Study on antioxidant activity of amino acids at frying temperatures and their interaction with rosemary extract, green tea extract, and ascorbic acid

Authors: Hwang, Hong-Sik; Moser, Jill; Liu, Sean

Submitted to: Journal of Food Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2019
Publication Date: 12/15/2019
Citation: Hwang, H., Winkler-Moser, J.K., Liu, S.X. 2019. Study on antioxidant activity of amino acids at frying temperatures and their interaction with rosemary extract, green tea extract, and ascorbic acid. Journal of Food Science. 84(12):3614-3623. https://doi.org/10.1111/1750-3841.14963.
DOI: https://doi.org/10.1111/1750-3841.14963

Interpretive Summary: Vegetable oils containing unsaturated fatty acids oxidize fast during frying. Oil oxidation generates potentially toxic oxidation products. The development of strong antioxidants is a solution for the oxidation problem of vegetable oil during frying. Synthetic antioxidants such as tert-butyl hydroquinone (TBHQ) are strong antioxidants, but they are potentially toxic to human. There have been tremendous efforts to develop new natural antioxidants that can replace synthetic antioxidants. Our research group recently discovered that some amino acids had strong antioxidant activity in frying oil. This study aimed to obtain further information to facilitate the practical application of amino acids as natural antioxidants for frying. To find out the optimal concentration, the activity at different concentrations of amino acids was examined. Antioxidant activity of amino acids continuously increased with increasing concentration up to about 0.3% in oil. A practical way to utilize a new antioxidant is to add it into an existing commercial antioxidant system to provide a synergistic or additive effect. Therefore, interactions with other antioxidants such as rosemary extract, green tea extract, and ascorbic acid were investigated. It is very difficult to predict whether mixing two antioxidants would give stronger activity or not. Caution should be exercised when mixing two antioxidants because sometimes mixing two antioxidants reduces the activity of each antioxidant, and the interaction of two antioxidants must be evaluated before mixing. The effectiveness of amino acid was also examined in other vegetable oils. The amino acid showed strong antioxidant activity in six other vegetable oils. This study provides some guidelines for the practical use of amino acid in frying.

Technical Abstract: 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 ºC within the concentration range of 0-15 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 alpha-tocopherol at high concentrations in SBO while it significantly increased the activity at each concentration of alpha-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 (R^2^ = 0.45-0.52) with the ratio of unsaturated fatty acids to saturated fatty acids indicating the effectiveness may be related to the fatty acid composition of oil.