Solutions of calcium, sodium and potassium ascorbate used in
the study were about pH 7 and that of ascorbic acid was about pH 3
and the greater inhibition of browning induced by the ascorbate
salts over ascorbic acid could be due to a pH effect on PPO activity.
All solutions were outside the optimal pH for PPO activity in apples
of 4e5 (Richard-Forget, Goupy, Amiot, & Aubert, 1994) and could
thus be expected to inhibit PPO activity. However, Li et al. (2014)
reported that PPO activity of a solution at pH 7 was only 70% that
of a solution at pH 3, suggesting a lower rate of phenol oxidation
occurred at pH 7.