The Research Of Combined Vacuum Freeze And Hot-air Drying For Moso-bamboo Shoots

Vacuum freeze drying technology have been recognized as the best dehydrated methodat home and abroad because of keeping its nature color, aroma, taste, shape andphysiological components. However, high energy consumption and expensive cost haverestricted extensively applications in agricultural products processing field. The objective ofthis paper was to reduce vacuum freeze drying energy consumption and improving hot-airdehydrated products qualities, study the effects of combined drying of vacuum freeze andhot-air drying on the qualities of dehydrated bamboo shoot slices and energy consumptionand probe into its heat and mass transfer models and investigated storage stability ofdehydrated bamboo shoot slices.In order to obtain the optimal technology, Moso-bamboo shoots applied by southernYangtze factories as experimental materials, A three-factor, five-level combinedexperimental design, was chosen to evaluate dependent variables affecting moisture content,rehydration ratio and energy consumption by Response Surface Methodology (RSM). Theindependent variables respectively were heating plate temperature, loadage and drying time.The optimum vacuum freeze drying conditions parameters were obtained.In order to obtain foundational parameters required by drying mechanisms analysis,thermal physicochemical properties parameters of moso-bamboo shoots were tested.Crystal , melting temperatures and specific heat capacity (Cp) were determined bydifferential scan calorimeter(DSC) and effects of added different solutes on them werediscussed;Specific heat capacity (Cp) were determined by DSC and effects of moisturecontent and temperature on it were discussed;Conductivity coefficients ofmoso-bamboo shoots were determined by flat plate steady heat exchange and effects oftemperature on it were discussed.In order to reduce energy consumption of vacuum freeze drying, the new combinedvacuum freeze and hot-air drying was investigated particularly. Combination method andconversion point were obtained. A three-factor, five-level combined experimental designbased on the single factor experiments, was chosen to evaluate dependent variables affectingmoisture content, rehydration ratio and energy consumption by Response SurfaceMethodology (RSM). The independent variables respectively were conversion moisturecontent, hot-air temperature and hot-air velocity. The results showed the optimumcombination drying conditions were conversion moisture content of 20% (w.b.), hot-airtemperature of 55℃-65℃ and hot-air velocity of 1.5m/s-2.0m/s.Effects of different drying methods on the quality, microstructure and energyconsumption were investigated by experimental determinations and SEM. The resultsshowed that combination dried bamboo shoot slices were close to vacuum freeze dried onesand energy consumption was reduced by 41% compared with vacuum freeze drying.A combined vacuum freeze and hot-air drying model for moso-bamboo shoots wasestablished on the basis of mass and energy conservation. At the first stage, moisture andtemperature distribution rules were followed by heat and mass transfer mechanisms ofvacuum freeze drying;At the second stage, they were followed by heat and mass transfermechanisms of hot-air drying. A good agreement between experimental and predicted datawas obtained.A one-dimensional heat and mass transfer coupling model during the course ofcombined vacuum freeze and hot-air drying for moso-bamboo shoots was established on thebasis of mass and energy conservation. Moisture content and temperature distribution ofvacuum freeze drying at the first stage of combined drying were simulated. Coeffients ofhot-air drying models at the second stage of combined drying were gained. A goodagreement between experimental and predicted data was obtained.In order to study storage stability of dehydrated bamboo shoots, the adsorptionisotherms of dehydrated bamboo shoots were determined. The data were fitted to sevenmodels at four temperatures. Adsorption characteristics, based on the GAB model wereanalyzed and the monolayer moisture content and adsorption energy were obtained. Thechanges of dehydrated bamboo shoot slices' quality at different storage conditions during 6months were studied, including moisture, water activity and color difference value.