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.