Why is peroxidase activity measured to check whether blanching was effective?

Peroxidase activity is measured to determine the effectiveness of blanching because this enzyme is particularly heat-stable.1

During vegetable processing, blanching is used to inactivate natural enzymes that cause undesirable changes, such as discoloration, off-flavours, and the loss of nutrients during storage. Because peroxidase is one of the most heat-resistant enzymes present in plant tissues, its successful inactivation serves as a reliable indicator that other, less heat-stable enzymes responsible for food deterioration have also been effectively neutralized. If residual peroxidase activity is detected, it indicates that the thermal treatment was insufficient to ensure the quality and shelf stability of the product.231