Effects Of Pulsed Electric Field Pretreatment On Microwave Vacuum Drying Of Yam

Yam is consumed and regarded as medicinal food in traditional Chinese herbal medicine,was seasonal foods and easily deteriorate during storage.It is of great importance to prolong the storage of yam for supplying in the off-season and without losing nutritional functionality.In order to inprove its dryingeffects,in this paper,the processing method of pulsed electric field pretreatment combined with different drying methods was used to dry yam,the effects of different drying methods on the drying characteristics and quality of yam were studied,and the mechanism of PEF was analyzed.The main conclusions are as follows:The effects of pulsed electric field(Pulsed electric field pretreatment,PEF)pretreatment combined with hot air drying,vacuum freeze-drying,microwave-vacuum drying and heat pump drying on the quality and drying characteristics of yam were compared.The results of the study showed that the PEF pretreatment of yam had a greater influence on the hardness,and the iodine blue value was greater than that of the sample without pretreatment.The rehydration rate of vacuum freeze-drying was better than other methods.The contents of nutritional components,active ingredients and antioxidant capacity of the pretreated samples were slightly lower than those of the untreated samples.PEF pretreatment destroyed the cell structure of yam,and its starch structure order was lower than the untreated sample.The initial temperature T_o,peak temperature T_p and enthalpy value△H in thermal properties were slightly lower than the untreated sample.The vacuum freeze-drying yam with pretreatment had the highest pasting temperature and peak viscosity,92.83±0.98℃and 7170±93.3 cp respectively,while the heat pump drying with pretreatment had the largest setback value and breakdown value,1802±138.7 cp and 1400±93.5 cp respectively.There were significant differences in RDS,SDS and RS of yam from different drying methods(P<0.05).With the increase of frequency,the elasticity modulus and viscosity modulus increased gradually.The results of principal component analysis and cluster analysis showed that the two drying methods of microwave-vacuum drying and vacuum freeze-drying were superior to the other two.In the comprehensive comparison of the four drying methods,microwave vacuum drying was the best,and microwave vacuum dried yam with PEF pretreatment was superior to the other three methods.The effects of different electric field strengths and different times of pulses on microwave-vacuum drying of yam were studied.The results showed that under the different electric field strengths,with the increase of the electric field strengths,the hardness and color L~*values of fresh yam decreased,while the△E value of dried yam increased.Under the different times of pulses,the hardness and L~*value of the dried yam decreased when the times of pulses increased.The more electric field strengths and pulsed times,the more severe destruction of cell structure,and the starch granules were also damaged,the starch structure order was also reduced.In terms of thermal properties,with the increase of the electric field strengths,the onset temperature T_o,the peak temperature T_p and the enthalpy value△H also decreased.Therefore,PEF pretreatment had a significant effect on the structure of yam.