J. Noronha

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.

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.

Simultaneous optimisation of surface quality during the sterilisation of packed foods using constant and variable retort temperature profiles

Abstract The differences in temperature dependence between spore inactivation and degradation of quality factors allow the optimisation of thermal processes in terms of maximisation of quality retention by choosing an optimum heating profile. While the maximisation of the retention for a single quality factor has received considerable attention in research, the possibilities of simultaneously maximising the retention of different quality factors have up to now not been addressed. In this article the possibilities of the simultaneous optimisation for more than one quality factor were theoretically assessed. The use of both constant and variable retort temperature profiles was considered. A special emphasis was given to the formulation of appropriate objective functions for the simultaneous optimisation of the surface retention of quality factors. For the simultaneous optimisation of quality factors the objective functions should be formulated in terms of maximising final retention and not, as in the case of single component optimisation, in terms of minimisation of cook values. The use of variable retort temperature profiles was shown to be particularly interesting for the simultaneous optimisation of more than one quality factor, as the final retention calculated compared well with the maximum retention achieved using individual calculated optimum constant retort temperature control for each of the components.

A critical analysis of mathematical procedures for the evaluation and design of in‐container thermal processes for foods

Kinetic data on thermal destruction of spoilage and quality factors coupled with the temperature history of a product during a heat sterilization cycle are the basic information needed for the evaluation and the design of thermal processes through a physical-mathematical approach. A critical review on the available physical-mathematical procedures used for thermal process calculations of in-container processed foods is presented. The origin and the limitations of each method are discussed. The equations associated with each method, for internal product temperature predictions, are explicitly given. The relative performance of selected methods under identical processing conditions is illustrated. Several problems associated with thermal process calculations are discussed.

Evaluation of process deviations, consisting of drops in rotational speed, during thermal processing of foods in rotary water cascading retorts

Abstract Taking into account the similarities of broken-line heating curves and heat-penetration curves obtained when a product undergoes a drop in rotational speed during thermal processing in a rotary water cascading retort, a semiempirical method used for handling broken-line heating behaviour is suggested as a method of dealing with this type of process deviation. For this purpose, the possibility of extrapolating the empirical heating rate parameter f lv determined on the heat-penetration curve of white beans in brine processed in a still retort, to the ‘still’ conditions associated with a rotational speed drop in a rotary water cascading retort, was investigated. Following this approach, safe product temperature predictions were obtained. The main criterion influencing the quality of the evolution was the determination of the correct empirical parametric value to be used. This means, when determining this parametric value, care is required when selecting the correct temperature interval on the heat-penetration curve.

Combined use of the equivalent point method and a multicomponent time-temperature integrator in thermal process evaluation: influence of kinetic characteristics and reference temperature☆

Abstract Possibilities for and restrictions to the combined use of a multi component time-temperature integrator (TTI) and the equivalent point method (EPM) to quantify the impact of a thermal process were considered in this theoretical study. The impact on Clostridium botulinum spores, chosen as target quality attribute in this study can be represented fairly by an ‘equivalent point’ when the activation energies of the constituents making up the multicomponent TTI are close to the activation energy of the target quality attribute (error ⩽1%) but substantial errors (up to 60%) may arise when the activation energies of target and multicomponent TTI constituents differ largely. By reconsidering the temperature histories with three different reference temperatures (∞, 423K, 394.1 K). the importance of this rescaling parameter was found to be negligible. The combined use of a multicomponent TTI and the EPM to quantify the impact of a thermal process will require careful verification.