Effect Of Low Frequency Thermosonication On Efficiency And Thermoelectric Physical Characteristics Of Agaricus Bisporus Under Microwave Vacuum Drying

Agaricus bisporus is the most widely cultivated and consumed edible mushroom in the world.A.bisporus is rich in nutrients,and has high protein,low fat and calorie characteristics,and contains many kinds of bioactive substances.However,A.bisporus is perishable after harvest,especially with a large number of polyphenol oxidase(PPO),which is easy to cause browning and nutritional flavor deterioration.Enzyme inhibition and dehydration drying can prolong the shelf life of Agaricus bisporus and facilitate storage and transportation,which is of great significance to ease the contradiction between production and marketing.Thermosonication and microwave vacuum drying is the predominant green enzyme inhibition pretreatment and dehydration technology of A.bisporus respetively,but the low frequency ultrasound may affect the microstructure and material properties of A.bisporus materials,and the further drying process.It is not clear that the characteristic of thermosonication treatment on A.bisporus materialsand its microwave vacuum drying process.In this paper,the mass transfer dynamics,energy consumption characteristics,water distribution and migration characteristics,glass transition temperature change,dielectric properties and thermal physical properties during the microwave vacuum drying process of A.bisporus treated by thermosonication was studied.The purpose is to lay a theoretical foundation for developing potential green process technology,which is thermosonication combined with microwave vacuum drying of A.bisporus.The specific research contents are as follows:1.Effect of thermosonic pretreatment on drying kinetics and energy consumption ofmicrowave vacuum dried Agaricus bisporus slicesThe effects of low frequency thermosonic pretreatment on the kinetics,energy consumption and color of microwave vacuum dried A.bisporus were evaluated by weighing method,mathematical calculation method and chromatic aberration analysis.It was found that thermosonic pretreatment removed approximately 40%-45%of water in A.bisporus slices.The experimental data of the followed microwave vacuum drying process was fitted and Page model showed excellent for explaining the drying characteristics of microwave vacuum dried A.bisporus slices both untreated and thermosonically treated.A simplified and modified form of Fick's second law was primarily used to calculate the moisture diffusivity during microwave vacuum drying,which was observed to vary between 3.68×108 and 1.52×107 m2/s,with the achtvaiton energy varying between 41.87 and between 3.68×10 and 1.52×107 m2/s,with the activation energy varying between 41.87 and 49.52 kJ/mol.In addition,the input energy and specific energy requirements for drying of the thermosonically pretreated A.bisporus samples were respectively determined to be within 0.30-0.54 kWh and 4.97-9.97 kWh/kg for power supplies ranging between 481 and 865 W and a constant vacuum degree of 70 kPa.Thermosonic pretreatment not only improved the product color the microwave-vacuum-dried A.bisporus slices,but also reduced the energy consumption of the drying process.2.Effect of thermosonic pretreatment on water state and glass transition temperature of microwave vacuum dried Agaricus bisporus slicesThe effects of low frequency thermosonic pretreatment on water state and glass transition temperature(Tg)of A.bisporus slices during microwave vacuum drying were studied using differential scanning calorimetry(DSC),low field nuclear magnetic resonance(LF-NMR),and magnetic resonance imaging(MRI).Results showed that four population groups were contained in the initial distribution of transverse relaxation time(T2)data of fresh A.bisporus slices:T21(0.38-7.05 ms),T22(9.33-32.75 ms),T231(37.65-265.61ms),and T232(305.39-811.13 ms).Thermosonic pretreatment significantly decreased the initial free water content of Agaricus bisporus sample,but accompanied by a sharp increase in its immobilized water."Semi-bound water transfer"appeared during microwave vacuum drying(MVD)at moisture contents(Xw)of 0.70 and 0.60(g/g,w.b.)for untreated and thermosonically treated samples,respectively.MVD caused dramatic changes in the water state and enhanced the Tg by decreasing the content and mobility of immobilized water in A.bisporus tissues.The mobility of semi-bound water for thermosonic-and MVD-treatedsamples was higher than MVD-untreated samples,resulting in decreased Tg values by approximately 2-11.5 ?,but the uniformity of water distribution in thermosonic-and MVD-treatedsamples is better at Xw?0.52(g/g,w.b.).3.Effect of thermosonic pretreatment on dielectric properties of microwave vacuum dried Agaricus bisporus slicesThe effects of low frequency thermosonic pretreatment on the dielectric properties of Agaricus bisporus slices under different moisture contents and temperatures were evaluated by using a network analyzer and an open-ended coaxial-line probe at microwave frequency(915 and 2450 MHz).Meanwhile,the permittivity at 25? was also correlated to the water state determined by low field nuclear magnetic resonance(LF-NMR)analysis.Results showed that the dielectric constant ?' and loss factor ?" of A.bisporus slices were affected significantly by the composition of water state in the sample.Thermosonic pretreatment changed the water state in sample slices during microwave vacuum drying(MVD),and enhanced the influence of semi-bound water(M22)and bound water(M21)on the permittivities.Thermosonically treated samples tended to have lower ?",values and higher microwave penetration depths.The dependency of permittivities on the moisture content and temperature at 915 and 2450 MHz for thermosonically treated A.bisporus samples could be described by third-degree pol·yinomla dmoleis,wth R2 higher than 0.998.Infrared could be described by third-degree polynomial models,with R2 higher than 0.998.Infrared thermal imaging analysis confirmed that thermosonic pretreatment is a potential technology to promote the uniformity of A.bisporus slices during the MVD process.4.Effect of thermosonic pretreatment on thermo physical properties of microwave vacuum dried Agaricus bisporus slicesThe effects of low frequency thermosonic pretreatment on the density(p),specific heat(Cp),thermal conductivity(?)and diffusivity(a)of A.bisporus samples were evaluated by instantaneous linear heat source method based on thermal characteristic analyzer.Results showed that,at moisture content(MC)ranging from 0.16 to 0.89(w.b.)and temperature from 25 to 65?,the density(?)of thermosonically treated A.bisporus samples decreased with temperature and MC,ranged from 946.43 kg/m3 to 1223.23 kg/m3.The ? of thermosonically treated A.bisporus samples increased with temperature and MC,and ranged from 0.305W/m·K to 0.624W/m·K.The Cpof thermosonically treated A.bisporus samples increased with MC,but the effect of temperature is not significant,and ranged from 2149.19 to 3862.85 J/kg·?.The a of thermosonically treated A.bisporus samples were calculated from the measured ?,Cp and ?,and ranged from 0.1048 mm2/s to 0.1537 mm2/s.The ? of thermosonically treated A.bisporus samples exhibited increasing trends with increasingMC and temperatures.Based on the experimental data,regression models for allthe properties were developed.Multiple regression models with high R2 values were developed to correlate these properties as a function of input variables.