Brown, Zoe Katherine (2010)
Eng.D. thesis, University of Birmingham.
Food drying techniques such as air and freeze drying are not ideal: high temperatures used during air drying result in degradation of nutrients and sensorial properties, while freeze drying is expensive and therefore only applicable to high value foods. As an alternative to such drying techniques, drying with supercritical carbon dioxide was investigated here. Initially, carrot was dried using this technique. Addition of a co-solvent (ethanol) to the supercritical fluid was used as a method to increase the water solubility in the supercritical fluid and therefore aid drying. Analysis of the dried and rehydrated product structure, rehydration properties and mechanical properties was carried out which gave an indication of product quality. Drying of agar, containing varying concentrations of sugar was carried out on a laboratory and pilot plant scale. Gel structure and gel properties were studied. Addition of sugar to agar gel pieces improved structural retention considerably during drying. Fourier transform infrared analysis was used to investigate interactions that may be responsible for structural differences seen during supercritical drying. Changes in experimental parameters such as flow rate and depressurisation rate did not appear to have a significant effect on the dried gel structure. The supercritical drying technique investigated allowed food products to be dried and unique structures to be created with different rehydration and textural properties to the equivalent food products dried by air or freeze drying.
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