The effect of free fatty acids (FFA) on lipid oxidation in water-in-stripped walnut oil emulsions was investigated. The formation of the primary oxidation products (hydroperoxides) and the secondary oxidation product (headspace hexanal) increased with increasing addition of oleic acid to the emulsions. However, oleic acid at high concentrations was primarily involved in accelerating hydroperoxide formation at early stage and then hydroperoxide decomposition rapidly after hydroperoxides reached the maximum point. The prooxidant effect of saturated FFA was dependent on their chain length with lipid oxidation rates being in the order of lauric acid>palmitic acid>stearic acid. The highest prooxidant activity of lauric acid among these FFA was probably due to its largest polarity and making the water droplet interface more negatively charged than the others when the aqueous phase pH was 7.0, which was above its pKa, thereby attracting prooxidant metals to the water droplet surface. The highest ability to promote lipid oxidation in water-in-oil emulsions was shown by linolenic acid, followed by linoleic and oleic acids, indicating that the oxidative capacity increased with increasing degree of unsaturation. The prooxidant effect of FFA with the cis double bonds was lower than those with the trans ones when oleic acid (18:1, cis) was compared to elaidic acid (18:1, trans) and linoleic acid (18:2, cis–cis) was compared to linoelaidic acid (18:2, trans–trans), which suggested that geometric isomeration of FFA influenced lipid oxidation rates of water-in-oil emulsions.
Practical applications: FFA promote lipid oxidation in bulk oil and oil-in-water (O/W) emulsions. From the present study, it became clear that FFA were also strong prooxidants in water-in-oil (W/O) emulsions. This research pointed out the importance of using oils and fats containing minimal FFA in W/O emulsion formulation to obtain the highest quality.