Antioxidant activity of amino acids in soybean oil at frying temperature: Structural effects and synergism with tocopherols

Authors: Hwang, Hong-Sik; Moser, Jill

Submitted to: Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/7/2016
Publication Date: 11/9/2016
Citation: Hwang, H.-S., Winkler-Moser, J.K. 2017. Antioxidant activity of amino acids in soybean oil at frying temperature: Structural effects and synergism with tocopherols. Food Chemistry. 221:1168-1177.

Interpretive Summary: Due to the vulnerability of vegetable oils such as soybean oil to oxidation, the use of these oils for frying is limited although they are well recognized as healthy oils due to high contents of polyunsaturated fatty acids. The development of new natural antioxidants is critical for the successful utilization of vegetable oils for frying. Amino acids are known to be antioxidants in oils and fats and are widely available and inexpensive, but no one examined the antioxidant activity of amino acid under frying conditions. We conducted oven-heating studies at 180 °C as well as frying studies using potato cubes in soybean oil at 180 °C to understand antioxidant activities of 20 amino acids. This study clearly proved that several amino acids had strong antioxidant activities indicating that these amino acids had high potential as natural antioxidants for frying. At the five-fold higher concentration, these amino acids showed much stronger antioxidant activities than a synthetic antioxidant, TBHQ. We also studied on the synergism with tocopherols that inherently exist in soybean oil and found that antioxidant activity of amino acid largely relied on the synergism with tocopherols in oil although amino acids themselves could act as primary antioxidants in the absence of tocopherols. In the frying experiment with potato cubes fried in SBO at 180 °C, methionine had the strongest antioxidant activity.

Technical Abstract: The purpose of this study was to evaluate amino acids as natural antioxidants for frying. Twenty amino acids were added to soybean oil heated to 180 ºC, and the effects of amino acid structure on the antioxidant activity were investigated. Amino acids containing a thiol, a thioether, or an extra amine group such as arginine, cysteine, lysine, methionine, and tryptophan had the strongest antioxidant activities. At 5.5 mM, these amino acids had stronger antioxidant activities than 0.02% (1.1 mM) tert-butylhydroquinone (TBHQ). A functional group such as an amide, carboxylic acid, imidazole, or phenol appeared to negatively affect amino acid antioxidant activity. Synergism between amino acids and tocopherols was demonstrated, and we found that this synergistic interaction may be mostly responsible for the antioxidant activity that was observed. In a frying study with potato cubes, 5.5 mM L-methionine had significantly stronger antioxidant activity than 0.02% TBHQ.