Rosmarinus officinalis essential oil vs. synthetic antioxidants: are they equally efficient on limiting lipid oxidation of feeding oils?

The aim of this work was to evaluate the effect of a Rosmarinus officinalis essential oil (EO at 3 different concentrations: 0.05%, 0.1% and 0.2%, w/w) on the oxidative stability of salmon 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 subjected to thermoxidation in an oven at 60°C for 30 days and sampled at 4 different times (0, 7, 15, 30 days). To assess the oxidative stability of the oils, several analysis were carried out: peroxide value (PV), oxidative stability index (OSI time), volatile compounds (VOCs), fatty acids, sterols and their oxidation products (SOPs). In both tested oils, none of the added antioxidants had a significant effect on the fatty acid composition, even though a general protective tendency towards PUFA was observed. Regarding wheat germ oil, EO showed to be effective in counteracting lipid oxidation, especially in terms of OSI time (over the whole 30-day treatment), VOCs (hexanal level 7-fold lower in OE 0.2% samples than in control ones) and SOPs formation. Among synthetic antioxidants, ETO showed the best performance in limiting secondary lipid oxidation, especially SOPs formation (1.1-fold lower content than in control ones). In salmon oil, EO was not able to reduce lipid oxidation at all tested concentrations. On the contrary, ETO and BHA were able to keep a stable OSI time for up to 7 days of thermoxidation; moreover, they significantly reduced sterol loss over time (-0.5%) with respect to OE-added oils (-25%). It must be highlighted that, in all oils tested (with and without antioxidants), the sterol oxidation ratio (OR) was always below 1%, threshold value that is recommended not to be exceeded in oils destined to animal feeding and pet food.