Mathematical models for combined high pressure and thermal plasmin inactivation kinetics in two model systems.

The combined high-pressure thermal inactivation kinetics of plasmin was studied in 2 model systems. The first system contained both plasmin and plasminogen, whereas, in the second system, all plasminogen was converted into plasmin, with urokinase, before the inactivation studies. High-pressure treatments were conducted in the range of 300 to 800 MPa combined with temperatures from 30 to 65 degrees C. Under all conditions of pressure and temperature (isobaric-isothermal) studied, for both systems, first-order inactivation was observed. A third-degree polynomial model (derived from thermodynamic principles) successfully described the temperature and pressure dependence of the inactivation rate constant over the entire experimental domain. The antagonistic effect and the stabilization effect observed above a threshold pressure value of 600 MPa were thought to be related to the disruption of disulfide bonds in plasmin and plasminogen.

High pressure thermal inactivation kinetics of a plasmin system.

A crude plasmin extract was prepared from milk by ultracentrifugation and was partially purified using ammonium sulfate precipitation. Isothermal and high-pressure inactivation of this plasmin system at pH 6.7 could be described by a first-order kinetic model. As expected, the plasmin system displayed a high thermostability. High-pressure treatments were conducted in the 300- to 800-MPa pressure range, combined with temperatures from 25 to 65 degrees C. The plasmin system was very pressure stable at room temperature, but inactivation occurred with combined high-pressure/temperature-treatments. The influence of temperature at different constant pressures on the inactivation rate constant was quantified using the Arrhenius equation. At all temperatures studied, a synergistic effect of temperature and high pressure was observed in the 300- to 600-MPa pressure range. However, an antagonistic effect of temperature and pressure appeared at pressures above 600 MPa.

Purified tomato polygalacturonase activity during thermal and high-pressure treatment.

Extracted tomato polygalacturonase was purified by cation-exchange chromatography (and gel filtration) and characterized for molar mass, isoelectric point, as well as optimal pH for polygalacturonase activity. The enzymatic reaction of purified tomato polygalacturonase on polygalacturonic acid as substrate was investigated during a combined high-pressure/temperature treatment in a temperature range of 25 degrees to 80 degrees C and in a pressure range of 0.1 to 500 MPa at pH 4.4 (the pH of tomato-based products). The optimal temperature for initial tomato polygalacturonase activity in the presence of polygalacturonic acid at atmospheric pressure is about 55 degrees to 60 degrees C. The optimal temperature for initial tomato polygalacturonase activity during processing shifted to lower values at elevated pressure as compared with atmospheric pressure, and the catalytic activity of pure tomato polygalacturonase decreased with increasing pressure, which was mostly pronounced at higher temperatures. The elution profiles of the degradation products on high-performance anion-exchange chromatography indicated that for both thermal and high-pressure treatment all oligomers were present in very small amounts in the initial stage of polygalacturonase activity. The amounts of monomer and small oligomers increased with increasing incubation times, whereas the amount of larger oligomers decreased due to further degradation.

Effects of combined pressure and temperature on enzymes related to quality of fruits and vegetables: from kinetic information to process engineering aspects.

Throughout the last decade, high pressure technology has been shown to offer great potential to the food processing and preservation industry in delivering safe and high quality products. Implementation of this new technology will be largely facilitated when a scientific basis to assess quantitatively the impact of high pressure processes on food safety and quality becomes available. Besides, quantitative data on the effects of pressure and temperature on safety and quality aspects of foods are indispensable for design and evaluation of optimal high pressure processes, i.e., processes resulting in maximal quality retention within the constraints of the required reduction of microbial load and enzyme activity. Indeed it has to be stressed that new technologies should deliver, apart from the promised quality improvement, an equivalent or preferably enhanced level of safety. The present paper will give an overview from a quantitative point of view of the combined effects of pressure and temperature on enzymes related to quality of fruits and vegetables. Complete kinetic characterization of the inactivation of the individual enzymes will be discussed, as well as the use of integrated kinetic information in process engineering.

Development of a novel methodology To validate optimal sterilization conditions for maximizing the texture quality of white beans in glass jars

Optimal thermal processes were designed for white beans in glass jars heated in a still and end-over-end rotary pilot water cascading retort. For this purpose, isothermal kinetics of thermal softening of white beans were studied in detail using a tenderometer and a texturometer. The fractional conversion model was applied in both cases to model the texture degradation. The Arrhenius equation described well the temperature dependence of the reaction rate constant. With regard to the heat transfer, heat penetration parameters (fh and jh) were experimentally determined from 100 containers under static as well as rotational (end-over-end) conditions at 4, 7, 10, and 15 rpm. Theoretical optimal temperatures, maximizing volume average quality retention, were calculated using a computer program valid for conduction heating foods. Experimental verification of the calculated results was conducted. Considering the finite surface heat transfer coefficient, theoretical and experimental optimal temperatures were of the same order of magnitude, around 130 degrees C, while for an infinite surface heat transfer coefficient the calculated optimum temperature was much lower than the experimental value. The type of reaction kinetic model, fractional conversion or first-order models, does not significantly affect optimal sterilization temperatures. Although some differences were found, the developed theoretical approach was successfully applied to convective and mixed heating mode products. The use of the correct surface heat transfer coefficient is crucial to design optimal processing conditions.

Recent advances in process assessment and optimisation.

After stating the general principle of food preservation, this paper focuses on currently available methods to evaluate quantitatively the integrated time temperature impact during and/or after a thermal preservation process. In this context, both the physical-mathematical approach and the use of time temperature integrators are briefly reviewed and recent evolutions are indicated. Also new trends with regard to thermal process optimisation are highlighted.