Effect of the pH on the lipid oxidation and polyphenols of soybean oil-in-water emulsion with added peppermint (Mentha piperita) extract in the presence and absence of iron

Abstract

This study evaluated the pH effect on the lipid oxidation and polyphenols of the emulsions consisting of soybean oil, citric acid buffer (pH 2.6, 4.0, or 6.0), and peppermint (Mentha piperita) extract (400 mg/kg), with/without FeSO₄. The emulsions in tightly-sealed bottles were placed at 25 °C in the dark, and lipid oxidation and polyphenol contents and composition were determined. The lipid oxidation was high in the emulsions at pH 4.0 in the absence of iron, however, iron addition made them more stable than the emulsions at pH 2.6 or 6.0. Total polyphenols were remained at the lowest content during oxidation in the emulsions at pH 4.0, and iron reduced and decelerated polyphenol degradation. The results strongly suggest that polyphenols contributed to decreased lipid oxidation of the emulsion via radical scavenging and iron-chelation, and rosmarinic acid along with catechin, caffeic acid, and luteolin were key polyphenols as radical scavengers in the extract.

Keywords: Emulsion lipid; Iron; Peppermint extract; Polyphenol; pH.

Fig. 1

HPLC chromatogram of polyphenols of 75% ethanol extract of peppermint (1; danshensu, 2; catechin, 3; unidentified, 4: caffeic acid, 5; luteolin-rutinoside, 6; unidentified, 7; rosmarinic acid, 8; salvianolic acid B, 9; isosalvianolic acid A, 10; luteolin, 11, 12; unidentified)

Fig. 2

Effect of pH on the lipid oxidation of soybean oil-in-water emulsion (4:6, w/w) with added peppermint extract (400 mg/kg) during oxidation at 25 °C in the dark (A), (B); No iron (C), (D); with iron (filled square; pH 2.6, filled circle; pH 4.0, triangle; pH 6.0). Different letters on the line are significantly different values among samples by Duncan’s multiple range test at 5%

Fig. 3

Effect of pH on the polyphenol content of soybean oil-in-water emulsion (4:6, w/w) with added peppermint extract (400 mg/kg) during oxidation at 25 °C in the dark (A) without or (B) with added iron at 5 mg/kg (filled square; pH 2.6, filled circle; pH 4.0, triangle; pH 6.0). Different letters on the line are significantly different values among samples by Duncan’s multiple range test at 5%