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An innovative method was developed to study the specific uncoupled effect of temperature and water activity (Aw) during mango drying. Thermal treatment of reconstituted lyophilized mango powder at targeted Aw enabled evaluation of the inactivation of polyphenol oxidase and peroxidase activity in a range 0.98–0.6 Aw and a time-temperature range of 1–4 h and 50–60°C. We established that thermal treatment of the enzymatic extract was not the same as the inactivation phenomena that occurred in the mango matrix, mainly due to differences in the pH of the buffer extract (6.4) and reconstituted food matrix (3.8). Even if the inactivation of these two enzymes was directly correlated with the temperature of the treatment, this behavior was also strongly linked to the Aw. A two-step change in the fruit was observed at high Aw (i.e., at 0.98 in fresh fruit), polyphenol oxidase (PPO) was shown to be more thermostable than peroxidase (POD), whereas when evaporation reduced the Aw to around 0.6, the opposite behavior was observed. As a consequence, enzymatic inactivation during drying should be considered as the result of the coupled effects of time-temperature and water activity. [ABSTRACT FROM AUTHOR]

Abstract: The effects of conventional heating (C) or ohmic heating (O) treatment on the degradation of sugar and ascorbic acid (AA) were evaluated in chunky fruit desserts prepared with apple puree (Golden Delicious var.) and chunky peach (Panavi var.) pieces. Concentrations of 5-HMF, furfural (F), 3-hydroxy-2-pyrone (3H2P) and 2-furoic acid (FA) were determined to assess the importance of oxidative and/or thermal reactions during processing. Depending on the thermal processing, 5-HMF and F levels in samples ranged from not detected to 3260 and from not detected to 570 μg/100 g FW respectively. Production of 3H2P and FA ranged from 480 to 2670 and from 84 to 420 μg/100 g FW respectively. Results showed that the impact of final heating treatment on the formation of AA degradation products (3H2P and FA) was minor, if any. Thus, AA degradation during apple products processing mainly depended on oxidative reactions pathway. Likewise, ohmic heating processing had very little effect on the formation of 5-HMF and F, in opposite to conventional heating. [Copyright &y& Elsevier]

Abstract: Pulsed electric field (PEF) treatment (E =1000V/cm, f=200Hz and t p =100μs) was applied to French cider apple mash pumped into a collinear treatment chamber at the flow rate of 280kg/h. Juices were recovered continuously using a single belt press. PEF treatment of the mash (32ms and 46kJ/kg) increased the juice yield by 4.1%. The content of total native polyphenols decreased by 17.8% in the treated juice due to oxidation by polyphenoloxidase. Meanwhile, the activity of this enzyme in PEF-treated juices was also decreased by 18.3%. It was suggested that PEF treatment enhanced the oxidation of native polyphenol compounds in cells because of electroporation of the inner cell membrane. The loss of PPO activity was related to the inhibition of the enzyme by the oxidised phenolic compounds. For this reason, one of the oxidation markers (molecular ion [M-H]− of chlorogenic acid dimer m/z=705) was monitored in the juices and electric treatment was observed to increase the content of this compound by 6.8% in the treated samples. A significant difference in the colour (DE= 6) in the L*a*b* space was detected between the control and treated juices. The colour of the treated juice was the most appreciated attribute among the sensorial panel when compared to the control. The overall chemical composition of the treated juices was not different when compared with the respective controls. Industrial relevance: The mechanical expression of fruit juices assisted by pulsed electric fields (PEF) has received considerable attention over the past few years and will most likely be scaled up for industrial applications. Consequently for the legal approval of such a process the in-depth characterisation of the treated product is required. Through this study, a high potential of PEF treatment for the sensorial improvement of juices was found and limited differences in the nutritional quality characteristics between untreated and PEF-treated apple juices were observed. However, more work regarding the mechanisms of changes during PEF treatment is required on both the pilot and industrial scales. [Copyright &y& Elsevier]

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]

The effects of Maillard reaction products (MRPs), synthesized from equimolar glucose or fructose with l-cysteine (1 mol l−1) aqueous model mixtures, by modulating pH and temperature of heating, according to a two-factor and five-level experimental design, were investigated on polyphenoloxidase (PPO) activity from apple. Final pH and absorbance measurements at 350 nm were also selected as indicators of the Maillard reaction development and checked. In general, inhibitory potency (IP) of the mixtures increased with the increase in temperature (80–120°C) and the decrease in pH (pH 2.0–12.0) of the reaction medium. A linear relationship between the IP and heating time (0–48 h) or Abs.350 nm (0–70 AU) was demonstrated for glucose/cysteine system heated from 80°C to 120°C. Polarographic and spectrophotometric data were used to calculate kinetic constants and activation energy (Ea) values of inhibitory MRPs formation versus PPO activity and of those compounds absorbing at 350 nm. Ea values for these reactions were close, being 191 and 124 kJ mol−1, respectively. The experimental design allowed to conclude that linear effects of both factors as well as a quadratic effect of pH were significant, leading to optimum conditions for the production of glucose-derived MRPs inhibitors. In most cases, glucose produced MRPs with higher IP compared to counterpart fructose–cysteine MRPs. [Copyright &y& Elsevier]

This review summarizes the life and work of Louis-Camille Maillard, the most worldwide renowned French biochemist of the 20th century. It focuses on the main events in Maillard's life and the reaction sugar–amino acids in his work, and offers an overview of the Maillard reaction in vitro and in vivo. [ABSTRACT FROM AUTHOR]

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]

Polyphenol oxidase (PPO) from eggplant was extracted and partially purified by a two-step fractionation-precipitation using ammonium sulfate and phenylsepharose hydrophobic interaction chromatography. The eggplant PPO extract was characterized concerning its kinetic properties. Optimal conditions to obtain Maillard reaction products (MRPs) with a maximal inhibitory potency (IP) toward PPO activity were determined using the surface response methodology and a four-factor and five-level experimental design. The MRPs were prepared from cysteine (0.25 M) and glucose (0-1 M), at several initial pH values (2-6) and at differing heating times (3-19 h) and temperatures (95-115 degrees C). The maximal IP was obtained after heating a model system of glucose/cysteine (1/0.25 M) at pH 2 for 3 h 20 min at 115 degrees C. The soluble part of this MRP, called MRP(IPmax), was a noncompetitive inhibitor toward eggplant PPO. The IP of MRP(IPmax) on PPO activity was very potent as compared to that displayed by benzoic, p-coumaric, and t-cinnamic acids, as well as sorbic acid and 4-hexylresorcinol. The activity of preincubated PPO at 0 degrees C with MRP(IPmax) was only slightly restored after dialysis or gel filtration.