The flow of liquid foods in an agitated vessel using PEPT: Implications for the use of TTI to assess thermal treatment

The motion of Time Temperature Integrators (TTI) has been assessed within a scale model vessel designed for thermal processing of viscous complex food fluids using Positron Emission Particle Tracking (PEPT). The vessel, based on the 'Vesuvio' vessel made by Giusti Ltd. was filled with water and starch solutions of varying rheology and the effect of rotational speed and fill height was assessed. PEPT was used to measure both the motion of the fluid and the TTI separately, since for the TTI to give a reliable measure of the thermal processing it should be isokinetic and follow the fluid streamlines. For low viscosity fluids, significant settling of the particle relative to the fluid was observed. This did not occur for the more viscous starch solutions where the TTI and fluid behaved similarly regardless of the mixing quality, which was significantly affected by changing fill height and rotational speed. Sub-circulatory regions observed close to the impeller could lead to overprocessing of the food fluids based on TTI measurements in these regions, since they would be expected to be in the coldest part of the vessel. This study shows that TTI can be applied to the processing of viscous fluids within agitated thermal processes, provided that requisite knowledge of the fluid motion and likelihood of settling of the TTI is assessed in tandem. Industrial relevance: Time Temperature Integrators (TTI) are potentially very valuable tools for the measurement and validation of thermal processes. In this paper, conditions under which TTI would be expected to give valid thermal measurements in agitated batch vessels are assessed by measurement of their relative flow to the fluid using a novel visualisation technique. As well as ensuring the correct implementation of TTI, this work could be used to improve vessel design. (C) 2009 Elsevier Ltd. All rights reserved.