| Heat pump drying is widely used in aquatic dehydration.However,the drawbacks of this method are the long drying time at low drying temperature and high energy consumption.Osmotic dehydration prior to heat pump drying was applied for partial removal of water,which can reduce energy consumption in subsequent heat pump drying and improve the quality of dried products.However,due to the extensive salt uptake during osmotic dehydration of aquatic products,the nutrition and flavor of the final products will be significantly reduced.Therefore,this research applies a certain processing methods(edible coating pretreatment)to monitor the penetration of solute inside foodstuffs during osmotic dehydration,in order to improve the quality of dehydrated products.Additionally,the drying kinetics and quality properties of scallop adductors dried by heat pump drier were investigated in this paper.The moisture adsorption isotherms and thermodynamic properties of scallop adductors were analyzed.The effects of edible coating pretreatment on moisture adsorption isotherms and thermodynamic properties of scallop adductors were also discussed.The glass transition temperature of scallop adductors was measured and the state diagrams were plotted.The effect of edible coating pretreatment on storage stability of scallop adductors with heat pump dried was investigated.The main study contents and results are as follows:(1)Effects of 3 coating treatments(viz.,chitosan,low methoxyl pectinate,and sodium alginate)on mass transfer process during osmotic dehydration of scallop adductors were investigated.Peleg's equation was employed to model the mass diffusion and its kinetics.Crank's analytical solution to Fick's second law was also adopted to evaluate the diffusivities of water(Dew),solute(Des)and Dew/Desratio.Osmotic dehydration process was significantly influenced by the types of coating materials,temperatures and salt concentrations.Scallop adductors coated showed higher water loss(WL)and lower solids gain(SG)values than those of non-coated samples.Effective diffusivity coefficients increased with increasing temperature and were found to range from 1.224 to 2.321×10-9m2/s for water and 1.152 to 1.894×10-9m2/s for salt.Sodium alginate and low methoxyl pectinate coating provided a good barrier against salt uptake and led to higher Dew/Des ratio.(2)The effects of sodium alginate(SA)coating pretreatment,drying temperatures and air velocities on the drying kinetics and quality characteristics of osmotically dehydrated scallop adductors and single SA films were evaluated.Drying took place in the falling rate period.Drying time decreased with increasing temperature,air velocity and SA coating.Among the eight mathematical models selected,the Two term model and Wang and Singh model gained the best fit for thin layer drying of scallop adductors and SA film,respectively.Effective moisture diffusivity coefficients increased with temperature and SA coating.Whereas the activation energies decreased with SA coating.Dehydrated scallop adductors with SA coated exhibited a significant lower value of shrinkage rate and hardness and higher value of toughness than uncoated ones at 30°C and 2.0 m/s(p<0.05).SA coating is a promising pre-treatment in improving heat pump drying efficiency and enhancing quality attributes of scallop adductors.(3)The effect of SA coating on water adsorption isotherms of scallop adductors were determined at different temperatures(15,25 and 35°C)using a gravimetric method.Several saturated salt solutions were selected to obtain different water activities(aw)in the range from0.11 to 0.90.Water adsorption isotherms of tested scallop adductors were classified like type III isotherms.Equilibrium moisture content decreased with increasing temperature at each aw.Seven models were employed to fit the experimental data and statistical analysis indicated that GAB model gave satisfactory results.The net isosteric heat of adsorption(differential enthalpy)and differential entropy decreased with increasing moisture content.The spreading pressure increased with increasing aw,but decreased with increasing temperature.SA coating can decrease the net isosteric heat of adsorption,differential entropy and the spreading pressure.The mechanism of reaction is enthalpy driven.(4)Differential scanning calorimetry(DSC)was used to deterrmined the glass transition temperature(Tg)of scallop adductors.Fitting the equation with the experienmental data could obtain the state diagram of scallop adductors.Tgof uncoated samples,coated samples and single coating decreased with increasing moisture content.Tgincreased with the application of SA coating.At 15°C,the critical water contents that ensures the glassy state of the dried samples during storage were 0.070,0.075,0.081 g/g(d.b.)for uncoated samples,coated samples and single coating,respectively,while the critical water activities were 0.080,0.230,0.395,repectively. |
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