Optimisation and modelling of aroma recovery by pervaporation
Abstract: During production of concentrated fruit juices, both physical and chemical losses of aroma compounds occur due to heat treatment such as pasteurisation and evaporation. This leads to an inferior quality of the final product. By recovering the aroma complex from fresh juice and then adding it back to the processed juice, heat treatment can be avoided. Hydrophobic pervaporation is an emerging membrane technique, which provides an interesting alternative when volatile organic compounds are to be separated from dilute aqueous mixtures. The intention of this thesis was to model and to optimise the pervaporation process for the application of aroma recovery from fruit juices, with focus on apple juice. Pervaporation was shown to be a promising alternative for the application of aroma recovery from natural apple juice, as it offers very high separation efficiency at mild process conditions. The operating conditions were shown to have a major impact on the performance of the process. Improved hydrodynamic conditions in the feed channel, increased feed temperature and decreased permeate pressure favour the aroma recovery. However, this has to be balanced with the energy input. Furthermore, due to the heat sensitivity of the aroma compounds, the temperature has to be kept at a reasonable level. There is also a possibility of controlling the composition of aroma compounds in the permeate via the process conditions. Optimum separation properties could be obtained when applying membranes designed with relatively thin selective layers using a modified silicone rubber polymer with a low degree of crosslinking in combination with a porous support of low resistance. A model for a poly[octylmethyl siloxane] membrane, which can predict the influence of permeate pressure for any permeant within the chemical groups of alcohols, aldehydes and esters was developed. The model provides a better understanding of the plastication phenomena inside the membrane and its effect on permeabilities. By use of the hydrophobicity, the molecular size and the ability to form hydrogen bonds, it was possible to predict the general trends regarding the permeabilities and their influence on temperature for the organics studied.
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