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