Antioxidant activity of black glutinous rice Oryza sativa L. extracts

Abstract

The objectives of this research were to determine the appropriate solvent and the optimum extraction condition to prepare crude extract from black glutinous rice and to determine the application of the crude extract as an antioxidant and colorant in fish oil enriched mayonnaise. Firstly, the black glutinous rice flour was extracted twice with 50% ethanol (v/v), 70% acetone (v/v), and 50% ethanol (v/v) followed by 70% acetone (v/v), at neutral (pH 6.8) and acidic (pH 2) conditions, for 2, 4 and 8 hours of total extraction times. Total phenolic content, total monomeric anthocyanin content and antioxidant activities, determined by DPPH and FRAP methods, of the crude extracts were measured. Statistical analysis showed that there were significant effects of all three main factors, which were type of solvent, pH of solvent and extraction time, as well as significant two-way and/or three-way interactions of those main factors on each parameter measured (p[is less than or equal to]0.05). Since the best extraction conditions for total phenolics, total monomeric anthocyanins, FRAP and DPPH assays were not agreeable, the most suitable extraction condition was selected from the shortest total extraction time which gave an acceptable amount of total phenolics and total monomeric anthocyanins, and high antioxidant activity. Therefore, the most suitable solvent and condition was the twice extraction with neutral 50% acetone at 4 hour total extraction time. The crude extract from the selected condition was then used to study its application in fish oil enriched mayonnaise. The freeze-dried extract was added into the mayonnaise at two different concentrations, 500 ppm and 1000 ppm (based on oil weight in the mayonnaise). Lipid oxidation in the oil phase of mayonnaise samples stored at 30˚C was determined every 5-7 days up to 30 days by measuring conjugated diene hydroperoxides (CDH) and thiobarbituric acid reactive substance (TBARs). Color of the mayonnaise was also measured in the Hunter (L, a, b) system. The results showed that the crude extract could retard an increase in both CDH and TBARs in the oil phase of the mayonnaise samples during storage. Crude extract at 1000 ppm showed higher antioxidant activity. However, lightness (L) and yellowness (b) of all mayonnaise samples decreased whereas redness (a) of all samples increased during storage which indicated the instability of anthocyanin colorant in the product. The overall results from the application study showed that although the crude extract exhibited great antioxidant activities in fish oil enriched mayonnaise, it could not serve as a good colorant in this product since its pink or light purple color was not stable.