Lettuce Osmotic Dehydration Mass Transfer Kinetics And Infiltration After Hot Air Drying Characteristics

Osmotic dehydration is based on the principle that a driving force for water removal sets up because of the high osmotic pressure of the hypertonic solution when porous material (such as fruits and vegetables) are immersed in a hypertonic aqueous solution. Osmotic dehydration could minimize the heat damage to quality and reduce total drying time. But the main research materials about osmotic dehydration are fruits. Because results from those studies could not be used in vegetables absolutely and models of osmotic dehydration could have a further study. So the object of this paper is to study the kinetics of mass transfer during osmotic dehydration of asparagus lettuce and following air-drying characteristic so as to provide references for research and actual processing.In this paper, asparagus lettuce was chosen to be experimental subject. And experimental indicators included water loss, solid gain, effective diffusion coefficients, wet basis and drying rate. Regression orthogonal rotatable design and uniform design were used in this study. Besides, SPSS, Matlab and SYSTAT were used to statistical analysis, drawing and optimization.First, a quadratic regression orthogonal rotatable design (factors including glucose concentration, sodium chloride concentration, temperature, thickness and dehydration time) was used to study the regression equations of water loss and solid gain on mass transfer of osmotic dehydration and processing parameters were optimized. Secondly, glucose concentration and temperature were decided to be factors. According to the results of uniform design, kinetics models of asparagus lettuce during osmotic dehydration were built and regression equations which were related to effective diffusion coefficients and factors were obtained. Last, effects of air temperature, air velocity and paving density on drying characteristics of asparagus lettuce treated with osmotic dehydration were studied. Also, drying characteristics, exterior quality of products and rehydration characteristics of samples with and without treated by osmotic dehydration were investigated.The results showes that, first, water loss and solid gain increase significantly by the increasing of glucose concentration and time. In the combined of glucose and sodium chloride solution, the increasing of sodium chloride concentration does not affect water loss apparently but increasing solid gain significantly. The increasing of temperature could decrease water loss but it increases solid gain. Thickness has negative effects on two indicators. And the optimum operating conditions for osmotic dehydration of asparagus lettuce are found to be glucose concentration of 32.5%, sodium chloride concentration of 2%, temperature of 35℃, thickness of 5mm and dehydration time of 139min. At this optimum point, predict and experimental values of water loss are 52.08% and 53.54% respectively and the values of solid gain are 9.83% and 9.14%. So regression equations of water loss and solid gain can predict the mass transfer of osmotic dehydration of asparagus lettuce.Secondly, Azuara model could calculate water loss and solid gain of equilibrium. The two-term exponential model is more suitable for the kinetics of osmotic dehydration than Page model in this research. Regression equations based on surface fitting represent the relationship between effective diffusion coefficients and factors (glucose concentration and temperature) well. And effective diffusion coefficient does not have direct relationship to water loss or solid gain of equilibrium, however, it represent the rate of water loss and solid gain reaching to the equilibrium.Thirdly, the increasing of temperature and air-velocity could reduce drying time, but paving density has a negative effect on it. Samples pre-treated with osmotic dehydration consume less drying time, but they have a lower drying rate comparing with untreated ones. And the exterior qualities of pre-treated samples after air-drying or rehydration are better than those untreated samples. The pre-treated samples rehydrate faster in water solution of high temperature, however, untreated samples are suitable for rehydrating under room temperature. In addition, pre-treated samples have a better rehydration capability than untreated samples. For samples which are treated by osmotic dehydration, the rehydration time can be controlled within 1h.