INTRODUCTION
Frying is a popular cooking method, which causes deterioration of oil quality and formation of toxic oxidation products (Guillén & Uriarte, 2012). Vegetable oils containing high levels of unsaturated fatty acids have many health benefits, yet they are prone to oxidation due to those unsaturated fatty acids (Choe & Min, 2006). In the past, hydrogenation has been used to decrease unsaturation in oil to decrease the oxidation rate. However, this process generates trans fats, which are associated with the development of cardiovascular disease (Angell et al., 2009). Many manufacturers of fried foods have switched to oils containing high levels of saturated fatty acids in response to recent regulations requiring removal of trans fats in food products, although highly saturated fats are less desirable for human health than vegetable oils containing high contents of unsaturated fatty acids (Karupaiah & Sundram, 2007). To use vegetable oils such as soybean oil for frying, strong antioxidants are needed to prevent their oxidation during frying. Generally, synthetic antioxidants are strong antioxidants and widely used for frying, of which the maximum concentration is limited by law to 0.02% of the total lipids due to potential toxicity (Ito, et al., 1986).
Amino acids are reported to have antioxidant activity in edible oils, which was attributed to synergism with tocopherols (Marcuse, 1962; Seher & Löschner, 1986), chelation of metals (Decker, et al., 2001), and radical scavenging activity (Marcuse, 1960). Earlier studies conducted with emulsion systems because amino acids are soluble in water, but not in oil (Marcuse, 1960, 1962; Riisom, Sims, & Fioriti, 1980). Recent studies found that amino acids were very effective in preventing oil oxidation at frying temperatures when they were directly added as powder in oil and that amino acids containing a thiol, thioether, or amine group had stronger activity than other amino acids (Heng, Ong, & Chow, 2020; Hwang & Winkler-Moser, 2017). Methionine effectively prevented the oxidation of soybean, olive, corn, peanut, high oleic soybean, canola, and sunflower oils at 180 ºC (Hwang, Winkler-Moser, & Liu, 2019).
Two major functional groups in amino acid are amine (-NH2 or -NH) and carboxylic acid (-COOH) groups. Amines are known to have radical scavenging activity and to produce new antioxidants by reactions with oxidation products during heating (Farag, et al., 1978; Choe & Min, 2009). However, in general, a carboxylic acid group in an antioxidant negatively affects its antioxidant activity, and the conversion of the carboxylic acid to a methyl or ethyl ester increases the antioxidant activity. For example, when the carboxylic acid group of gallic acid and protocatechuic acid was converted to a methyl or ethyl ester, the antioxidant activity increased in fish oil and its emulsion (Asnaashari, Farhoosh, & Sharif, 2014; Farhoosh, et al., 2016). In addition, when ferulic acid was converted to methyl ferulate, its antioxidant activity increased in a β-carotene-linoleate model system (Karamać, et al., 2005). The methyl ester of β-alanine also more effectively protected SBO from oxidation than β-alanine in a heating study at 180 ºC (Hwang, et al., 2019). Miura et al. (Miura, et al., 2014) reported that when the carboxylic acid group of cysteine derivatives was converted to the carboxylate group by mixing the cysteine derivatives in situ with triethylamine, their antioxidant activity in methyl linoleate at 36 ºC increased.
In this study, sodium or potassium salts of several amino acids were prepared as powder, suspended in soybean oil, and were evaluated for their antioxidant activities in SBO at 180 ºC in comparison with the corresponding amino acids. Oxidation of oil was evaluated by analyzing polymerized triacylglycerols (PTAG) using gel permeation chromatography (GPC). Losses of reactive protons including olefinic, allylic, and bisallylic protons in NMR spectrum of oil were also evaluated, which are known to be reliable markers for oil oxidation (Hwang et al., 2017; Pignitter & Somoza, 2012).
MATERIAL AND METHODS