Measuring antioxidant activity

Antioxidants are used in a wide variety of food products, and their activity may vary depending on the temperature, food composition, food structure and availability of oxygen. Temperatures at which antioxidant activity may be required range from 180–200°C for frying oils, to about 5 °C for products such as margarine or mayonnaise that are stored in the fridge. Besides the processing and storage temperatures to which these products are exposed, the accompanying constituents including water, proteins, carbohydrates, vitamins, minerals and other food components vary, and the physical structure of the food also varies. This can cause big changes in the activity of the antioxidant in different food systems. It is commonly observed that a non-polar antioxidant such as a-tocopherol is relatively ineffective in oil but is strongly effective in an oil-in-water emulsion. In contrast, a polar antioxidant such as ascorbic acid or trolox (a water-soluble derivative of a-tocopherol) is more effective in an oil than in an emulsion. This has been described as the polar paradox. Normally, a more rapid measurement of antioxidant activity is required than would be obtained by making the food product, storing it at ambient temperature and then measuring the oxidative state of the food. Consequently, there are three decisions to be made: