Influence of pH and high pressure on the thermal inactivation kinetics of horseradish peroxidase

Kinetic Modeling, Thermodynamic Approach and Molecular Dynamics Simulation of Thermal Inactivation of Lipases from Burkholderia cepacia and Rhizomucor miehei

The behavior against temperature and thermal stability of enzymes is a topic of importance for industrial biocatalysis. This study focuses on the kinetics and thermodynamics of the thermal inactivation of Lipase PS from B. cepacia and Palatase from R. miehei. Thermal inactivation was investigated using eight inactivation models at a temperature range of 40–70 °C. Kinetic modeling showed that the first-order model and Weibull distribution were the best equations to describe the residual activity of Lipase PS and Palatase, respectively. The results obtained from the kinetic parameters, decimal reduction time (D and t_R), and temperature required (z and z’) indicated a higher thermal stability of Lipase PS compared to Palatase. The activation energy values (Ea) also indicated that higher energy was required to denature bacterial (34.8 kJ mol⁻¹) than fungal (23.3 kJ mol⁻¹) lipase. The thermodynamic inactivation parameters, Gibbs free energy (ΔG^#), entropy (ΔS^#), and enthalpy (ΔH^#) were also determined. The results showed a ΔG^# for Palatase (86.0–92.1 kJ mol⁻¹) lower than for Lipase PS (98.6–104.9 kJ mol⁻¹), and a negative entropic and positive enthalpic contribution for both lipases. A comparative molecular dynamics simulation and structural analysis at 40 °C and 70 °C were also performed.

Effect of Water Content on the Thermal Inactivation Kinetics of Horseradish Peroxidase Freeze-Dried from Alkaline pH

The thermal inactivation of horseradish peroxidase freeze-dried from solutions of different pH (8, 10 and 11.5, measured at 25 C) and equilibrated to different water contents was studied in the temperature range from 110 to 150 C. The water contents studied (0.0, 1.4, 16.2 and 25.6 g water per 100 g of dry enzyme) corresponded to water activities of 0.0, 0.11, 0.76 and 0.88 at 4 C. The kinetics were well described by a double exponential model. The enzyme was generally more stable the lower the pH of the original solution, and for all pH values, the maximum stability was obtained at 1.4 g water/100 g dry enzyme. Values of z were generally independent of water content and of the pH of the original solution, and in the range of 15–25 °C, usually found in neutral conditions, with the exception of the enzyme freeze dried from pH 11.5 and equilibrated with phosphorus pentoxide, where a z-value of the stable fraction close to 10 C was found.

Effects of High Pressure Processing on Infectivity ofToxoplasma gondiiOocysts for Mice

High pressure processing (HPP) has been shown to be an effective non-thermal method of eliminating non-spore forming bacteria from a variety of food products. The shelf-life of the products is extended and the sensory features of the food are not or only minimally effected by HPP The present study examined the effects of HPP using a commercial scale unit on the viability of Toxoplasma gondii oocysts. Oocysts were exposed from 100 to 550 MPa for 1 min in the HPP unit and then HPP treated oocysts were orally fed to groups of mice. Oocysts treated with 550 MPa or less did not develop structural alterations when viewed with light microscopy. Oocysts treated with 550 MPa, 480 MPa, 400 Mpa, or 340 MPa were rendered noninfectious for mice. Mice fed oocysts treated with no or 100 to 270 MPa became infected and most developed acute toxoplasmosis and were killed or died 7 to 10 days after infection. These results suggest that HPP technology may be useful in the removal of T. gondii oocysts from food products.