Polyphenoloxidase activity and tissue browning in fresh-cut 'rocha' pear as affected by pH and phenolic substrate

Cell rupture in fresh-cut processing allows phenolic substrates and polyphenoloxidase (PPO), previously sequestered in different organelles, to react leading to undesirable color development. However, it is not always clear how PPO activity and phenolic content relate to tissue browning. We studied the effect of pH and phenolic substrates on PPO activity and on tissue browning in fresh-cut 'Rocha' pear, to help in the development of food additives that maximize the quality of fresh-cut pear. Substrates 4-methyl catechol, caffeic acid, (+)-catechin hydrate, catechol, chlorogenic acid, dopamine hydrochloride, and pyrogallol, were prepared in citric acid-phosphate buffer at pH values ranging from 3.0 to 8.0. PPO activity was assayed by measuring the rate of increase in absorbance at 420 nm wavelength at 25°C. Pear slices were covered with the buffered phenolic solutions and color change was assessed following 30 min at room temperature. pH optima for PPO activity depended on the phenolic substrate. Activity was optimal at pH 5.0 for catechol and 4-methylcatechol; pH 6.0 for chlorogenic acid; pH 7.0 for dopamine, caffeic acid, and catechin; and pH 8.0 for pyrogallol. The highest PPO activity at every pH tested was observed when catechol was used as substrate. Discrepancies were observed between the pH dependency of PPO activity and browning. Significant correlations were obtained between PPO activity and lightness (L*) or metric-hue difference (ΔH*) over the pH range 3.0 to 8.0 except for chlorogenic acid and 4-methylcatechol. With chlorogenic acid, the main PPO substrate in 'Rocha' pear, tissue browning was higher at pH 3.0 (higher ΔH*), but PPO activity was very low at the same pH. Similarly with the other phenolic substrates, browning at pH 3.0 was higher than the corresponding PPO activity. The pH of additives for cut pear should be corrected to reduce browning potential.