Effect of lipophilization of dihydrocaffeic acid on its antioxidative properties in fish oil enriched emulsion

The relative low intake of fish and the health beneficial n-3 polyunsaturated fatty acids (PUFA) in the Western countries has created a growing market for food products enriched with eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Since EPA and DHA are more susceptible to lipid oxidation than PUFAs from vegetable oils due to their highly polyunsaturated nature, it is necessary to develop methods to protect these PUFAs. Many food systems are emulsions. Due to the so-called polar paradox phenomenon, hydrophilic antioxidants may in many cases be better antioxidants in bulk oil than lipophilic compounds, whereas lipophilic antioxidants are more efficient than hydrophilic antioxidants in emulsions. This phenomenon has been explained by the affinity of the compounds towards the different phases in bulk oil and emulsions. The hydrophilic character of many naturally occurring antioxidants may cause a low efficacy in inhibiting lipid oxidation in food emulsions. However, lipophilization of the antioxidants with a fatty alcohol may alter their location in the emulsion matrix and thereby improve their efficacy. Evaluation of the effect of lipophilisation of selected antioxidants revealed that generally, lipophilized dihydrocaffeic acid and rutin increased the oxidative stability of o/w emulsions and fish oil enriched milk compared with their parent compound. The results supported a cut-off effect in relation to the acyl chain length esterified to the phenolic compound. Octyl dihydrocaffeate (C8 acyl chain) was a stronger antioxidant than oleyl dihydrocaffeate (C18 acyl chain) and rutin laurate (C12 acyl chain) was a stronger antioxidant than rutin palmitate (C16 acyl chain). Interestingly, it seemed that the cut-off effect not only is specific for the individual lipophilized phenolic compounds, but that it also depends on the emulsion system, i.e. the optimal chain length seems to vary between different emulsion systems.