Abstract
This study investigated the effect of metal ions on the oxidation of sterols and their kinetics in oil-in-water emulsions. Sterol substrates were added with different metal ions (Cu2+, Fe2+, Mn2+, Zn2+, Na+, and Mg2+) of five concentrations and investigated after 2 h of heating at 90 °C. The substrates added with Fe2+ and Cu2+ were heated continuously to evaluate the kinetics of four sterols and their corresponding sterol oxidation products (SOPs). Sterol oxidation increased as the metal ion concentration increased and the heating time was prolonged. The capability of the metal ions oxidizing sterols ranked as followed: Fe2+ > Cu2+ > Mn2+ > Zn2+ > Mg2+ ≈ Na+. 7-Ketosterol, 7β/7α-Hydroxysterol, 5β,6β/5α,6α-Epoxysterol, and Triols were the main oxides on the B ring, whereas 6β-Hydroxysterol was not or only slightly influenced. The acceleration of sterol degradation induced by Fe2+ and Cu2+, as well as the formation of oxidation products, followed first-order formation/elimination kinetics. The acceleration effect may be partly ascribed to the increase in elimination rate constant and formation rate constant. Transition metal ions can significantly induce sterol oxidation, which reduces food nutritional quality and triggers the formation of undesirable compounds, such as SOPs.
Highlights
This study investigated the effect of metal ions on the oxidation of sterols and their kinetics in oilin-water emulsions
The acceleration effect may be partly ascribed to the increase in elimination rate constant and formation rate constant
Sterol oxidation reduces the nutritional quality of food[5] and promotes the formation of compounds that exhibit pathological and toxic effects compared with unoxidized sterols
Summary
This study investigated the effect of metal ions on the oxidation of sterols and their kinetics in oilin-water emulsions. Oil-in-water emulsion is a common form of lipids in food, such as butter, margarine, milk, infant formula, salad dressing, mayonnaise, sauces, soups, beverages, cream, and some desserts[10]. These emulsions accelerate lipid oxidation because their large surface areas facilitate interactions between lipids and water-soluble pro-oxidants[11]. Much effect has been exerted to understand how metal ions accelerate the oxidation of unsaturated fatty acids in oil-in-water emulsions[11,17]. Few studies have focused on the factors that influence sterol oxidation in emulsified oils
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