Factors affecting antioxidant activity of amino acids in soybean oil at frying temperatures

Authors: Hwang, Hong-Sik; Moser, Jill; Doll, Kenneth - Ken; GADGIL, MAYURESH - Bradley University; Liu, Sean

Submitted to: European Journal of Lipid Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2019
Publication Date: 6/4/2019
Citation: Hwang, H.-S., Winkler-Moser, J.K., Doll, K.M., Gadgil, M., Liu, S.X. 2019. Factors affecting antioxidant activity of amino acids in soybean oil at frying temperatures. European Journal of Lipid Science and Technology. https://doi.org/10.1002/ejlt.201900091.
DOI: https://doi.org/10.1002/ejlt.201900091

Interpretive Summary: This research determined several factors affecting antioxidant activity of amino acids in soybean oil at 180 ºC. Since healthy vegetable oils such as soybean oil are very susceptible to oxidation, the food industry and food service industry tend to avoid the use of soybean oil as frying oil and favor unhealthy saturated fats such as imported palm oil, which are more resistant to oxidation. To overcome this problem, it is imperative to develop strong natural antioxidants to facilitate the utilization of healthy and domestic vegetable oils for frying. We previously discovered that amino acids were excellent new natural antioxidants for frying oils. Therefore, we conducted this study to understand the factors affecting the activity, which will enhance the practical application of amino acids as antioxidants for frying. We found that amino acids with better solubility and higher basicity had higher antioxidant activity. One of the proposed mechanisms for the activity of amino acids was the antioxidant activity of secondary products produced by reactions between an amino acid and oxidation products. This study revealed that this mechanism was not the essential mechanism for the activity under the conditions used in this study. We also excluded another proposed mechanism, metal chelation, as the major mechanism for the activity. Protein hydrolysates and peptides, which are composed of many different amino acids, also are strong antioxidants for soybean oil at frying temperatures. However, their activity was not as strong as some single amino acids. Therefore, this work is a guide to selection of an amino acid, which is needed for practical application. All these findings are critical information desperately needed by the industry in order to increase the use of healthy vegetable oils.

Technical Abstract: This study elucidates mechanisms and factors affecting antioxidant activity of amino acids in soybean oil at 180 ºC. Methylation of the carboxylic acid group of beta-alanine increased the activity, presumably, due to increased lipophilicity and/or deprotonation of the ammonium group. The significant antioxidant activity of 3-(dimethylamino)propionic acid indicated that formation of antioxidant substances by reactions of the amino group (-NH2) with oxidation products was not an essential factor for the activity of amino acids, which was further evidenced by strong activity of 1,8-bis(dimethylamino)naphthalene. Although, in general, antioxidant activity of amino acid increased with increasing lipophilicity, DL-2-aminobutyric acid (C4) and DL-norleucine (C6) had higher activity than other analogues with a longer alkyl chain. We found no evidence that chelation of metals was the major factor for the activity of amino acids. Additionally, dipeptides did not show stronger activity than the two constituent amino acids alone in soybean oil.