Study On Microwave Combined With Hot Air Drying And Aroma Control Of Food

The microwave combined with hot air drying method combines the drying principle of microwave drying and hot air drying.It is a drying method that simultaneously applies microwave drying and hot air drying to dry materials at the same time.Aroma is an important quality of dried products.It is very important to study the changes of aroma in the drying process.How to reduce the aroma diffusion of the food and avoid the generation of char smell in the drying process has not been solved.Therefore,this paper builds microwave combined with hot air drying and aroma on-line detection system.In this paper,carrots are used as research materials to control the influence of the material center temperature,hot air temperature,and average temperature gradient on the color difference,rehydration rate,vitamin C content,sensory evaluation score,drying time,and aroma diffusion of the dried product as to explore a new high quality process.1.The laboratory-scale microwave combined with hot air drying and aroma on-line monitoring system based on temperature control were designed and constructed.The results show that the temperature of the hot air in the inflow tank of the system is adjustable;the output power of the microwave oven is adjustable so that the temperature of the material can be adjusted during drying;aroma can be online detected.The entire system can avoid the quality deterioration caused by the burnt and carbonization of the material in the late stage of drying.2.Experiments were conducted through the Box-behnken center combination experiment to set the material center temperatures（60?C,70?C,and 80?C）,temperature difference between the material temperature and the hot air temperature（10?C,20?C,and 30?C）,and material thickness（8 mm,13 mm,and 18 mm）.Effect on color difference,vitamin C content,rehydration rate,sensory score,and drying time of the dried carrot mass was studied.The results of the process variables obtained by optimizing the maximum desirability function are:the material temperature is 66.89?C,the temperature difference is 20.47?C,and the cube size is 10.82 mm×10.82 mm×10.82 mm.Drying experiments using the predicted optimal drying conditions were in agreement with the predicted values.3.Model for the mathematical shrinkage model is fitted and modified for microwave combined with hot air drying.The dimensional shrinkage parameters of the different material temperatures,different hot air temperatures and the thicknesses of the material predicted by the modified model were compared with the measured values in the experiment.The model verification shows that the model can well predict the shrinkage of the drying process.4.In combination with the dimensional shrinkage model,it was found that the optimized drying parameters had different temperature gradients during the drying process,and the temperature gradient influenced the drying rate.By controlling the temperature of the fixed material,the temperature of the hot air is adjusted to control the temperature gradient.The influence of different temperature gradient parameters on the microwave combined with hot air drying was investigated with emphasis on the optimization parameters of the response surface.And the quality of the dried product was compared with the composite index of color difference,vitamin C content,rehydration rate,sensory evaluation score,and drying time of the dried carrot.The average temperature gradient of 6℃·mm^-1 provides the best product quality.In order to achieve industrial production,further attempts were made to simulate the drying conditions at 6℃·mm^-1 without weighing the average temperature gradient using a linear control method.It was found that the product quality of the linear control,the quality of the temperature gradient of 6℃·mm^-1 were better than the product quality of the response surface optimization parameter drying.5.The aroma diffusion and quality of carrots by controlling the material temperature and temperature differences were studied.The emission of carrot and char peaks increases with the increasing material temperature.The spread of aroma is divided into two phases:the first phase is the change phase of the first hour;the second phase is the constant phase after the first phase.The best quality was obtained with material temperature at 70?C and hot air at 50?C.When the material temperature is the lowest at 60?C,with the least aroma emission,the best quality was obtained with air temperature at 40?C.Based on the fixed aroma release,the optimal solution was set during first hour using a material center temperature of 60?C and air temperature of 40?C.After one hour,the material temperature was set at 70?C and hot air temperature was set at 50?C.It was found that the quality and aroma of the optimized solution were better than those of the fixed material temperature and hot air temperature.6.By studying the aroma diffusion,a logistic function model of aroma diffusion was proposed and the model parameters were modified according to different material temperatures and hot air temperatures.The experimental values in the experiment and the simulated values in the model were selected to verify the results.The results showed that the modified aroma diffusion model can simulate the carrot odor diffusion during the drying process.7.As the average temperature gradient increases,more aroma diffuse during the drying process.The lower the average temperature gradient,the less the amount of aroma loss.However,the dry quality is poor due to the long drying time.This article develops a fuzzy logic controller to control the average temperature gradient.This new control strategy successfully improves the retention of volatiles and avoids scorching with acceptable quality.At the same time,further attempts were made to use a linear control method to simulate fuzzy logic control to achieve drying without the need for an electronic nose.The control effect of the linear control is equivalent to that of the fuzzy logic control and the quality is better than the sample quality dried at 6℃·mm^-1.