Effect of Residence Time Distribution in Flash Pasteurizers on the Thermal Death of Microorganisms

To date the calculation of the applied heat load in flash pasteurization plants in terms of Pasteurization Units (PUs) implies several inaccuracies. In this study, the residence time distribution (RTD), which is neglected in practice, was analyzed. Commonly for the determination of PUs the theoretical mean value of the residence time is used. This research investigates, exemplary in a pilot plant, the influence of the RTD on the resulting reduction of the cell count. RTDs were measured with model liquids for beverages (low viscosity) at different flow velocities and for syrups and beverage compounds (high viscosity). The data were used to estimate the microbial inactivation effect by calculations, considering the microbial inactivation kinetics following a first order reaction. For comparison also the survival kinetics were calculated from the mean holding time as result of the residence time distribution. A change in the flow rate, while still ensuring turbulence, leads to a decrease of the count reduction by up to 1.4 orders of magnitudes induced by the RTD. For laminar flows, due to high viscosities, a reduction of 5 orders of magnitudes was obtained, corresponding to a PU reduction of 20 % assuming a D (60 degrees C) - value of 1 min. In practice, safety margins in the PU parameterization can now better be adjusted with the knowledge about the RTD impact. The PUs can be individually adapted to the plant and to the product resulting in a safer and a more gentle processing combined with possible energy savings. Thus, the RTD provides a relevant mean for the practice of pasteurization in the case of low Re numbers and small D-values. The impact of RTD is hence not generally negligible.