Mass Transfer And Odor Analysis During Pretreatment Combined With Microwave Drying Of Perilla Leaves

Hot air drying and microwave drying are widely available in practical industrial production.However,due to the high temperature generated during drying,the products will gradually cause changes of the physical,chemical and nutritional in many aspects.This article utilized ultrasonic technology to pretreat the perilla leaves to improve the problem caused by the drying process,increase the drying rate and retention rate of nutrients,and explore the advantages and application of ultrasonic technology in terms of food drying.The cavitation and mechanical effect of ultrasound can shorten the drying time and enhancing product quality.Perilla is an excellent plant resource that has the concomitant function of both medicine and foodstuff.Mass transfer and quality of perilla leaves were investigate after different pretreatments and drying conditions.The changes of volatile compounds of the dried products were analyzed by PEN3 E-nose and GC-MS technology.According to the result,the optimal treatment conditions were selected.The main content of this paper is as follows:（1）The present work investigated the influences of blanching（B）and ultrasonic assisted blanching（UB）as pretreatment on mass transfer,drying kinetics,color,contents of nutrient,and aroma compounds of hot-air dried perilla leaves.Fick’s second law was applied to calculate the effective moisture diffusivity(D_eff).The Azuara two-component model can predict moisture loss during the drying procedure.Principal component analysis（PCA）and hierarchical cluster analysis（HCA）was used to distinguish sample characteristics of different pretreatments.B70-1,UB70-3,and UB70-1 were classified in one category,it indicated that the similarity between the samples of three conditions.The GC-MS result showed that the changes of five main volatile compounds of perilla leaves before and after drying processing.According to the results,the quality of the dried perilla leaves was improved relatively after performing lower UB pretreatment temperature（70℃）and shorter pretreatment time.（2）Under the right pretreatment conditions（control,UB70-1,UB70-3）,perilla leaves were dried at constant temperature（50℃,60℃,70℃）with the Microwave hot air coupling drying equipment.The equipment can realize online detection of odors during drying and maintain drying temperature based on the temperature feedback.E-nose（Pen3）was used for analyzing flavor of different odor in the drying process.The result showed that the response value of the sensor S2,S6,S7,S9 is higher and sensitive to the volatile material composition in the drying process.The effective moisture diffusivity and activation energy of the samples were calculated.The volatile compounds and quality of the dried samples was analyzed and compared with that of the hot air drying.These results suggested that the pretreatment combined with microwave hot air coupling drying can save energy consumption and retain nutrients.It has advantages in production and processing.（3）This paper used the GC-MS technology for detecting the changes of volatile aroma compounds of the dried perilla leaves.A total of 70 volatile flavor compounds were identified,including 22 alkenes,11 alcohols,8 alkanes,7 aldehydes,6 esters,5 ketones,4 aromatics,3acids and 4 other compounds.The principal components and cluster analysis of E-nose and GC-MS showed that higher drying temperature and longer drying time had a negative effect on the retention of volatile compounds in perilla leaves.The correlation analysis showed that the signal strength of S1,S3 and S5 were positively correlated withα-pinene,β-caryophylene,α-rachtene,geraniene,perilla ketone,etc.,which verified that the sensors S1,S3 and S5 were sensitive to hydrocarbons and aromatic compounds.