New insights into RF and microwave drying of foods

Renshaw, Ryan C. (2017) New insights into RF and microwave drying of foods. PhD thesis, University of Nottingham.

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Abstract

This work reports a fundamental study of the science and economics of microwave and RF drying of foods, addressing current knowledge gaps and introducing new techniques to aid the development of new microwave and RF drying processes.

The present study has discovered that a correlation exists between the points of inflection of the moisture dependant dielectric properties and the moisture dependant water activity. Hence the moisture dependant dielectric properties are governed by the state of the water as defined by the sorption isotherms.

Water activity equations were mathematically modified in this study to successfully describe moisture dependant dielectric properties. This is a valuable contribution to science as there are currently no recognised standard equations for describing the moisture dependence of dielectric properties. These new proposed equations are of great value as they can be used in microwave drying models. This will enable engineers to optimise microwave and RF processes, leading to a reduction in the trial and error approach that currently prevails in industry. Improved optimisation may ultimately lead to more successful microwave and RF applications.

It is extremely important to be able to monitor key parameters such as porosity, oil and water content, when introducing a new food processing technique. To this end, a new and novel technique for quantifying the porosity of thin irregularly shaped food has been established. This can be used as a diagnostic tool to assess and optimise processing changes such as the introduction of new microwave or RF drying processes. A good understanding of how RF/MW processes affects porosity can be used to optimise the drying process, leading to a greater probability of success.

Sorption isotherm measurements carried out in this study showed that the gelatinisation of starch in potato has a negligible effect on the isosteric heat of sorption. This is important for drying applications, as the isosteric heat of sorption is very significant compared to the latent heat at low moisture contents.

Dielectric properties were measured for potato crisps, biscuits, and pasta particulate at 915MHz, and were used to gain new insights into moisture levelling behaviour through analytical analysis. Contrary to common belief, it was found that moisture levelling can be more effective below the critical moisture content, at lower moisture contents. Although moisture levelling limits the variability of the final moisture content, it was found that fine control of the final moisture content can only be achieved by minimising variation in electric field exposure.

Feasibility studies of microwave applications showed that finish drying microwave applications were far more practical and cost effective compared to applications that have to remove large quantities of water. Dry food is generally quite thin so that it is eatable, or for rehydration purposes. As a consequence conventional food drying processes tend to be sufficiently energy efficient so that even microwave and RF finish drying processes are comparatively expensive with respect to energy usage. Hence, microwave and RF food drying must be justified by improvements in quality.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Dimitrakis, George A.
Robinson, John P.
Kingman, Samuel W.
Keywords: Dielectrics, water activity, sorption, Micro-CT, porosity, microwave processing, food
Subjects: T Technology > TP Chemical technology > TP 368 Food processing and manufacture
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 43232
Depositing User: Renshaw, Ryan
Date Deposited: 13 Jul 2017 04:40
Last Modified: 22 Aug 2017 11:56
URI: http://eprints.nottingham.ac.uk/id/eprint/43232

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