Activity, electrophoretic characteristics and heat inactivation of polyphenoloxidases from apples, avocados, grapes, pears and plums

Abstract

Polyphenoloxidases (PPO) were extracted from apples, avocados, grapes, pears and plums. In avocados, higher PPO activity was observed than in plums, which showed higher PPO activity than apples, grapes and pears. For all cases studied, enzyme activity was maximal around neutral pH (pH 6-7). Apple, pear and plum PPO appeared to have a molecular weight exceeding 750 kDa, which might be due to aggregation phenomena taking place during extraction. For avocado, in addition to a large aggregate (MW > 750 kDa), two smaller PPO molecules with a MW of 58.5 and 66.5 kDa were detected. Plum PPO seemed to have at least two isoenzymes (pI 4.8 and 5.2). In the case of avocado PPO, pI values equalled 4.6 and 4.7. Thermal inactivation followed first-order decay for all PPOs studied. Except for apple and pear PPO, the relation between decimal reduction time and inactivation temperature was log-linear in the temperature domain studied. In the case of apple and pear PPO a 'breakpoint' was noted at 72.5 and 75°C, respectively. Above and below these temperatures a log-linear relation between decimal reduction time and inactivation temperature was noted. Thermal resistance of PPO seemed to be dependent on its origin. Among the polyphenoloxidases studied, grape PPO was most heat sensitive. © 1998 Academic Press Limited.