Effects of microwave drying on the mechanical and optical properties of handsheets made from kraft and chemi-thermomechanical pulps were studied experimentally. The quality of paper dried in a microwave field of 2450 MHz is compared with that of paper dried by conventional method under standard conditions. Key physical properties measured include burst index, density, tear index, breaking length, zero-span tensile strength, double fold, STFI compressibility and optical properties include brightness, opacity and scattering coefficient. All properties were found to be either enhanced or at the same level as those obtained under standard conditions. Furthermore, it is suggested that microwave drying could replace the conventional drying method in the standard testing of pulp and paper samples for quality control purposes.

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published_or_final_version / Botany / Doctoral / Doctor of Philosophy

No description available.

Modern food industry dictates strict conditions on energy use and application, preventing unnecessary energy dissipation. Energy demanding processes such as distillation and drying have to be optimised to the highest extent, while retaining or improving the final product quality. Pretreatments to drying can be used in order to optimize drying, and some of cranberry pretreatments such as chemical, mechanical and osmotic dehydration were optimized. Chemical pretreatment consisted of dipping cranberries into solution of ethyl oleate and sodium hydroxide at different temperatures, and process times. Mechanical pretreatment was cutting of berries into halves or quarters. Tested parameters for osmotic dehydration were the duration of process, osmotic agent type and its concentration. / Once the appropriate pretreatment was selected, cranberries were subjected to hybrid drying under subatmospheric pressure and using microwaves as an energy source. Evaluated process parameters were microwave power level, microwave power mode, and the operating pressure of process. This drying method showed good potential, but in order to verify the results obtained, it was compared to microwave/convective drying. Slight advantages of the microwave/vacuum process over the microwave/convective process were apparent in almost all product quality parameters, as well as in process efficiency.

Microwave drying of corn was investigated at selected levels of initial moisture content, absorbed power by grain, and temperature and superficial velocity of air at inlet. A surface-wave applicator was used to couple microwaves with the corn. / The drying rate curves indicated that the microwave drying of corn took place in the falling rate period. It was hypothesized that diffusion is the controlling mechanism for moisture transfer from within the kernel in microwave drying of corn. / A mathematical model was developed to describe the change in moisture content at the surface as a function of the free moisture content of corn. The diffusion model employing varying surface conditions was used to describe the microwave drying process. An Arrhenius type equation was developed to describe the relationship between the diffusion coefficient and the outlet air temperature. The diffusion coefficient values varied from 0.0008 to 0.0082 cm$ sp2$/h when constant levels of microwave power were applied continuously for drying corn. Equilibrium moisture content was determined and regression equations were developed to describe the EMC with microwave power and air velocity. / The diffusion coefficient increased with the levels of absorbed power, decreased with increasing air velocity but remained insensitive to the inlet air temperature when microwaves were applied continuously for drying corn. The increased drying rates at higher power levels reduced the drying time considerably but at the cost of energy loss through the passing air and reduced germination and bulk density of dried corn. Application of absorbed microwave power at 0.25 W/g resulted in greater than 92% germination of dried corn. Deleterious effects on product quality was observed when the applied power exceeded 0.75 W/g. / Pulsed and variable microwave power effects were investigated in order to optimize the drying process. Time for drying corn increased but the effective duration for which microwaves were applied and the energy requirement in the pulsed mode was lower compared to both continuous and variable microwave operation.

Microwave assisted osmotic dehydration (MWOD) under continuous medium flow conditions is a new process with good potential for quality optimization. It combines microwave process with osmotic dehydration and improves the mass transfer rate of osmotic dehydration process and product quality. The thesis describes the design and development of this process. / Preliminary studies on osmotic dehydration were carried out in two parts. First, the effects of processing time, temperature and solution concentration on mass transfer under conventional osmotic dehydration process were investigated and suitable ranges of parameters: 40-60°C, 40-60°Brix and 3h, for further osmotic dehydration kinetics study were identified. Then, the osmotic dehydration efficiency under continuous flow condition process was evaluated. For this, a continuous flow osmotic contactor was developed and found to be an efficient process in terms of osmotic dehydration of apple cylinders. Solids diffusivity (Ds) was lower in continuous flow osmotic dehydration process compared with conventional osmotic dehydration correspondents (P<0.05). Being a separate operation unit, the dehydration process and solution management can be done in a more efficient way in this process. / Following the preliminary studies, the osmotic contactor was relocated under a microwave oven so that heating and mass transfer operations could be facilitated by continuous microwave treatment providing a microwave assisted osmotic dehydration (MWOD) process. Compared with conventional osmotic dehydration (COD), moisture loss (ML%), solids gain (SG%) and mass transport coefficients (km and ks) of MWOD were improved, the average k m was increased 80% and the average ks was decreased 20%, respectively. Moreover, product rehydration property and color profile were improved. Microwave heating had an important effect on water transfer during the osmotic dehydration. Application of microwave heating to osmotic dehydration process facilitated in increasing moisture loss from the sample and simultaneously restricted the product's solute gain. Higher moisture loss in mass transfers area helped to control and strongly counters the solids gain. / Modeling of the mass transfer phenomenon is necessary to optimize osmotic dehydration processes to have a high product quality at minimum energy costs. To explain the simultaneous mass-flow in an osmo-dehydration process, evaluation of equilibrium kinetics is important. Pseudo-equilibrium (practical equilibrium) and dynamic period data are necessary for estimating the time of osmotic process, and ultimate mass transport of the solutes and water, and hence these data were gathered. / The effect of osmotic dehydration treatment on sample subsequent air drying behavior and product quality parameters were investigated. Compared with control samples, osmostically treated samples moisture diffusivity during subsequent air drying process was reduced over same moisture content range: from 1.18*10-9m2/s to 0.77*10-9--1.07*10 -9 m2/s. Drying rates of MWOD pretreated samples varied depending on treatment conditions. MWOD pretreatment shifted product's color profile to those that can be achieved under freeze drying conditions. / Sorption isotherms induced by osmotic dehydration were studied, using a gravimetric-static method, and fitted to GAB model. Adsorption isotherms of products were affected by drying method and osmotic dehydration pretreatment conditions. Adsorption isotherms of osmo-air dried apple cylinders followed type II isotherms (Sigma shaped curve). Monolayer (Mm) values of the osmo-air dried products were reduced. Sorption isotherms of osmotically treated-air dried products were shifted from the control isotherms. / Overall, this work has demonstrated potential of microwave heating for improving moisture transfer during osmotic dehydration and microwave osmotic treatment on subsequent air drying and resulting product quality, as well as the importance of equilibrium kinetics study in process modeling.

Since cranberry skin is thick and has low porosity, skin pretreatments were considered before drying in order to facilitate water diffusion. Mechanical and chemical pretreatments were considered, by cutting the fruits in half, by making pin holes in each cranberry, and by dipping the fruits in an alkaline solution. / A second pretreatment seemed necessary when drying cranberries in order to reduce their tartness. Cranberries were osmotically dehydrated, by immersing them in osmotic solutions of sucrose or high fructose corn syrup. / Once the pretreatment conditions were selected, four drying methods were tested on the pretreated cranberries. These methods included hot air drying, freeze-drying, vacuum drying and a combination of hot air and microwaves. A quality evaluation of the dried samples was also performed, including evaluation of overall appearance, taste, color, water activity, rehydration capacity and texture. (Abstract shortened by UMI.)

The current study was conducted for evaluating the effects of two different strategies viz., phase control and cycle control, on the microwave/air drying process. A phase-controlled electrical power regulator was developed and connected in series with the original cycle-controlled power regulator of an existing domestic microwave oven. The microwave oven was further modified such that combined microwave and convectional drying can be accommodated. / The system performance was evaluated. It was observed that phase-controlled power regulator could be successfully used for quasi-continuous (fast-switching) power regulation with the maximization of power efficiency. The degradation of output microwave power was recorded and the nonuniform distribution of microwave field in the cavity was also verified. / The effects of phase control and cycle control were evaluated through combined microwave and convective drying of potato samples. Results showed that different power control methods had different impacts on drying kinetics and product quality. In both drying modes, the drying time increased with the decrease of microwave power density and the increase of air velocity. The drying rates of cycle-controlled drying are significantly higher than those of phase-controlled drying. In terms of rehydration capacity the phase-controlled drying mode produced better results. The product colour and sensory attributes were independent of the power control methods.

Thesis (Ph.D.)--University of Melbourne, Institute of Land and Food Resources, 2005. / Typescript. Includes bibliographical references (leaves 106-111).