Thermal treatments

Thermal technologies, including retorts and heat exchangers, are the foundation of food preservation and production methods (Richardson, 2001). Among the newer thermal technologies, radio-frequency processing causes direct heating within the volume of the food material (Rowley, 2001). Energy is absorbed by a dielectric material although the mechanism is different for radio-frequency processing compared with that using microwave frequencies. The radiation produces more uniform temperature and moisture profiles for drying and heating, with associated higher quality products; however, the equipment cost mitigates against its use. Microwave technology produces more rapid heating rates for the same power and the technology is more compact although the complexity is greater and the smaller penetration depth of the lower wavelength limits its use.

Ohmic heating (Ruan et al, 2001) is a high-temperature, short-time process more suited to liquid-particulate mixtures than scraped surface heat exchangers which cannot achieve sterilisation temperature in the centres of larger particles. A continuous flow ohmic heater is not restricted by heat transfer from the outside of particulates and heats the food as an electrical resistance in a circuit connected to an AC power supply. In contrast to radio-frequency or microwave heating it requires contact with electrodes, and temperature non-uniformities can arise through heterogeneity of the heating medium.

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