Mid-inrfared Drying Shiitake Mushrooms And Its Hybrid Drying Study

In this research, shiitake mushrooms were used as raw materials and the feasibility ofmid-infrared drying (MIRD) technology in terms of dehydration was discussed. Thedehydration characteristic and technology of mid-infrared drying on mushrooms were studiedsystematically, and then the drying kinetics model of MIRD process was established. Incomparison with several different drying techniques, we had a more in-depth understanding ofthe MIRD technology. A two-stage combined drying method of MIRD and freeze drying (FD)was applied, in order to improve the quality of MIRD products while reducing the energyconsumption of FD.Firstly, based on the infrared absorption properties of the mushrooms, the feasibility ofmid-infrared drying that the wavelength was2.4~3μm of mushrooms was confirmed. Theeffect of some factors such as heating temperature, wind velocity, radiation distance andloading coefficient on dehydration characteristics of MIRD were studied. The results showedthat there was only a deceleration stage. The higher the temperature, the greater the windvelocity, the smaller the radiation distance and the lower loading coefficient, the greater ofdrying rate and the shorter the drying time was. With drying time, unit energy consumptionand sensory quality as indicator, by orthogonal experiment, the results showed that theoptimum drying conditions: heating temperature was60°C, wind velocity was1.4m/s, theradiation distance was14cm and loading coefficients was2.0kg/m2. For the purpose ofunderstanding the law of dehydration during mid-infrared drying process, three commonlyused drying kinetics models were used to validate the relationship between the moisture ratiosafter mid-infrared drying. We know the Page equation was the best model in the study ofmid-infrared drying kinetic of shiitake mushrooms. Then the characteristic parameters of themathematical model were got by using multivariate linear regression analysis, and it couldshow the change of moisture variation during MIRD process accurately.Secondly, the effect of MIRD, hot-air assisted radio frequency drying (HRFD), coupledhot air and microwave drying (HMVD), hot air drying (HD) on drying characteristic, energyconsumption, surface temperature and uniformity, color, texture, shrinkage, rehydrationcapacity, nutrition retention and microstructure of shiitake mushrooms were studiedcomparatively based on differences in the way energy transfer. These four drying tests wereconducted at fxed air temperature, as well as identical the power level. The results showedthat the drying time of HMVD was the shortest (90min), and energy consumption was thelowest, followed by MIRD methods, which was200min, while the drying time of HD was450min. During drying, the surface temperature of HD and MIRD samples was lowerrelatively, but the drying uniformity was good, while the surface temperature of HMVDsamples increased sharply and overheating phenomenon seriously. MIRD mushrooms showedbetter appearance, larger rehydration capacity and lower shrinkage due to it obvious poroushoneycomb network structure on the section, while cellular structure of HMVD and HRFDmushrooms had a seriously collapse, resulting in poor rehydration and larger shrinkage.Comprehensive comparison, MIRD mushrooms had lower hardness after rehydration, but other qualities were optimal, and the energy consumption was lower. Therefore, it was a gooddrying method worthy of popularization and application.Finally, MIRD was applied before or after FD of shiitake mushrooms to enhance thequality of MIRD products. The effect of application of MIRD before freeze drying(MIRD+FD) and after freeze drying (FD+MIRD) on drying time, energy consumption, color,rehydration capacity, apparent density, nutritional ingredient and aroma compounds et al.were compared with the effect of FD on these parameters. The results showed that thecombination of FD (for6h) followed by MIRD saved33%time and40%unit energyconsumption compared to FD while keeping the product quality to acceptable level. TheMIRD+FD combination was found to be inferior compared to the FD+MIRD as the formertended to produce products with collapsed surface layer and poor rehydration capability. Thecombination of MIRD with FD had a significant effect on nutrition and aroma retention andcaused an increase of sulfur compounds, especially FD+MIRD. Comprehensive efficiency,energy consumption and quality analysis, FD+MIRD was a new technology to improve thequalities of MIRD products and reduce the energy consumption of FD method, and worthpromoting.