Graphical abstract

The effect of heat treatment on the activities of three quality related enzymes:-peroxidase (POD), polyphenol oxidase (PPO), and lipoxygenase (LOX), from edible white yam (Dioscorea rotundata) was studied over a temperature range of 50 to 80°C using mathematical analysis of the kinetic and thermodynamic parameters for the thermoinactivation of the enzymes. Denaturation of these enzymes, measured by loss in activity, could be described by a simple first-order reaction that was resolved into biphasic inactivation curves. This indicates the existence of two isoforms of different thermal stabilities with k-values between 0.032 and 0.525. D-values decreased with increasing temperature, indicating faster inactivation of the enzymes at higher temperatures. Results suggested that peroxidase is relatively more thermostable than polyphenol oxidase and lipoxygenase with a Z-value of 4.11, Ea of 2510kJ mol -1 for the first phase of the biphasic inactivation reaction. The Gibbs free energy (ΔG), values range from -552.95 to 279.01kJ/mol for the three enzymes. The results indicate that the oxidation reactions were: (1) not spontaneous (ΔG > 0) for peroxidase at low temperatures, (2) spontaneous (ΔG < 0) for lipoxygenase (3) slightly endothermic (ΔH > 0) for lipoxygenase and (4) reversible (ΔS < 0) for all the three enzymes at all temperature. The high z-value obtained for peroxidase in the first phase of inactivation indicates that a high amount of energy was required to initiate its denaturation, and supports why it is used as a marker for inactivation of quality-related enzymes. Materials and Methods Preparation of homogenatesFresh uninfected yam tubers were peeled, washed and diced into Citation: Eze SOO, Chilaka FC, Nwanguma BC (2010) Studies on Thermodynamics and Kinetics of Thermo-Inactivation of Some Quality-Related Enzymes in White Yam (Dioscorea rotundata). J Thermodyn Catal 1:104.

Polyphenol oxidase and peroxidase were puri®ed from white yam (Dioscorea rotundata) using DEAE-cellulose ionexchange chromatography. Thermoinactivation curves for polyphenol oxidase showed monophasic kinetics, while those for peroxidase were biphasic. Urea partially stabilised peroxidase against irreversible thermoinactivation, but did not do so in the case of polyphenol oxidase. Only peroxidase was capable of regenerating activity after thermoinactivation. The results showed that thermoinactivation of peroxidase was mainly due to conformational changes, while that of polyphenol oxidase was probably due to covalent damage. Peroxidase reactivation might play an important role in the browning of processed yam. INTRODUCTIONDespite the importance of yam as a major food crop in West Africa, its postharvest biochemistry has been poorly studied. 1 As a consequence, postharvest losses due to sprouting, physical damage, pests and pathogens have remained relatively high. 2 One relatively successful procedure for improving the postharvest storage of white yam (Dioscorea rotundata) tubers involves processing into¯our. This consists of (1) peeling off the outer covering, (2) cubing, (3) boiling in distilled water for 10±15 min, (4) drying and (5) milling into¯our. Unfortunately, consumer acceptability and the shelf-life of the¯our are adversely affected by the inability of the¯our to retain the typical white colour of yam which is highly desired by consumers. 3,4 Colour changes in freshly damaged plant materials have been attributed to the activity of polyphenol oxidase (PPO; o-diphenol:O 2 oxidoreductase, EC 1.10.3.2), which catalyses the oxidation of polyphenols to o-quinones. 5,6 Peroxidase (Px; donor: hydrogen peroxide oxidoreductase, EC 1.11.1.7) catalyses the oxidation of a number of aromatic compounds and has been associated with darkening in fresh and processed vegetables and fruits. 7 Both PPO and Px are known to be highly heat-resistant but each displays a different capability for reactivation after thermoinactivation. 5,8±11In D rotundata, which contains PPO, 3,4,12 correlating PPO activity with tissue browning was shown to be contraindicative. 13 The purpose of the present study was to relate the thermoinactivation processes of PPO and Px to the discolouration of processed yam.

There is an increasing demand for amylolytic products such as glucose syrup in biotechnological industries. Starch liquification by α-amylase must take place at temperatures higher than the gelatinization temperatures and the use for thermostable α-amylase is therefore required. Two (or) isotypes namely amy-1 and amy-2 of α-amylase from pearl millet have been investigated for their thermostability and thermodynamic characterization.Thermal inactivation of α-amylase follows first order kinetics which varies with time and product during inactivation process. The half-life of α-1 was 198 mins at 50ºC. The Ea (inact) was calculated using Arrhenius plot gave 22.00 KCalmol K -1 at 50ºC. This suggest that the α-1 and α-2 are thermostable.

An acidic peroxidase was extracted from garlic (Allium sativum) and was partially purified threefold by ammonium sulphate precipitation, dialysis, and gel filtration chromatography using sephadex G-200. The specific activity of the enzyme increased from 4.89 U/mg after ammonium sulphate precipitation to 25.26 U/mg after gel filtration chromatography. The optimum temperature and pH of the enzyme were 50°C and 5.0, respectively. The Km and V max for H2O2 and o-dianisidine were 0.026 mM and 0.8 U/min, and 25 mM and 0.75 U/min, respectively. Peroxidase from garlic was effective in decolourizing Vat Yellow 2, Vat Orange 11, and Vat Black 27 better than Vat Green 9 dye. For all the parameters monitored, the decolourization was more effective at a pH range, temperature, H2O2 concentration, and enzyme concentration of 4.5–5.0, 50°C, 0.6 mM, and 0.20 U/mL, respectively. The observed properties of the enzyme together with its low cost of extraction (from local sources) show the potential of this enzyme for practical application in industrial wastewater treatment especially with hydrogen peroxide. These Vat dyes also exhibited potentials of acting as peroxidase inhibitors at alkaline pH range.