Inhibitory effect of unheated and heated d-glucose, d-fructose and l-cysteine solutions and Maillard reaction product model systems on polyphenoloxidase from apple. I. Enzymatic browning and enzyme activity inhibition using spectrophotometric and polarogr

To demonstrate whether caramelization products (CP) and Maillard reaction products (MRP) could inhibit enzymatic browning and/or inactivate apple polyphenoloxidase (PPO), l-cysteine, d-glucose, d-fructose aqueous solutions and equimolar mixtures (1 M) of hexose/cysteine were each tested on purified enzyme activity, using both spectrophotometric and polarographic methods. Inhibition was evaluated as a function of temperature (80–110 °C), time (0–48 h) of heating and concentration of reactants. High (1–2.7 M) concentrations of hexoses were needed to develop a slight inhibiting effect on PPO activity. Heating at 90 °C for extended time periods, increased their inhibitory effect. Unheated or heated cysteine solutions exhibited an inhibitory effect in terms of browning intensity, without noticeably affecting PPO activity. Conversely, MRP showed a very strong inhibitory potency. Inhibition efficiency increased with heating time and temperature. The extent of inhibitory effect was positively correlated with absorbance measurements of MRP at 350 nm, used as an indicator of the Maillard reaction development.