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Content archived on 2024-04-16

Ohmic heating of foods


A pressurised ohmic heater has been constructed in which food can be electrically heated to commercial temperatures of 130 to 150 C. This heater was constructed and tested to the required levels of temperature and pressure by the University Engineering Department.

The pressurised cell allows up to 6 temperatures to be monitored continuously during heating. Provision has been made for rapid cooling by incorporating a heated lid onto which food can be discharged by inverting the pressure vessel at the end of operation, making it possible to model the heating and cooling cycle of commercial products.

Preliminary work and investigation of food conductivities showed that the heating rate of food materials was reasonably uniform from 100 C to 140 C; no changes in conductivity have been identified owing to changes in material chemistry in meats or vegetables other than those identified in earlier work.

One area which was especially studied is the ohmic processing of milk based fluids. At temperatures in the region of 125 C the casein micelle becomes thermally unstable and begins to disintegrate. It was found that the conductivity of milk did increase over the relevant temperature range but that the effect could be swamped by the presence of about 0.5% salt.

The processing of wheat grains has also been studied. In cereal processing, process steam is used both to supply water for gelatinisation of the starch within the grain, and for the process heating. Electrical heating allows separation of these 2 operations; the food can be heated using electricity and the water required for gelatinization can be supplied at ambient temperature.

It is important to identify the differences between conventional and electrical processing.
A number of possible effects of the electric field were considered possible;
enhancing the transport of charged species, within the food or outside;
affecting the texture of the material by changing cell membrane structure;
enhancing th e destruction of nutrients owing to enhanced ionic reactions.

Work has concentrated on an investigation of the first two effects and results show that the electric field does affect the movement of charged species within foods undergoing electrical processing.

Prior to this work, extensive 2-dimensional modelling of the electrical heating process had been undertaken and this has been extended to 3-dimensional modelling, using the commercial program ANSYS, which allows heat generation and transfer to be simulated in complex geometries.

Modelling flow and heat transfer throughout the whole geometry of the heater is too complex; it is necessary to model representative sections of the flow to allow calculations to be completed in a realistic time. Flow experiments have shown that under many practical circumstances it is possible to consider the flow as a homogeneous mixture of solids and liquids. It is thus possible to consider a 'unit cell' of the fluid, defined by analogy with the use of unit cells in geology and crystallography (ie, a cell which contains a pattern of solid and liquid) which is repeated throughout the food material. This unit cell can be modelled and taken as typical of the whole heating process.

The model allows the relative magnitude of heat transfer and heat generation terms to be assessed. A more complete model could be written in which the conduction equation is solved for each particle (ie where the solid particle does not have a uniform temperature) however, calculations on realistic cases have shown that heat generation sufficiently exceeds heat transfer that in particle variation can be neglected.

The model allows rapid simulation of the overall process and is simple enough for use in design.
Further modelling work is now underway to determine the extent to which physical properties, particularly solid and liquid electrical conductivity and the solid liquid heat transfer coefficient, affect the heating rate.
In addition, preliminary work is underway to model the effect of flow in the heater.

Significant experimental work on the flow of food materials has been undertaken, using both single particles and high solids fractions.
It is vital to be able to monitor the velocities of particles through the commercial system, to ensure that the fastest moving particle is sterile and the slowest is not overcooked. A detection system has been designed in which the passage of particles doped with metal tracer can be detected by coils placed outside of the pipe through which the food is flowing. Preliminary experiments demonstrated that the sedimentation properties of the particles was indistinguishable from those of food particles. Particle densities between 1010 and 1200 kg/m{3} have been used, similar to those of commercial food solids.
Preliminary experiments were carried out to develop equations that correlated the flow of a single particle through the system. The flow of single particles in water was correlated in terms of the modified particle Froude number.

With respect to multiple particles, flows at solids fractions between 3 and 35% have been investigated. The carrots used contain a natural distribution of densities, between 1030 and 1080 Kg/m{3}. In horizontal flow, flow stratification occurs; a sedimented bed of slow moving particles is produced together with a stream of low solids fraction which travels at high velocity above the bed. In order to maintain continuity, the velocity of the flow above the packed bed is significantly higher than the mean velocity.


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University of Cambridge
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Pembroke Street
CB2 3RA Cambridge
United Kingdom

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