Fernando D. Goffman and Christine J. Bergman
Rice (Oryza sativa L.) bran contains several classes of antioxidants, including tocols, oryzanol and phenolic compounds. Antioxidants contribute to the protective effects against oxidative damage, which has been implicated in a range of diseases, including cancer, cardiovascular disease and aging (Kehrer 1993). Antioxidants are also one of the principal ingredients that protect food quality by preventing oxidative deterioration of lipids (Shahidi et al 1992). In recent years the powerful antioxidant capacity of the phenolics has been attracting much attention, and there is a general belief that the phenolics present in plant foods contribute positively to long-term human health (Parr and Bolwell 2000). In spite of that plant phenolics have been widely recognized as potent antioxidants, less is known about the concentration of phenolics in rice as well as the potential of these compounds as free radical scavengers.
Characterize sixteen rice varieties differing in bran color regarding their total phenolic content and to elucidate if a relationship exists between total phenolics and antiradical activity.
MATERIALS AND METHODS
Plant material in 1999/2000 sixteen
rice cultivars (Table 1
) differing in bran color were grown in Puerto Rico in single plots. Plants were harvested at physiological maturity and grain samples were taken for chemical analyses. The grains were dehulled and all broken or immature kernels were removed. About 50 g of dehulled kernels were milled using a McGill Mill #1 (position 12) for 30 s. The bran fraction was collected and sieved through a 840 µm-sieve. Bran samples were conserved in freezer until analysis.
Total phenolics determination: Total phenolics content was measured using the Folin-Ciocalteu reagent with gallic acid as a standard. About 100 mg of bran were weighted into a test tube and extracted overnight with 7.5 mL methanol at room temperature. The tubes were then centrifuged at 4000 rpm for 10 minutes and the supernatants filtered through 1 µm glass filters. Aliquots of 10, 20 or 100 µL of the extracts were diluted with distilled water to a final volume of 1.2 mL and mixed with 500 µL of a 5-fold diluted Folin-Ciocalteu reagent solution. One mL of a 0.5 M ethanolamine solution was then added. The mixture was read after exactly 30 min at 600 nm. Total phenolics were expressed as mg of gallic acid per g bran.
Antiradical efficiency: The antiradical efficiency was determined against the stable radical DPPH (2,2-diphenyl-1-picrylhydrazyl), by monitoring the reduction of its absorbance (515 nm) after adding an aliquot of the extract. 10 to 20 µL of the methanolic extracts were added to 2.88 mL of the DPPH solution. Absorbance values were registered during 30 minutes and plotted against time, the resulting curves being then integrated using SigmaPlot software. The integration values of DPPH after adding the extracts were compared to a blank solution of DPPH (zero antiradical activity). Antiradical efficiency was expressed as % of reduction of the integration values.
RESULTS AND DISCUSSION
The entries showed extreme diverging values for total phenolic contents as well as antiradical efficiency (Table 1). Total phenolics were in the range from 3.5 to 65.3 mg of gallic acid equivalent per g bran (mg GA eq./g). The white, light brown and speckled brown bran lines showed quite similar low phenolic contents (mean = 4.7 mg GA eq./g), whereas those with darker bran showed a greater range in phenolic concentration. Specifically, the two genotypes with brown bran had 5.3 and 65.3 mg GA eq./g, and two with purple bran had 3.8 and 18.7 mg GA eq./g. The results indicate that rice lines exhibiting colored bran appear to be good candidates for detecting further variation for both phenolics content and antiradical efficiency, and for finding genotypes displaying higher values of these traits. By using methanol as extraction solvent, other antioxidants than phenolics are also being extracted, which includes tocopherols, tocotrienols and oryzanol. Those antioxidants also exhibit antiradical effect against DPPH. We have found that the antiradical efficiency was highly correlated with total phenolics (Figure 1, r= 0.99), even when the lines displaying low phenolic contents were considered alone (Figure 2, r2= 0.67). That suggests that phenolics are the main compounds responsible for the free radical-scavenging activity in methanolic rice bran extracts. This suggestion is also supported by the fact that phenolic compounds exhibit up to 4-times higher antiradical activity against DPPH as compared with alpha-tocopherol (Sánchez-Moreno et al 1998). Further investigations are needed for evaluating the antiradical efficiency of individual classes of rice phenolics.
*mg gallic acid equivalent per g dry weight bran.
†Antiradical efficiency, expressed as % of depleted DPPH area.
Values above 100% were calculated from 4-fold dilutions of the corresponding extracts.
Correlation between total phenolic content and antiradical efficiency.
Correlation between total phenolic content and antiradical
efficiency in low phenolics lines.
Kehrer, J.P. 1993. Free radicals as mediators of tissue injury and disease. Crit. Rev. Toxicol. 23, 21-48.
Parr, J.A. and G.P. Bolwell. 2000. Phenols in the plant and in man. The potential of possible nutritionall enhancement of the diet by modifying the phenols content or profile. J. Sci. Food Agric. 80, 985-1012.
Sánchez-Moreno, C., Larrauri, J.A. and F. Saura-Calixto. 1998. A procedure to measure the antiradical efficiency of polyphenols. J. Sci. Food Agric.76, 270-276.
Shahidi, F. and P.K. Wanasundara. 1992. Phenolic antioxidants. Crit. Rev. Food Sci. Nutr. 32, 67-103.
Goffman, F.D., Bergman, C.J.
Rapid degradation of lipids after milling makes rice bran unsuitable for human consumption. To better understand these lipolytic processes, bran from a conventional U.S. long ('Cypress') and medium grain ('Earl') rice cultivar were stored at room temperature for six months, and the changes in triacylglycerol content, fatty acid composition and free fatty acids (FFAs) were followed. Cypress showed a faster rate of triacylglycerol degradation compared to Earl, which apparently was due to its higher lipase activity (ca. 26% higher). The palmitic acid percentage was similarly reduced in both cultivars to about 80% of its initial concentration. Oleic and linoleic acids remained unchanged. The final content of FFAs was ca. 58% higher in Cypress than in Earl. The difference appeared to result from both the higher initial triacylglycerol content (23.5 and 18.3 mg triacylglycerol per 100 mg bran in Cypress and Earl, respectively) and lipase activity of Cypress. The results suggest a means of reducing rice bran rancidity by selecting breeding lines with lower bran oil content and lipase activity.
Goffman, F.D., Bergman, C.J.
Hydrolytic rancidity restricts the utilization of rice bran, reducing its potential value in human nutrition. In the present study, three groups of eight rice cultivars each displaying different levels of oil concentration (high-, medium- and low-oil) were cultivated under field conditions and evaluated for bran oil content, hydrolytic rancidity and esterase activity. Genotype effects were statistically significant for all measured traits, whereas environment (year) was nonsignificant. Hydrolytic rancidity was strongly correlated with esterase activity but not with oil concentration. A wide variation was found for both hydrolytic rancidity and esterase activity, which ranged from 6.8 to 56.0 mg C8:0/g bran and from 4.3 to 22.8 mg C8:0/g bran, respectively. Red bran displayed the lowest values for both hydrolytic rancidity and esterase activity. Apparently, the low values for hydrolytic rancidity were related to the inhibitory effect of bran tannins on lipase activity. In conclusion, cultivar variation was detected for both hydrolytic rancidity and esterase activity in the studied genotypes, esterase activity being the principal factor explaining the variation found for the former trait. Therefore it may be possible to create new cultivars with increased stability against hydrolytic rancidity by selecting for lower esterase activity.
Goffman, F.D., Pinson, S.R., Bergman, C.J.
Rice bran contains valuable nutritional constituents, which include lipids. A germplasm collection consisting of 204 genetically diverse rice accessions was grown under field conditions and evaluated for total oil content and fatty acid composition. Genotype effects were highly statistically significant for lipid concentration and fatty acids Environment significantly affected oil content, stearic, oleic, linoleic and linolenic acids but not palmitic acid. The oil content in rice bran varied a great deal, ranging from 17.3 percent to 27.4 percent wt/wt. Major fatty acids of bran oil were palmitic, oleic and linoleic acids, which were in the ranges of 13.9-22.1 percent, 35.9-49.2 percent and 27.3-41.0 percent, respectively. The ratio of saturated to unsaturated fatty acids was highly related to the palmitic acid content. The data suggests it is possible to select rice breeding progeny with enhanced oil content and fatty acid profile.
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