Preservation of alpha-tocopherol in sunflower oil by herbs and spices

The ability of some commercially available herb and spice extracts to preserve alpha-tocopherol in sunflower oil during heating at 85-105 degrees C was assessed using sunflower oil as a model system. The Rancimat was evaluated for the heating stage and was used throughout as it was shown to be viable: alpha-tocopherol did not evaporate under the test conditions. The delay in the onset of rancidity was found to be directly related to the initial alpha-tocopherol concentration (P < 0.01). Rosemary, thyme, turmeric, sage, oregano and cumin extracts (2000 mg.kg-1) delayed rancidity (P < 0.01) and preserved alpha-tocopherol (P < 0.01). Some preservation was observed with clove extract but coriander and cardamom extracts were pro-oxidants. With thyme extract, the log of the induction time (as an indicator of the delay in rancidity) was directly proportional to the temperature (85-100 degrees C). The ethyl acetate, hexane and methanol extracts of fresh sage were effective for preserving alpha-tocopherol (P < 0.01). With thyme, rosemary and sage extracts, the increase in the preservation of alpha-tocopherol was directly related to the concentration of the herb extract (P < 0.01) and was quite effective even at 100 mg.kg-1. The increased delay in the onset of rancidity was due directly to the improved preservation of alpha-tocopherol (P < 0.01). In further experiments, the preservative effect of turmeric was shown not to be due to its reported major antioxidant, curcumin, even though it delayed rancidity. When herb/spice extracts were examined mixed with thyme, bay and turmeric showed synergism (P < 0.01) whereas bay alone was slightly inhibitory. The mode of action appeared to be due to free radical activity rather than through singlet oxygen generation.