A. Van Loey

Effects of high electric field pulses on enzymes

Abstract The application of high electric field pulses (HELP) in food processing offers potential both as a preservation technology, and as an adjunct to other processes, such as drying, extraction. In respect of the use of pulsed electric fields as preservation technology, the impact of this technology on food quality degrading enzymes is of particular interest. This paper presents a detailed literature review on the effects of pulsed electric fields on enzymes, complemented with recent results on the impact of this HELP-technology on enzymes, obtained within EU project ‘High electric field pulses: food safety, quality and critical process parameters’ (contract FAIR-CT97-3044).

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

ABSTRACT 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.

Modeling Conductive Heat Transfer and Process Uniformity during Batch High-Pressure Processing of Foods

A numerical model for predicting conductive heat transfer during batch high hydrostatic pressure (HHP) processing of foods was developed and tested for a food simulator (agar gel). For a comprehensive evaluation of the proposed method, both “conventional” HHP processes, HHP processes with gradual, step‐by‐step pressure buildup and pressure release, and pressure cycling HHP processes were included. In all cases, good agreement between experimental and predicted temperature profiles was observed. The model provides a very useful tool to evaluate batch HHP processes in terms of uniformity of any heat‐ and/or pressure‐related effect. This is illustrated for inactivation of Bacillus subtilis α‐amylase, an enzymatic model system with known pressure‐temperature degradation kinetics.

Quantitative evaluation of thermal processes using time-temperature integrators

Abstract The further development of new heating technologies, process assessment and process optimization in the thermal processing of foods is limited by the applicability of currently used process evaluation methodologies. Therefore, considerable effort has been and will continue to be put into the development of specific sensors — time-temperature integrators (TTIs) — which can allow faster, easy and correct determination of the impact of a process on a product attribute without the need for detailed knowledge of the actual time-temperature history of the product. This article presents the current state of the art regarding TTI development to monitor thermal processes, as well as a discussion of the possible applications and limitations of several types of TTI systems.

Thermal and combined pressure--temperature inactivation of orange pectinesterase: influence of pH and additives.

Inactivation of commercially available orange pectinesterase (PE) was investigated under isothermal and isothermal-isobaric conditions. In both cases, inactivation data could be accurately described by a fractional conversion model. The influence of enzyme concentration, pH, Ca(2+) concentration, and sucrose on the inactivation kinetics was studied. Enzyme stability against heat and pressure increased by increasing enzyme concentration. An increased Ca(2+) concentration caused sensitization to temperature and increased the residual fraction active PE after thermal treatment. To the contrary, in the case of pressure treatment, decreasing Ca(2+) concentrations increased pressure inactivation. The remaining fraction active PE after pressure treatment was not influenced by the addition of Ca(2+) ions. Acidification accelerated thermal as well as pressure-temperature inactivation, whereas in the presence of sucrose an increased temperature and pressure stability of orange PE was observed. Sucrose had no influence on the remaining activity after thermal treatment, but it increased the residual fraction after pressure treatment. The remaining fraction was for all additives studied independent of the pressure and temperature level applied except for the inactivation in an acid medium, when a decrease of the residual fraction was observed with increasing temperature and pressure.

New semi-empirical approach to handle time-variable boundary conditions during sterilisation of non-conductive heating foods

Semi-empirical methods for the prediction of time-temperature histories in conductive and non-conductive (convective and mixed mode) heating foods subjected to a time-variable processing temperature are proposed. Four alternatives are considered: (i) Hayakawas method (Duhamels theorem and Hayakawas formulae); (ii) Duhamels theorem with analytical solution; (iii) numerical solution with apparent time (time shift); (iv) numerical solution with apparent position. The incorporation of the empirical heating characteristics f and j in conductive models was accomplished by evaluating the existing analogies with thermophysical properties in the solutions of the Fourier equation. Approaches using Duhamels theorem or finite difference solutions were used to handle variable boundary conditions. The application of the models in the calculation of processing values for thermal processes with different come up times and different boundary conditions during come up time and thermal processes with process deviations is discussed. The numerical solution with apparent position was preferred because it combines accuracy and flexibility.

Single, Combined, or Sequential Action of Pressure and Temperature on Lipoxygenase in Green Beans (Phaseolus vulgaris L.) : A Kinetic Inactivation Study

The objective of this investigation was to study kinetically the effect of pressure and temperature either as a single, as a combined, or as a sequential action on lipoxygenase (LOX) inactivation in crude green beans extract. The LOX isozymes in green beans extract had a different heat sensitivity but a similar pressure stability: two fractions following apparent first‐order reactions, i.e., a heat‐labile fraction and a heat‐stable fraction, were observed in studies on its thermostability, whereas only one fraction following a first‐order reaction was noticed in studies on its pressure stability. At ambient pressure, irreversible LOX inactivation was studied in a temperature range from 55 to 70 °C. At room temperature, pressures around 500 MPa were required in order to inactivate LOX in green beans extract. The effect of a pressure or a thermal pretreatment on LOX thermo‐ or barostability, respectively, was also investigated but no significant differences in inactivation kinetics due to the pretreatment were observed.

Evaluation of process value distribution with time temperature integrators

Abstract The performance of an enzyme-based Time Temperature Integrator (TTI) in monitoring the spatial distribution of processing values was evaluated. After proper kinetic calibration, processing values calculated from the response of free α-amylase ( z = 7°C, 90–115°C) were found to match processing values obtained from a variable temperature history with the general method in laboratory heating experiments where TTIs were embedded in a spherical teflon food model particle. These systems were then applied as wireless monitors of processing value distributions under various processing conditions (viscosity of the brine, End-Over-End rotation, headspace) in a pilot retort. The particle-to-particle variation in processing values for an in-pack heat treated food model system could readily be determined with this TTI. The possibilities of such wireless systems in heat penetration studies, design and optimization for processing conditions which are not readily accessible to physical experimentation are indicated.

Kinetics of quality changes of green peas and white beans during thermal processing

Abstract The design of optimal thermal processes relies on relevant and accurate kinetic data for bacterial inactivation and quality evolution. By use of taste panels, kinetic data for quality evolution of peas (Pisum sativum L., var. Wavette) and white beans (Phaseolus vulgaris, subsp. nanus Metz., var. Manteca de leon) were obtained. The thermal destruction kinetics of these quality attributes was described using two alternative models, the thermal death time model (z- and D-value) and the Arrhenius model (Ea- and k-value). Kinetic parameters were estimated by use of three least squares methods: two-step linear regression, multiple linear regression and nonlinear regression. Multilinear regression analysis was selected for parameter estimation. The following temperature coefficients were obtained: colour of peas (z = 26.4 °C; E a = 102.4 kJ mole ), hardness of peas (z = 28.5 °C, E a = 94.9 kJ mole ) hardness of beans (z = 21.3 °C, E a = 130.8 kJ mole ) and appearance of beans (z = 24.3 °C, E a = 118.7 kJ mole ).

Influence of rotational speed on the statistical variability of heat penetration parameters and on the non-uniformity of lethality in retort processing

Abstract In a case study on white beans, the effect of rotation on the statistical variability of heat penetration parameters and on the non-uniformity of lethality in industrial-scale retort processing was investigated. In addition, the influence of process time on the non-uniformity of lethality was evaluated. No clear relation between the non-uniformity of the heating parameters and the rotational speed was observed. Rotation seemed to influence process lethality variability throughout the retort by its effect on non-uniformity in heating characteristics and in retort temperature. Absolute non-uniformity (i.e. standard deviation) increased with increasing holding time, whereas relative non-uniformity (i.e. coefficient of variation) decreased as holding time increases.