Heating study at 180 °C
It has been a problem that most conventional methods for lipid oxidation assessment detect either primary or secondary oxidation products, which often lead to inconsistent conclusions (Gray, 1978). In-vitroantioxidation activity tests determining the radical scavenging ability of an antioxidant do not well correlate with the antioxidant activity under frying conditions because these tests are typically conducted at room temperature (Hwang et al., 2012). Among many analytical methods, NMR, HPLC, and size-exclusion chromatography methods are known to be the most reliable methods (Pignitter & Somoza, 2012). Recently, an NMR method specifically designed for the assessment of frying oils has been developed, which was found to reliable, and to be faster and more convenient than other conventional methods (Hwang et al., 2017). Therefore, in this study, PTAG using the size-exclusion chromatography and the NMR method were used to determine oil oxidation (Table 1). The results with an amino acid salt were compared with those of the corresponding amino acid, TBHQ, and control, and they were marked with letters, a, t, and c, respectively when the mean values were significantly different (P < 0.05). When the values of an amino acid potassium salt were significantly different from those of the corresponding sodium salt, they were marked with a letter n.
Alanine Na, glutamic acid di-Na, phenylalanine Na, and proline Na had significantly higher antioxidant activity compared to their corresponding amino acids resulting in lower losses of olefinic H and bisallylic H and PTAG. Arginine Na, asparagine Na, glutamic acid Na, and tyrosine Na prevented oxidation of SBO slightly better than their corresponding amino acids. The antioxidant activity order of glutamic acid and its mono- and di-sodium salts was di-sodium salt > mono sodium salt > glutamic acid indicating that the activity was the highest when all the carboxylic acid groups were converted to carboxylate groups. Antioxidant activity of glutamine Na was not stronger than that of glutamine. This indicates that not all the amino acid salts have stronger antioxidant activity than the corresponding amino acids, for which the reason is not clear at the present stage. Further studies with more amino acid salts will be needed to fully understand the effectiveness of amino acid salts.
Potassium salts tested in this study had even stronger antioxidant activity than sodium salts. Especially, alanine K and phenylamine K had significantly lower losses of reactive protons (olefinic H and bisallylic H) and PTAG than their corresponding amino acids. It can be explained by higher lipophilicity of potassium salts than sodium salts (Yamaguchi et al., 1995) since it was reported that more lipophilic amino acids had stronger activity than less lipophilic ones (Hwang, et al., 2019). It is noteworthy that the effectiveness of 5.5 mM alanine K, phenylalanine K, and proline K in preventing the oxidation of SBO was greater than 0.02% (1.1 mM) TBHQ.