Polyphenoloxidase activity and browning in fresh-cut "Rocha" pear as affected by pH, phenolic substrates and antibrowning additives

tThe effect of pH, phenolic substrates, and food additives on polyphenoloxidase (PPO) activity and ontissue browning was studied in fresh-cut ‘Rocha’ pear. Substrates 4-methylcatechol, caffeic acid, (+)-catechin hydrate, catechol, chlorogenic acid, dopamine hydrochloride, and pyrogallol, were preparedin citric acid-phosphate buffer at pHs ranging from 3.0 to 8.0. pH optima for PPO activity dependedon the phenolic substrate. Activity was optimal at pH 5.0 for catechol and 4-methylcatechol; pH 6.0 forchlorogenic acid; pH 7.0 for dopamine, caffeic acid, and catechin; and pH 8.0 for pyrogallol. Discrepancieswere observed between the pH dependency of PPO activity and browning, as assessed by objective colormeasurement. Significant correlations were obtained between enzyme activity and metric-hue difference( H*) over the pH range 3.0–8.0 for four of the eight phenolics. Chlorogenic acid, the main PPO substratein ‘Rocha’ pear, induced high tissue browning but very low PPO activity at pH 3.0–4.0. Chemical inhibitionof PPO was tested using catechol as substrate, and buffer solutions containing 250 mM Ca2+in four salts(ascorbate, chloride, lactate and propionate), 57 mM ascorbic acid, 61 mM N-acetyl-l-cysteine and 3 mM4-hexylresorcinol. PPO inhibition by additives was affected by the pH of the buffer, and was more effectivewith ascorbic acid, N-acetyl-l-cysteine and calcium ascorbate. It was concluded that inferences on tissuebrowning based on PPO activity can be misleading. Measurement of tissue color is proposed as a reliablemeans to assess the antibrowning effectiveness of additives and the pH of additives for cut pear shouldbe corrected to reduce the browning potential