Thesis (PhD) - Swinburne University of Technology, Industrial Research Institute Swinburne, Faculty of Engineering and Industrial Sciences, 2006. / [A thesis submitted for the degree of Doctor of Philosophy], Industrial Research Institute Swinburne, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, 2006. Typescript. Includes bibliographical references (p. 157-165)

Thesis (Ph. D.)--University of Hong Kong, 1999. / Includes bibliographical references (leaves 137-140).

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The potential of dielectric heating with microwaves at 2450 MHz for drying grapes into raisins was studied. Feasibility studies in a conventional microwave oven were successful. A new microwave drying system equipped with specialized instrumentation and data acquisition components and permitting full control of microwave power levels and duration of application was then developed and used for detailed experimental work. / It was found that when the grapes were dipped in surfactants, as is common practice in the raisin industry, microwave drying was not only faster than convective drying but also had a much lower specific energy requirement. However, it was also possible to obtain raisins of adequate quality without dippings. Good quality light coloured raisins were obtained without sulphur dioxide fumigation. Thus, microwave drying has potential in reducing both the quantity of chemicals entering at this point in the food chain and the energy consumed for food preservation. / The shrinkage and density of grapes were found to be linearly related to moisture content; initial size and method of drying had no influence. (Abstract shortened by UMI.)

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This work was aimed at obtaining high quality dried strawberries using microwaves to assist convection air of 2 m/s at 30--45°C. Preliminary trials with whole strawberries were unsuccessful. Fruits would cook rather than dry at low microwave power levels, and burst at higher powers. This was due to the inhibition of moisture movement by the waxy cuticle. Slice and pureed strawberries dried, but were of lower quality than freeze-dried / A treatment consisting of dipping the berries in a solution of ethyl oleate and sodium hydroxide was studied. Such treatments are used in industry to reduce the skin resistance to moisture diffusion. Result showed that the treatment greatly enhanced the drying rates of whole berries in convection and microwave regimes. A 1% concentration of ethyl oleate was sufficient for maximum reduction of drying time, and it is possible that even lower concentrations could be used for strawberries. Rehydration was similar to that of the dipped and freeze-dried samples, but the microwaved samples were a bit softer, and had less aroma, colour and flavour. / Osmotic dehydration was then studied as a technique of binding flavours and aromas and of reducing the time required for finish drying with microwaves. These studies were performed on strawberries and blueberries. Results showed berries that were dipped and then osmotically dehydrated for 24 h in sucrose, yielded a microwave-dried final product that was equal to the freeze-dried one in terms of quality, and this, with a much lower time for finish drying. / The shrinkage ratio of strawberries has a straight line relation to the moisture ratio. The reduction in equivalent diameter is well-described by a reciprocal logarithmic function. / The results of these major aspects of the research suggest that microwave-drying could be a viable and more rapid alternative to freeze-drying when berries are first subjected to a pretreatement of ethyl oleate and partial dehydration by osmosis. It was also found that if microwave energy is applied in continuous mode, the initial applied power should not exceed 0.2 W g--1, otherwise burning may occur. It is also recommended that osmotic dehydration be limited to not much longer than 24 hours, since off-odours, develop. The results apply to convective regimes with inlet air temperatures of 45°C and inlet velocity of 2 m/s.