Abstract
The antioxidant activity of methanolic extracts from Sclerocarya birrea kernel oil meal, extracted using two different methods was evaluated. The extraction was carried out using magnetic stirring of the material in methanol/water (80:20 v/v) overnight followed by two ultra-sonic treatments for 45 min. (Overnight extract, ONEXT) and three ultra-sonic treatments for 45 min. only (Ultra-sonic extract, USEXT), respectively. Three fractions were obtained,from each extract and the contents of total phenolic compounds were determined in each fraction according to the Folin-Ciocalteau method as 34.6, 54.8, and 58.6 mg/g. of dry product in ONEXT and 29.6, 84.8, 143.9 mg/g in USEXT, respectively. The antioxidant activity of the extracts was evaluated according to the beta-carotene-linoleic acid assay, where the extracts and their fractions showed significant effect (p < 0.05). The antioxidative properties of the extracts obtained from the two extraction methods described were similar. The AAC (antioxidant activity coefficient) of these extracts and their fractions increased with an increasing concentration of the extract. The effect of ONEXT and USEXT at the 0.2 and 0.8% levels on the oxidative stability of sunflower oil at 70 degrees C was tested in the dark and compared with the commonly used synthetic antioxidant BHA. The oil peroxide values (PVs) were significantly (p < 0.05) lower with the addition of extract in comparison to a control. In comparison to BHA (0.02%) the increase of PVs after the addition of ONEXT (0.2% and 0.8%) and USEXT (0.8%), respectively, was reduced. The oxidation of sunflower oil, treated with 0.2%, 0.5%, and 1.0% of ONEXT and USEXT, respectively, was tested using the Rancimat test at 120 degrees C. Both extracts increased the induction time compared to a control and BHA, and the stabilization factor F increased with the concentration.