Effect of the phytochemicals curcumin, cinnamaldehyde, thymol and carvacrol on the oxidative stability of corn and palm oils at frying temperatures.

Several active components naturally available in plants are strongly considered as good antioxidants to retard the lipid oxidation. Response surface methodology was used to investigate the effects of frying temperature (150-180 °C) and concentration of four plant-based active components (60-350 mg/kg oil); curcumin, cinnamaldehyde, thymol and carvacrol on oxidative stability of corn and palm oils. According to induction time values, the stability of oils drastically decreased with increasing temperature. Curcumin and cinnamaldehyde showed no significant effect (p > 0.05) on both oils. Carvacrol significantly increased induction times of corn and palm oils, but thymol was effective in palm oil only (p < 0.05). An actual frying experiment was carried out with only corn oil to confirm efficiency of carvacrol. The free fatty acid (%), peroxide value (meq/kg), para-anisidine, and total polar component values (%) of the fresh oil were 0.080, 2.55, 2.85, and 7.5, respectively. These values changed to 0.144, 1.47, 12.01, 10.0, respectively for the control oil; 0.138, 2.27, 11.49, 10.0 for BHT-added oil; 0.132, 1.42, 5.66, 9.5 for carvacrol-added oil after 30 frying cycles. Therefore, carvacrol could be considered as a good alternative to BHT for preservation of oils at frying temperatures.