Rosmarinus officinalis essential oil as a natural antioxidant in feeding oil: effect on oxidative stability compared to synthetic antioxidants

Lipids play a fundamental role in animal nutrition due to their deep influence on animal growth, feed quality and stability. However, lipids are prone to oxidation as a result of the processing techniques employed in feed formulation. Therefore, in order to preserve feed quality and enhance the quality of animal-derived food, the inclusion of antioxidants (natural and/or synthetic) is necessary during feed formulation. The aim of this work was to evaluate the effect of Rosmarinus officinalis essential oil (REO at 3 concentrations: 0.05%, 0.1% and 0.2%, w/w) on the oxidative stability of linseed and wheat germ oils; its performance was compared with those of 3 synthetic antioxidants (BHA, BHT and ethoxyquin (ETO), 150 mg/kg). Oils with and without antioxidants (control) were thermoxidized in an oven at 60°C for 30 days and sampled at 4 different times (0, 7, 15 and 30 days). The oxidative stability of the oils was evaluated through several analysis: peroxide value (PV), oxidative stability index (OSI time), volatile compounds (VOCs), sterols and their oxidation products (SOPs). In both tested oils, REO had no effect in contrasting primary lipid oxidation, but showed an important effect on limiting secondary lipid oxidation. In wheat germ oil, REO showed to be effective in counteracting lipid oxidation, especially in terms of OSI time (over the whole 30-day treatment), VOCs (hexanal: 6-fold lower in REO 0.2% samples than in control) and SOPs (about 13% lower in REO 0.2% samples than in control). In particular, the impact of REO 0.2% on SOPs formation was similar to that of ETO and BHT. Regarding linseed oil, REO was not as effective as the synthetic antioxidants (ETO and BHT) in reducing lipid oxidation in terms of OSI time, but it limited sterol loss and VOCs formation (hexanal: 4 times lower in 0.2% REO samples than in control). In all oils tested (with and without antioxidants), the sterol oxidation ratio (OR) was below 1%. Considering these results and the recognised variability among essential oils, further studies will be necessary to evaluate diverse Rosmarinus officinalis essential oils to validate our results, and to clarify the interactions between endogenous and exogenous antioxidants.