Combined Effects Of Pulsed Electric Field And Ultrasound Pretreatments On Mass Transfer And Quality Of Mushrooms

Shiitake mushroom(Lentinus edodes),one of the most common edible fungus,is popular due to its delicious flavor,low calorific,and high nutritional value.However,fresh mushrooms are highly perishable,so it is necessary to process after post-harvest as soon as possible.Due to the low cost of equipment and operating difficulty,hot-air drying is the most common processing method for mushrooms.However,this technology also has shortcomings such as long processing time and high energy consumption.In addition,long-term hot-air drying will lead to darken in color,decrease in rehydration ability,and loss in nutrition,which cannot meet the growing demand of consumers for the high-quality food.Therefore,there is an urgent need for a drying pretreatment method to solve the limitations of the traditional hot-air drying.In recent years,due to the advantages of high efficiency and low impact on the nutrients in samples,several non-thermal technologies,such as pulsed electric field(PEF)and ultrasound(US),have been developed as promising methods in the field of fruit and vegetable drying.The experimental object is mushroom.The moisture changes of different pretreatment methods(PEF,US,PEF-US)during the drying process of mushrooms were compared.Related drying kinetic models were established,and the mechanism of different pretreatment methods to enhance mass transfer were discussed.At the same time,the effects of the different pretreatment methods on the changes in moisture content,state and distribution of mushrooms during rehydration were studied using the non-destructive low-field nuclear magnetic resonance(LF-NMR)and magnetic resonance imaging(MRI)techniques.Besides,the effects of different pretreatment methods on the tissue structure,appearance,color,nutrients,volatile substances and antioxidant activity of dried mushrooms were also studied.The main research contents and conclusions are as follows:(1)Through single factor experiment on electric field intensity,pulse number and temperature and orthogonal experiment,it is concluded that shiitake mushrooms have the shortest hot-air drying time under the parameters of electric field intensity of 1 k V·cm^-1,pulse number of 40 times,and drying temperature of 65°C.The disintegration coefficient of mushrooms after PEF treatment was calculated by measuring the electrical conductivity.It was found that as the number of pulses increased,the cell disintegration index was increased,while the intensity of the electric field had no significant effect on the cell disintegration index.Therefore,the above parameters were selected as the conditions for subsequent researches.(2)Different pretreatment methods were carried out before drying:Control,PEF,US and PEF-US.It was found that pretreatments can shorten the drying time and increase the drying rate.Among them,the PEF-US group had the shortest drying time(4.7 h),which is 21.7%shorter than the control group(6 h).In addition,four common drying kinetic models were used to fit the hot-air drying data of mushrooms.The results showed that the Page model can better describe the changes in moisture ratio and drying rate during the hot-air drying of mushrooms.At the same time,it is found that the combined pretreatment of pulsed electric field and ultrasonic wave significantly enhances the mass transfer and effective water diffusion coefficient of the processed samples by calculating the effective water diffusion coefficient and activation energy.The D_eff was 8.67×10^-9 m²s^-1 and the E_a was 11.3939 k J·mol^-1.(3)The influence of different pretreatment methods on the tissue structure of mushrooms was studied.It can be found that the hardness and shrinkage of mushrooms in all treaed groups were significantly lower than those in the Control group.Scanning electron microscopy(SEM)analysis of the four groups of samples revealed that the cell structure of the Control group samples collapsed and the fiber structure was layered,while the sample surface of the pretreatment group showed a porous structure with large cavities.Among them,the PEF-US group has the largest pore size.Moreover,the moisture content,state and distribution of dried mushrooms during the rehydration process were studied by the low-field nuclear magnetic resonance technology.It was found that the content of non-flowing water in the mushrooms was increased as the rehydration time increased,while there were no significant changes in both bound water and free water.In addition,the rehydration ability of all treated samples was stronger than the Control group,especially the PEF-US group.The results further showed that the porous structure produced by the pretreatment also had a positive effect on the rehydration ability of the mushrooms.(4)The effects of different pretreatment technologies on the qualities of dried mushrooms were discussed.The results showed that the PEF-US group can better retain the color,flavor and nutritional quality of mushrooms.Compared with other groups,the highest amount of soluble sugar(3.90?g·m L^-1),polysaccharides(44.72?g·m L^-1),total phenolics(224.17?g·?L^-1)and total flavonoids(33.21?g·?L^-1)and the strongest DPPH free radical scavenging ability were observed in the PEF-US group.In addition,dried mushrooms pretreated by PEF-US had the least loss of characteristic flavor substances.