Fifty grams of potato or apple were homogenized for 30 s with 50 ml of a cold 0.1 M sodium phosphate buffer (pH 7.0, 4 C) containing 10.0 g/L polyvinylpyrrolidone and 5.0 g/L Triton X-100. The homogenates were centrifuged for 30 min (4 C) at 8000g (Sorvall RC 5 C Refrigerated Centrifuge, Du Pont, Newtown, CT). The supernatant was collected and stored at 30 C for use as crude PPO enzyme.