Study On Moisture Transfer And Quality Of Lycium Barbarum Under Freeze-vacuum Drying

With the general improvement of people’s living standards and the gradual increase of self-health awareness,functional foods like Lycium barbarum（LB）,which are used for both medicine and food,are gradually gaining popularity.As a high water content berry,there is a certain challenge of storage problems brought about after harvesting.The traditional natural sun drying and mechanical hot air drying can no longer meet people’s needs due to the shortcomings of nutrient flavor loss and color and shape change,and new technologies such as freeze-vacuum drying are gradually accepted by people.In this paper,freeze-vacuum drying of LB is studied from the following two aspects.1.The analysis of heat and mass transfer in the freeze-vacuum drying process（1）Simulation:Firstly,establish mathematical model as well as physical model through pre-experiment,image processing and parameter measurement;Secondly,set up thermal-water-mechanical two-way coupling to analyze the temperature change of the whole process,the stress-strain inside LB in the pre-freezing stage and the moisture change of the drying process;Finally,divide the grid to solve the model.（2）Experimental aspects:Through the freeze-drying machine to obtain the real-time temperature of LB freeze-vacuum drying,and compare with the simulation value;Through the freeze-vacuum drying microscope system to obtain real-time images of LB in the freeze-vacuum drying process,combined with parameters,statistical indicators to analyze the stress strain of LB;Through low-field nuclear magnetic technology nondestructive detection of LB internal moisture distribution after drying,combined with empirical equations to predict the LB freeze-vacuum moisture transport in the drying process.2.The quality analysis after freeze-dryingHot air drying experiments were introduced for comparison,and the feasibility of freeze-vacuum drying was specifically analyzed by starting from drying time,physicochemical indexes,and nutritional composition.The results of the study showed that.（1）The comparison of experimental and simulated results of the internal temperature of LB showed a high value of the coefficient of determination R~2 with a good fit.By increasing the temperature rise rate during drying,the overall freeze-drying time can be shortened.（2）Freeze-vacuum drying has a certain effect on the deformation of LB cells.The shape factor（S_f）and length shrinkage（S_l）showed a strong positive correlation,while the shape factor（S_f）and equivalent diameter shrinkage（S_D）were weakly negatively correlated during freeze-drying.The shape coefficient of LB cells increased and roundness was greater after freeze-drying.（3）The Wang and Singh model fits better with the simulated moisture ratio in the freeze-drying process.Increasing the heating rate during the drying process instead led to poor dehydration of the freeze-dried LB.（4）The freeze-vacuum-dried LB is fuller and rounder than the hot air-dried LB,with larger volume and smoother surface;lower water content,crispier when chewing,higher sweetness and good retention of fresh wolfberry odor,these characteristics can better meet the needs of consumers nowadays.（5）In the process of freeze vacuum drying,some LB are easy to produce sugar,and because of its high brittleness value,it cannot be stored in bags,which increases the packaging cost.At the same time,for hot air drying,once the temperature is increased,the drying time can be greatly shortened,while the freeze vacuum drying time cannot be greatly shortened.