Evaluation of Natural Antioxidants in Food Systems.
Edwin Frankel
Department of Food Science and Technology
University of California, Davis, CA
Oxidation of lipids not only produces undesirable rancid flavors in foods but can also decrease their nutritional quality and safety by the formation of secondary oxidation products after cooking and processing. Enormous worldwide attention has been given recently to the use of natural phenolic antioxidants to control lipid oxidation in foods and biological systems. Flavonoids are phytochemicals with potent antioxidant activity that are found in many fruits and vegetables and beverages (red wine, grape juice and tea). Phytochemicals are now being widely investigated for their nutritional benefits. Although much progress has been achieved in determining the relative absorption of flavonoids and metabolism in man, very little is known about their bioactivity. Natural phenolic antioxidants inhibit lipid oxidation in complex foods by a multi-step mechanism that requires more than one specific evaluation methods. Unfortunately, many contemporary studies tend to use non-specific indicators of oxidation and antioxidation. When testing natural antioxidants it is important to consider the food system composition, the type of oxidizable substrate, the mode of accelerating oxidation, the methods to assess oxidation, and how to quantify antioxidant activity. Antioxidant effectiveness is also determined by the heterogeneity nature of the system, the type of lipid substrate including its physicochemical state and degree of unsaturation, the presence and types of initiators, notably transition metals and their possible interaction. Each evaluation should be carried out under various conditions of oxidation, using several methods to measure different products of oxidation. Because most natural antioxidants and phytochemicals are multifunctional, a reliable antioxidant protocol requires the measurement of more than one property relevant to foods. There is a great need to standardize antioxidant testing to minimize the present chaos in the methodologies used to evaluate antioxidants.