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Evaluation of the thermal and mixing performance of an agitated vessel for processing of complex liquid foodstuffs

Mehauden, Karin (2009)
Ph.D. thesis, University of Birmingham.

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Abstract

Thermal treatment is the most common method used by industry to ensure food is safe for consumption and to increase its storage life. To ensure safety, food is often overprocessed which can significantly affect its nutritional value as well as taste and flavour attributes. In this study, the heating and mixing efficiency of a bespoke vessel used for heat treatment of complex foodstuffs (250 litre ‘Vesuvio’ vessel manufactured by Giusti Ltd) was investigated. Enzymatic Time Temperature Integrators (TTIs) were used to determine the heat treatment efficiency. TTIs are small unattached measurement devices which contain a thermally labile enzyme: determination of the degree of degradation of the enzyme at the end of the thermal process enables the integrated temperature history to be obtained. TTIs can be used for process validation, particularly when the processing environment is inaccessible for fixed devices such as thermocouples. The reliability and accuracy of the TTIs was determined by exposure to various non isothermal industrially relevant temperature profiles using a Peltier stage and Polymerase Chain Reaction (PCR) device. The integrated temperature histories obtained by the TTIs’ correlated generally well with data obtained from thermocouples installed in parallel, although the error increases with holding time of the heat treatment. The work showed that the TTIs can be used reliably over a range (e.g. Enzymatic TTI made from the α-amylase from the Bacillus Licheniformis can reliably used from 5 to 30 minutes at 85°C) which is relevant for conditions of thermal pasteurisation of interest to this study. The range of time temperature profiles that enzymatic TTIs can monitor depends on the thermal resistance of the enzyme. The heat treatment efficacy of the ‘Vesuvio’ vessel was evaluated using TTIs and two thermocouples fixed onto the vessel wall and impeller shaft at the centre of the vessel. In addition to the plain or ‘free’ TTIs, a new TTI was developed where it was placed at the centre of an open structure to prevent intimate contact between the surface of the TTI and the vessel wall (‘Golf Ball’ and ‘Tie Clip’ TTIs). The food fluid could, however, penetrate the structure. The parameters examined in the study were fluid rheology, fill level (100% and 120% filling level) and the heating options (steam heating via wall jacket or direct injection). The study showed that the thermal process efficiency is lowered as the fluid viscosity increases when the wall jacket was used alone; this was observed by greater differences between the temperatures recorded by the thermocouples between the centre and the vessel wall. This was overcome by using direct steam injection into the vessel contents. Overfilling the vessel was also found to affect performance. The ‘free’ TTIs were found to have a higher thermal treatment than the TTIs which could not directly contact the wall. Under perfect mixing conditions, the ‘free’ TTIs and the TTIs placed inside the open structure should both give close results. However, this is not the case and it can be seen that the discrepancy increases when the mixing conditions worsen (increase of the fluid viscosity, no use of steam injection). The reliability of the TTIs as a validation tool is dependent upon their following the same path as the food fluid, i.e. they should be isokinetic and follow the fluid streamlines. To investigate this issue, the flow of both fluid and TTIs was examined on a reduced scale version of the ‘Vesuvio’ vessel using Particle Image Velocimetry (PIV) and Positron Emission Particle Tracking (PEPT). The effect of changing fluid rheology, agitation speed and filling level were investigated on the basis of a scaling at constant power per unit mass. The PIV experiments showed that the flow was laminar/transitional through bulk of vessel, with significant flow instabilities at the free surface and at the trailing edge of the impeller. Bulk mixing can therefore be expected to occur by laminar mechanisms with some mixing by eddy diffusion present at the free surface. The mixing pattern was not affected by rheology or agitation speed, however, overfilling of the vessel appeared to move the centre of the fluid rotation to above the impeller shaft, as verified using PEPT. PEPT was also applied by inserting either the free tracer into the fluid or placing it within a TTI. Significant differences in the path taken by the TTI and the fluid were observed when the TTI had a significant settling velocity in the fluid. Hence TTIs cannot be assumed to give reliable results in low viscosity fluids (e.g. water).

Type of Work:Ph.D. thesis.
Supervisor(s):Fryer, P. J. and Simmons, Mark J. H. and Cox, Philip William
School/Faculty:Colleges (2008 onwards) > College of Engineering & Physical Sciences
Department:School of Engineering, Department of Chemical Engineering
Keywords:Time Temperature Integrator, Heat treatment efficiency, Positron Emission Particle Tracking, Mixing efficiency
Subjects:TP Chemical technology
QD Chemistry
Institution:University of Birmingham
ID Code:297
This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder.
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