Working Groups, San Diego, CA, Feb. 2008.

Interpretive Summary:

Technical Abstract: Rice bran is a by-product of the rice milling process. It is rich in protein, fat, crude fiber, minerals and vitamins, and is a valuable source of antioxidants. It can be used for ingredients, be developed into a functional food, and can be extracted for rice bran oil production. However, rice bran becomes rancid rapidly after milling due, in part, to the activity of the lipase enzyme, which hydrolyzes triglycerols into free fatty acids. Stabilizing the bran by inactivating the lipase through a heating process can prevent oil deterioration, but might reduce the levels of heat-labile antioxidants. Brown rice has a short shelf life (three to six months) because of the hydrolytic and oxidative deterioration of bran oil. It was suggested that de-hulling paddy or rough rice to produce brown rice disrupts the outer bran layer resulting in the contact of lipase enzymes with the lipid. Selection for rice genotypes that are more stable against hydrolytic rancidity might be an alternative solution. This report presents the genotypic diversity of hydrolytic stability among 148 genotypes, a subset of rice germplasm from the National Small Grain Collection. The rice accessions were grown and harvested in Stuttgart, AR in 2002, and were stored at 4 degrees C and 20% relative humidity in their paddy form. The hydrolytic rancidity of the rice bran was assessed by measuring the quantity of the free fatty acids (ffa) in the bran (expressed as mg of C18:1 equivalent/g of rice bran), i.e. the products of hydrolytic deterioration of bran lipid, after having subjected the bran to a 48h period of storage at elevated temperature. The rice stored in paddy form for 5 years at 4 degrees C were low in lipid deterioration with a mean ffa of 3.1mg/g bran (ranged from 0.98 to 7.42). The deterioration of lipid in the milled bran fraction increased 4.3 fold on average when stored at 35 degrees C for two days; the ffa ranged from 3.7 to 50.1 mg/g bran with a mean value of 13.2. The purple and red bran genotypes had the lowest hydrolytic rancidity among all genotypes of different bran color classes. The mean +/- SD ffa was 7.1 +/- 0.5 for the purple bran, and 6.9 +/- 1.9 mg /g bran for the red bran genotypes. The ffa of other color-bran classes were: brown, 16.5 +/- 8.2, light brown, 17.1 +/- 9.2, and white bran, 15.6 +/- 7.4 mg /g bran. The range of ffa in the light-brown bran class, which is the typical bran color class of US cultivars, was 5.38 to 50.15mg/g bran. The two Arkansas cultivars, Wells and Francis, which were grown in the same field and had gone through the same post-harvest management and storage, had the ffa of 13.8 and 21.2 mg C18:1 mq/g bran, respectively. Comparing the US cultivars with the germplasm evaluated in this study, it is evident that opportunity exists for improving the hydrolytic stability of brown rice and its bran fraction through the use of breeding techniques.