Study On Drying Mechanism And Technology Of Berry Pulp Under Microwave Assisted Foam-mat Drying

Berries are rich in a variety of nutrients with high edible and healthcare values.However,berries are easily rotted and are difficult to store for long-term,limiting the market promotion of berries.Therefore,investigation of the technology of post-harvest storage of berries is needed.Drying is an effective method for extending the storage period of fresh berries and producing the variety of products.In addition,dried berry products can also meet the demands of market requirement for natural and safe foods.The microwave assisted foam-mat drying(MFD)method combines the advantages of microwave drying and foam-mat drying.Compared with the traditional drying methods,MFD has the advantages of high drying efficiency,premium product quality for drying berry materials.However,the drying process often causes over high local temperature and large degradation of bioactive ingredients due to the uneven distribution of electromagnetic field in microwave drying and the nature of materials.This research aims to clarify the mechanism of the effects of MFD on berries qualities,and effectively control the quality of berry dried products.In this study,the kinetic models of microwave energy absorption,mass and heat transfer process and degradation of bioactive ingredients in the foamed berry pulp in microwave field were studied.The mechanism of bioactive ingredients degradation in berries under microwave processing conditions was analyzed.Berry dried products with high bioactive ingredient content were obtained by optimizing the MFD technology parameters and process.The main conclusions obtained from this study are as follows:(1)The chaneges of temperature and moisture content of berry pulp under MFD conditions were studied.A dynamic model between physical properties of berry pulp and temperature and moisture content was established.The drying characteristics of berry pulp under different microwave intensities were also investigated.We found that the MFD process may be roughly divided into three stages: preheating drying stage,foaming drying stage,and rapid heating stage.The temperature of berry pulp was gradually increased to 70 °C during the preheating drying stage.The increase of microwave intensities can significantly reduce the time that is required for preheating drying stage.The temperature of berry pulp was between 75 to 80 °C during the foaming drying stage.Due to the massive evaporation of berry pulp,prolonging this foaming drying stage duration can improve the drying efficiency.During the rapid heating stage,the integral temperature of berry pulp increased and the moisture content continued to decrease till the safe moisture content.However,high microwave intensity may burn the center region,thus the temperature of the rapid heating stage should be controlled reasonably.By considering both drying efficiency and product quality,the microwave intensity in the range of 7-9 W/g was suitable for processing.A center combination experiment with three-factor and five-level was designed to study the dielectric properties(dielectric constant and dielectric loss factor)and thermal parameters(thermal conductivity,specific heat capacity,and thermal diffusivity)of pulp under different temperatures,densities and moisture contents.The change law of the pulp showed that both the dielectric and thermal properties of the pulp were positively correlated with the moisture content.In addition,the dynamic models of the change of the dielectric and thermal properties of the pulp were established,which can accurately analyze the microwave energy absorption and transformation process.These findings provided an optimized condition for establishing the heat and mass transfer process.(2)Heat and mass transfer models describing MFD process were established to study the mechanism of berry pulp heat and mass transfer in three drying stages.The heat and mass transfer model of the foamed berry pulp during MFD was established based on the dynamic model of berry pulp physical property parameters and the conservation equations of energy and mass.According to the drying characteristics of different drying stages,the mechanism of internal temperature and moisture content change of the berry pulp were discussed in different stages.The multi-physics coupling process among electromagnetic field,temperature field,and moisture concentration field in berry pulp were calculated by simulation software.The model prediction results were in a good agreement with the experimental results,suggesting that the models could be used to describe the MFD process.(3)The degradation regular pattern of bioactive ingredients during MFD was studied.The experimental results showed that the temperature and moisture contents of berry pulp during the drying process have both direct and indirect effects on anthocyanins and vitamin C.The direct path coefficients of moisture content on anthocyanin and vitamin C were 0.985 and 0.953,respectively.The direct path coefficients of temperature on anthocyanin and vitamin C were-0.933 and-0.951,respectively.The degradation ratio of active ingredients in berries increased with the elevating of temperature and the decrease of moisture content.In addition,the degradation regularity of bioactive ingredients was closely correlated to the drying stages.Through theoretical analysis,the ratio of moisture content to temperature was taken as the influencing factor to analyze the degradation kinetics of bioactive ingredients at different moisture content to temperature ratios.The moisture content of berry pulp in MFD process ranged from 15% to 90%,and the temperature ranged from 25 to 130 °C.The degradation activation energy of anthocyanin and vitamin C in preheating drying stage was 4.379 kJ/mol and 3.926 kJ/mol,respectively.The reaction heat of anthocyanin and vitamin C in foaming drying stage was-0.658 kJ/kg and-0.515 kJ/kg,respectively.In order to reduce the degradation of anthocyanin and vitamin C,the drying process should be adjusted to ensure the ratio of moisture content to temperature ratio between 0.96 and 3.60.The mathematical model between the drying parameters and the bioactive ingredients was obtained based on the combination of heat and mass transfer model during drying process.The predicted results of the validated model were in good agreement with the experimental results.This model can be used to optimize the berry drying process.(4)Optimizations of the MFD parameters were done in this research.In order to improve the drying efficiency and reduce the degradation of bioactive ingredients in berries,the vacuum drying conditions were introduced into MFD process.By reducing the pressure of drying chamber,the moisture in berry pulp was evaporated quickly under low temperature conditions during drying process to ensure the ratio of moisture content to temperature within a suiable range.The vacuum pressure,microwave intensities and material thickness were taken as the influencing factors.And the anthocyanin and vitamin C in the obtained berry powder were taken as the target factors.The genetic algorithm was combined with neural network(BPNN)to optimize the process parameters of vacuum microwave foam-mat drying technology.The optimum process parameters for MFD were obtained when the vacuum pressure was 60 kPa,the microwave intensity was 9 W/g,and the material thickness was 8 mm.Compared with microwave assited foam-mat drying,vacuum and microwave assisted foam-mat drying can significantly reduce the degradation of anthocyanins and vitamin C in dried berry powder products,which improve the retention of nutrients in berry products.