A.A.P. de Alwis

A finite-element analysis of heat generation and transfer during ohmic heating of food

Abstract In ohmic heating an electrical current is applied to a flowing food stream, and the material is sterilised by internally generated heat. It is important to be able to predict the sterilisation effect in a practical situation, where complex food shapes are heated. A finite-element model has thus been developed to study the ohmic heating of foods. The equations solved by the model are described, and it is shown that the model predicts the behaviour of real systems. The use of the model to determine the correct size of particle for a commercial process is outlined and suggestions for further development discussed.

The use of direct resistance heating in the food industry

Abstract Direct resistance heating (DRH) offers the chance to process solid and liquid foods at the same rate, avoiding the delay due to thermal conduction which prevents the use of HTST technologies on paniculate foods. The attempts to exploit the advantages of DRH in food processing over the last century are reviewed. A successful DRH unit requires non-contaminating electrodes which have a good contact with the food material, control of the food heating rate and, if sterilisation is required, an efficient aseptic packaging process. Recent developments in these three areas mean that the advantages of direct resistance heating can now be commercially exploited.

Ohmic processing of solid-liquid mixtures: Heat generation and convection effects

Abstract Ohmic heating allows large food particles to heat at the same rate as liquids, and enables ‘high-temperature short-time’ sterilisation of foods containing up to 25-mm particles. The rate of heat generation depends on the local electric field strength. Around materials of high or low electrical conductivity, the electric field can be distorted. Depending on fluid viscosity and solids fraction, this distortion may cause local temperature variations in the liquid around the particle which might affect both the heating rates and thus the process time of food particles. Experiments have been conducted to determine the magnitude of temperature variations in the liquid both in ideal cases and for real foods. A number of possible models to predict liquid temperatures have been tested, in which various amounts of convection are assumed.

Operability of the ohmic heating process: Electrical conductivity effects

Abstract Ohmic heating is a novel commercial process in which an electric current is applied to a flowing food stream. The passage of current generates heat, which is used to sterilise the food; it is thus possible to sterilise particles as fast as liquids. The controlling parameter is the electrical conductivity of the solid and liquid phases. The range of conductivities which are acceptable to the ohmic process have been examined by finite element simulation. The variation of electrical conductivity in the components of a feed mix on the operability of the process is considered, together with the ohmic heating patterns of composite materials. Providing the conductivities of the two phases do not differ significantly even heating will result. Other process conditions are also briefly discussed.

Electrical conductivity meter for food samples

A description is given of a cell and associated electronic unit suitable for the measurement of the electrical conductivity of foods. Some measured conductivities are reported.

AN EXPERIMENTAL STUDY OF PARTICLE FLOW VELOCITIES IN SOLID-LIQUID FOOD MIXTURES

Abstract The design of equipment to sterilise solid-liquid food mixtures continuously requires that the flow properties of such mixtures be understood. Little information is available on food flows, which can consist of high solids fractions of particles of a range of densities in non-Newtonian and viscous liquids. A metal detection system which can log two types of particle in the same experiment has been constructed and used in a loop flow rig to study flows of single particles and carrot-water mixtures. Results can be correlated using the particle Froude number; some preliminary analysis is presented to suggest that data can be correlated against 1/Frp . The effect of solids fraction on particle flow velocity has been investigated; greatest variations in the particle velocity appear for stratified flows for solids fractions between 10-20%.