Microwave assisted osmotic dehydration of apple cylinders under continuous medium flow conditions for improving moisture transfer rate and product quality.

摘要

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.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.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 (P0.05). Being a separate operation unit, the dehydration process and solution management can be done in a more efficient way in this process.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.