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Abstract: It was previously reported that Maillard reaction products (MRP) obtained from glucose with cysteine (1M/0.25M) mixtures and compounds generated during the mixing of heated cysteine with HMF solutions (Mix) were prone to inactivate various vegetal polyphenoloxidases (PPO). In this study, antioxidant properties of these model systems were compared using in vitro assays (AAPH°, DPPH° and TMM tests). Results showed that antioxidant activity observed in MRP and the Mix could be attributed to the sulfhydryl group of cysteine. The Mix behaved like a non-competitive inhibitor towards eggplant PPO (K i =0.7μM). A highly active fraction, devoid of thiol compound and HMF, was obtained after fractionation of the Mix by SPE. The Mix was as efficient as metabisulfite in preventing enzymatic browning of apple puree (CIE-La ∗ b ∗). MRP, the Mix and HMF could not be considered as mutagenic in the Salmonella microsome assay using Salmonella typhimurium TA98 and 102 strains. [Copyright &y& Elsevier]

Maillard reaction products (MRPs) prepared from aqueous equimolar glucose or fructose with glutathione (GSH) model solutions (0.25 M), when heated at 90 °C for 15–39 h, were previously recognized as strong apple PPO inhibitors. This paper reports the inhibition mode of the purified o-diphenoloxidase activity from apple, using 4-methylcatechol as the substrate. Assuming a reversible inhibition, Lineweaver-Burk plotting showed that both MRPs were mixed-type inhibitors, glucose/GSH being the most efficient model system, with Ki values ranging between 11.6 and 1.1 μl MRPs, according to the heating treatment and the sugar tested. Enzyme inactivation took place during pre-incubation (0–180 min) of PPO with various MRPs at 0 °C, and was only partly reversed by exhaustive dialysis or gel filtration. Activity initially lost was also partly restored when cupric ions (CuSO4) were added to the reaction medium, suggesting a chelating effect of copper ion at the active site of PPO, as already observed with cysteine-derived MRPs. MRPs derived from the tripeptide GSH were more potent inhibitors of apple PPO than those prepared from cysteine. [Copyright &y& Elsevier]

The effect of unheated and heated glutathione (GSH) and Maillard reaction products (MRPs), derived from equimolar mixtures (0.25 M) glucose or fructose with GSH, on purified apple polyphenoloxidase (PPO) activity was investigated by polarography and spectrophotometry, using 4-methylcatechol as the main phenolic substrate. When assayed alone, glutathione interacted with enzyme-generated o-quinones, giving rise to colourless conjugates, as demonstrated by high pressure liquid chromatography (HPLC). By polarography, increasing concentrations (0–300 mM) of GSH, resulted in a high inhibitory effect on PPO activity, mostly due to a drop of pH of the reaction solutions to acidic values. Upon heating GSH at 90 °C, thermal degradation product formation was responsible for a partial PPO inhibition. GSH-derived MRPs highly inhibited PPO activity, inhibition efficiency increasing with heating time (2–39 h) and temperature (80–110 °C). Compared with MRPs prepared from hexose with cysteine, those from GSH exhibited a more potent inhibitory effect, due to the presence of an additional carboxylic function on the tripeptide. Therefore, these MRPs could be considered as potential natural inhibitors for use with minimally processed fruits. [Copyright &y& Elsevier]

A previous study showed that Maillard reaction products (MRP) derived from equimolar glucose or fructose and l-cysteine model solutions (1 M) were potent inhibitors of polyphenoloxidase (PPO) from apple. In the present paper, a kinetic study was undertaken of the behaviour of a Michaelis–Menten PPO-catalyzed reaction in the presence of MRP heated at 90 °C for various times (2–39 h), using 4-methylcatechol as the substrate. Results obtained revealed that both MRP systems were mixed-type inhibitors, the glucose/cysteine model solution being the most effective one. Activity of preincubated PPO at 0 °C for different times with either heated l-cysteine or MRP was only slightly restored via exhaustive dialysis. Recovery of enzyme activity was more marked when cupric ions were added to the reaction medium. Possible mechanisms of inhibition of PPO activity by MRP are discussed, in relation to chelation of cupric ions. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]