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

Abstract

The effect of pH, phenolic substrates, and food additives on polyphenoloxidase (PPO) activity and on tissue browning was studied in fresh-cut 'Rocha' pear. Substrates 4-methylcatechol, caffeic acid, (+)catechin hydrate, catechol, chlorogenic acid, dopamine hydrochloride, and pyrogallol, were prepared in citric acid-phosphate buffer at pHs ranging from 3.0 to 8.0. 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. Discrepancies were observed between the pH dependency of PPO activity and browning, as assessed by objective color measurement. 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 substrate in 'Rocha' pear, induced high tissue browning but very low PPO activity at pH 3.0-4.0. Chemical inhibition of PPO was tested using catechol as substrate, and buffer solutions containing 250 mM Ca 2+ in four salts (ascorbate, chloride, lactate and propionate), 57 mM ascorbic acid, 61 mM N-acetyl-l-cysteine and 3 mM 4-hexylresorcinol. PPO inhibition by additives was affected by the pH of the buffer, and was more effective with ascorbic acid, N-acetyl-l-cysteine and calcium ascorbate. It was concluded that inferences on tissue browning based on PPO activity can be misleading. Measurement of tissue color is proposed as a reliable means to assess the antibrowning effectiveness of additives and the pH of additives for cut pear should be corrected to reduce the browning potential.

Evaluation of antibrowning and antioxidant activities in unripe grapes recovered during bunch thinning

“…Additionally, any evident antibrowning effect was observed on the zymogram of Abate pear PPO and on pear slices (Figure e) probably because of the low pH of the unripe grapes. In this regard, Gomes et al () demonstrated that the browning in fresh‐cut Rocha pear was not reduced by low pH values in the presence of catechol substrate. The variable antibrowning performance among plant PPOs confirmed that the inhibitory effectiveness was mainly related to the enzyme source, as spectrophotometrically demonstrated by some authors (Zocca et al , ).…”

“…The main agent responsible is polyphenol oxidase or tyrosinase (PPO; EC 1.10.3.1), a copper‐containing oxidoreductase that catalyses two different reactions involving the oxidation of phenolic substances and subsequent production of quinones that polymerise to brown pigments known as melanins (Seo et al ). The degree of browning is related to the type and concentration of endogenous phenolic substances; presence of oxygen, reducing substances and metallic ions; pH; and temperature that affects PPO activity (Nicolas et al , Gomes et al ). The enzyme reaction in agro‐food products leads not only to colour alteration but also to a reduction in the nutritional and sensory quality as a consequence of quinone condensation with amino acids and proteins (Rapeanu et al ) and of the degradation of phenolic substances, substrates recognised for their health benefits as antioxidants (Quideau et al , Mihaylova et al ).…”

The Correlation between Soil Nutrient and Potato Quality in Loess Plateau of China Based on PLSR

“…Determination of browning intensity was performed according to Nguyen et al [40]. The potato polyphenol oxidase activity was determined using the method described by Gomes et al [41] for the use of a 100 mM citrate acid-200 mM sodium phosphate buffer between pH 3.0 and 8.0. The enzyme extract (0.5 mL) was added to the quartz test tube and a buffer solution containing phenolic substrate (2.5 mL) was added.…”

Isolation, purification and identification of etiolation substrate from fresh-cut Chinese water-chestnut (Eleocharis tuberosa)

“…Sometimes, the CWC is also washed, peeled, sliced and packaged before being marketed as a ready-to-eat product. However, most of fruit and vegetables turn brown after being peeled (Sagar & Kumar, 2010), such as apples (Luo, Lu, Zhou, & Feng, 2011), pears (Gomes et al, 2014) and potatoes (ZvitovYa'ari & Nussinovitch, 2014). In contrast with other fruit and vegetables, fresh-cut CWC is prone to etiolation, reducing its shelf life and its commercial value (Ma, Wang, Hong, & Cantwell, 2010;Oms-Oliu et al, 2010;You et al, 2012).…”