Browsing by Author "Tobback, P."
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- Analysis of the inactivation kinetics of freeze-dried á-amylase from bacillus amyloliquefaciens at different moisture contentsPublication . Saraiva, J.; Oliveira, J. C.; Hendrickx, M.; Oliveira, F. A. R.; Tobback, P.The thermal inactivation kinetics of freeze-dried á-amylase in a solid matrix was studied at water contents ranging from 1.5 to 23.9 g water per 100 g dry solid. These conditions were obtained by equilibration in dry environments, with water activities ranging between 0.11 and 0.88. Isothermal inactivation experiments in the range 135 to 150 °C were performed. Results were analysed with both the Bigelow and Arrhenius models. It was concluded that there was no statistical significance to suggest that the water content influenced the kinetic parameters. An activation energy of 128 kJ/mol and pre-exponential factor with a logarithm of 33.9 described all the results well.
- Critical evaluation of commonly used objective functions to optimize overall quality and nutrient retention of heat-preserved foodsPublication . Silva, C.; Hendrickx, M.; Oliveira, F.; Tobback, P.Two objective functions—volume average retention and volume average cook-value—commonly used to optimize the overall quality and nutrient retention of heat-preserved foods, were studied and optimal processing conditions based on the two approaches were compared. Case studies from the literature were run and tested for the two objective functions. The influence of the decimal reduction time (at the reference temperature, Dref value) of the quality factor on the optimal processing temperature was evaluated. For high Dref values, e.g. for vitamins, the two objective functions give the same results and the Dref value has a minor influence on the optimum processing conditions. For low Dref values, such as for texture and colour, the optimal sterilization temperature is affected by this parameter and the use of volume average cook-value, as objective function, does not take this effect into account, which may result in underestimation of the optimal processing temperature.
- Generalized (semi)-empirical formulae for optimal sterilization temperatures of conduction-heated foods with infinite surface heat transfer coefficientsPublication . Hendrickx, M.; Silva, C.; Oliveira, F.; Tobback, P.A computer program was developed to model heat sterilization of one-dimensional conduction-heated foods with negligible surface resistance to heat transfer. The optimal sterilization temperature, defined as the processing temperature that results in a food product with minimum surface cook-value after achieving the desired degree of sterility, was calculated as a function of food properties (thermal diffusivity, Z-value of the quality factor), processing conditions (dimensions and geometry of the food or container, initial temperature of the product, heating medium come-up-time) and processing criteria (target F0-value). The initial temperature of the food and the heating medium come-up-time have a minor effect on the optimal temperature. Generalized (semi)-empirical formulae relating optimal sterilization temperatures and all relevant variables were developed.
- Optimal sterilization temperatures for conduction heating foods considering finite surface heat transfer coefficientsPublication . Silva, C.; Hendrickx, M.; Oliveira, F.; Tobback, P.Optimal sterilization temperatures are defined as the processing temperatures which result in a minimum surface cook-value after achieving the desired degree of sterility. They were calculated as a function of product heating rate, surface heat transfer coefficient, initial food temperature, heating medium come-up-time, z-value for the quality factor and target Fo-value. Different one-dimensional heat transfer shapes were considered. Compared to the other variables, initial temperature and heating medium come-up-time had little influence on optimal processing temperature. Regression equations were developed relating optimal temperatures with all relevant variables.
- Optimization of heat transfer in thermal processing of conduction heated foodsPublication . Hendrickx, M.; Silva, Cristina L. M.; Oliveira, F.; Tobback, P.Optimal sterilization temperature was defined as the temperature resulting in a minimum surface cook-value product together with the desired degree of sterility. Sterilization of conduction heated foods, with simple shapes as infinite slab, infinite cylinder and sphere, was modelled. The model assumed first order inactivation kinetics for microorganisms and quality. The temperature distribution was solved by an explicit finite difference numerical method with a non-capacitance surface node. Optimal temperatures were calculated as a function of : i) Food Properties - thermal diffusivity, Z-value for the quality factor, Processing Conditions - dimensions and geometry of the food or container, surface heat transfer coefficient, initial product temperature, heating medium come-up-time and Processing Criteria - target Fovalue.Initial food temperature and heating medium come-up-time have a minor influence on the optimal conditions. Generalized (semi)-empirical formulas relating quantitatively optimal temperatures and all relevant variables were developed. The generalized equations are a valuable tool to calculate or estimate processing sterilization conditions for regularly and irregularly shaped products. They were successfully applied to predict the optimal sterilization temperatures for conduction heated foods, packaged in typical cylindrical cans and retortable pouches, with different surface resistances to hea, transfer.
- Optimizing thermal processes of conduction heated foods: generalized equation for optimal processing temperaturesPublication . Hendrickx, M.; Silva, C.; Oliveira, F.; Tobback, P.Optimal sterilization temperatures, for maximum surface quality retention, were calculated as a function of : (I)product propertles (thermal diffusivity and z.value) (II) processlng conditions Igeometry and dlmensions of the food, surface heat transfer coefficient, initial product temperature and retort come up time) and z-value (III) processing criteria (target- Fo.value). From this theoretical study generalized empirical relations relating optimal temperatures and all relevant variables were formulated and presented. Applications in classical canning are lllustrated in detail.
- The influence of water activity on thermal stability of horseradish peroxidasePublication . Hendrickx, M.; Saraiva, J.; Lyssens, J.; Oliveira, J.; Tobback, P.The thermal stability of horseradish peroxidase in the solid state was studied as a function of water activity, from 0.11 to 0.88. At all activities the enzyme was found to be much more stable in the solid state than in solution. Inactivation temperatures were in the range of 140–160°C. Inactivation curves show a biphasic behaviour which can be described by a model assuming two fractions (heat labile and heat stable) with independent first order inactivation kinetics. The labile fraction represents approximately 30% of the total activity. The z-value for both stable and labile fractions depends on water activity (moisture content) and has a maximum at aw= 0.76 (44.4°C and 43.8°C, respectively).