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The effect of natural antioxidants was tested by oxidative stability index (OSI) in refined soybean oil (RSO) using Rancimat method. Phenolic compounds of Thompson grape pomace were extracted with 95:5 (v : v) ethanol : water. The antioxidant activity of grape pomace extract (GPE) obtained was compared with those of rosemary extract (RE), and tocopherols mix (TM) known as covi‐ox T‐70. The synergistic effect of citric acid was also evaluated. Results showed a total phenols content of 17.39 mg (+)‐catechin equivalents per g of GPE. Thompson GPE at 0.3 and 0.5% (w/w) exhibited greater antioxidant activity than TM. RSO containing GPE or RE at 0.5% (w/w) showed an OSI higher than 48 h at 110C. Citric acid did not show synergistic effect with GPE. However, a synergistic effect for TM at 0.02% (w/w) with citric acid at 1.0% (w/w) was observed. Citric acid at 2.0% (w/w) with RE at all concentrations tested also displayed synergism. PRACTICAL APPLICATIONS Synthetic antioxidants are widely used in the food industry to delay fat oxidation; however, their utilization has been questioned due to toxicology concerns. Researchers have recognized the need to identify new natural antioxidants to use as additives. In this study, we measured total phenolics content of Thompson grape pomace extract (GPE) and found that the antioxidant activity of this extract was stronger than tocopherols from soybean oil. We propose that GPE can have applications as a food antioxidant.

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The stability parameters of 22 samples of soybean oil produced in Mexico were determined. Samples were analyzed for moisture, color, free fatty acids, peroxide value, p-anisidine value, fatty acid profile, metals, flavor, and Rancimat test for oxidative stability. Results obtained were compared with the stability parameters of soybean oils produced in the United States and Costa Rica. The fatty acid profile in all samples analyzed corresponded to the expected profile for a 100% soybean oil. Sixtyfour percent of the oils had oxidative stabilities similar to those reported for soybean oils from the United States and Costa Rica. This suggests that in spite of the good quality, the soybean oil production process in Mexico needs further improvement. Especially important is maintaining appropriate control during the degumming and bleaching steps. Special consideration should be given to preserving the natural antioxidants present in the oil.