The effect of peroxidase on food flavor and quality is discussed, and its use as an index of blanching fruits and vegetables reviewed. Peroxidase is considered the most heat stable enzyme in plants. If inactivated, all ofher enzyme systems are usually inactivated in the product. There is an empirical relationship between residual peroxidase activity and the development of off-flavors and off-odors in foods. Peroxidase activity has been detected in products stored at — 18°C and under moisture conditions as low as 12.5% moisture. Any activity detected following severe heat treatments is usually due to the enzyme not being wholly inactivated. Inadequate thermal processing may result in regeneration of the enzyme.