Study on Antioxidant Activity of Amino Acids at Frying Temperatures and Their Interaction with Rosemary Extract, Green Tea Extract, and Ascorbic Acid

Some amino acids have strong antioxidant activity in frying oil. This study aimed to obtain further information including antioxidant activity at different concentrations and interactions with rosemary extract, green tea extract, and ascorbic acid. Antioxidant activity of arginine, cysteine, lysine, methionine, and tryptophan was examined by increasing the concentration in soybean oil (SBO) at 180 °C within the concentration range of 0 to 15 mM. These amino acids showed increased activity with increasing concentration without showing prooxidant activity at the given concentration range. Addition of 15 mM methionine did not inhibit the prooxidant activity of α-tocopherol at high concentrations in SBO while it significantly increased the activity at each concentration of α-tocopherol. Methionine showed an additive effect with a commercial rosemary extract while lysine had an antagonistic interaction in SBO at the total concentration of 5.5 mM. Mixtures of green tea extract and methionine did not show better activity than methionine alone in SBO and stripped SBO. (-)-Epigallocatechin gallate, the major active component in green tea, showed a synergistic effect with methionine in stripped SBO but there was no significant interaction effect in SBO. Although ascorbic acid had a synergistic effect with methionine in stripped SBO, it showed a significant antagonistic effect in SBO. Methionine had strong antioxidant activity in six other vegetable oils showing a moderate correlation (R2 = 0.45 to 0.52) with the ratio of unsaturated fatty acids to saturated fatty acids indicating the effectiveness may be related to the fatty acid composition of oil. PRACTICAL APPLICATION: Some amino acid such as methionine and lysine showed stronger antioxidant activity than the leading commercial natural antioxidant, rosemary extract. These amino acids showed great potential as a natural antioxidant in frying. The price of food-grade L-methionine is generally lower than rosemary extract and green tea extract. This paper provides information on the concentration effect and interactions with currently used antioxidants such as tocopherols, rosemary extract, green tea extract, and ascorbic acid.