Effects of interface generation, droplet size and antioxidant partitioning on the oxidation rate and oxidative stability of water–in–oil emulsions: A comparison of coarse emulsions and nanoemulsions

In order to gain an in–depth understanding of the factors affecting the oxidation of water–in–oil (W/O) emulsions, both initial oxidation rate and long–term oxidative stability were applied to assess the degree of oxidation of W/O emulsions. The results showed that the generation of the oil–water interface exacerbated lipid oxidation in W/O emulsions and the oxidation rate was independent of the initial droplet size (8 days). The difference in oxidative stability between coarse and nanoemulsions depended on the change in droplet size during storage and the formation of reverse micelles by the excess polyglycerol polyricinoleate (PGPR). The antioxidant (AO) esters with different alkyl chain lengths were added to the W/O emulsions to assess the effects of AO partitioning on lipid oxidation. The lipid oxidation results confirmed the applicability of the non–linear hypothesis in W/O emulsions. There was a critical chain length (CCL, octyl gallate, C8). Below or above CCL, the efficiency of the AOs decreased. C8 showed a 'dual effect', acting at both the oil–water and oil–air interface (interfacial tensions reduced by 0.36 and 4.66 mN/m, respectively). Future research should focus on means of effectively migrating AOs from the two phases to the oil–water interface to delay lipid oxidation. • The initial oxidation rate of W/O emulsion was independent of the droplet size. • Oxidative stability depended on droplet size changes and reverse micelles. • Applicability of the non–linear assumption in W/O emulsion was confirmed. • Optimal antioxidant activity occurred at intermediate alkyl chain length. • In W/O emulsions, C8 acted at both the oil–water and oil–air interfaces.