|Rice Quality Program Recent Accomplishments|
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.
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.
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
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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.
Hydrolytic Degradation of Triacylglycerols and Changes in Fatty Acid