Residence time distribution (RTD) in aseptic processing of particulate foods: a review

Abstract

Aseptic processing technique has been successfully applied to liquid foods and acid foods containing discrete particulates. However, the extension of aseptic processing to low-acid heterogeneous liquid foods containing discrete particulates has been difficult due to lack of data on critical factors such as interfacial heat transfer coefficient between the liquid and the particle ( h fp) as well as the residence time distribution of particles in the holding tube of the aseptic system. Conventional thermal processing calculation methodology cannot be employed for the establishment of these processes, because of the difficulties associated with gathering experimental time-temperature data at the particle centre as it travels through the aseptic system. Mathematical modelling followed by biological verification has been attempted as a possible alternative. These models require accurate data on the thermo-physical properties of the particles, associated fluid to particle heat transfer coefficient ( h fp) as well as residence time distribution (RTD), especially in the holding section of the system. Both h fp and RTD depend on several factors which may also be interdependent: Theological properties, flow rate, temperature, and density of the carrier fluid, shape, density and concentration of the solid particles, as well as holding tube diameter and length. Rheological properties of the carrier fluids have been shown to influence fluid velocity profiles in the holding tube, and intuitively the food particle RTD in both the heat exchangers and the holding tube. The foci of this review are residence time distribution (RTD) of food particles in aseptic processing systems and the different experimental techniques used to gather RTD data as well as mathematical models used to describe RTD curves.