Study On The Characteristics Of Hot Air Drying Of Sweet Potatoes And Its Application In Solar Drying Of Sweet Potatoes

Sweet potatoes have high water content and are easily susceptible to decay and spoilage during storage and transportation due to environmental factors.Dehydration of sweet potatoes not only extends their storage period but also facilitates transportation.Currently,hot air drying is the most commonly used drying method,which has advantages such as easy operation and large-scale processing.Solar energy,as a renewable energy source,can provide effective heat for hot air drying and reduce energy consumption,thus attracting considerable attention.However,the uncertainty and intermittency of solar radiation remain important factors limiting the development of solar drying.In this study,the drying characteristics and quality analysis of sweet potato slices were conducted around the hot air drying process of sweet potatoes,and the required temperature range for drying was obtained.Based on this conclusion,it was applied to the solar drying system of sweet potato slices.Firstly,a numerical model of a flat-plate solar collector was established,and its structural optimization was carried out to improve the outlet temperature and collection efficiency of hot air.Secondly,in order to avoid the serious impact of overheated air on product quality under the condition of strong solar radiation intensity in summer,a thermal storage unit was introduced to effectively store heat and improve the drying efficiency of the system.The main research contents and conclusions of the paper are presented as follows:(1)In order to obtain the optimal drying temperature for sweet potatoes,sweet potato slices were selected as the research object in this study.The effects of different hot air drying temperatures(50 ～ 100 ℃)and slice diameters(22 ～ 43 mm)on the hot air drying characteristics,rehydration ratio,color difference,and sensory evaluation of sweet potato slices were analyzed.The results showed that under the experimental conditions,the time required for complete drying increased with the decrease in hot air drying temperature and the increase in slice diameter.By comparing the prediction values of the artificial neural network model and the logarithmic model with the experimental values,the prediction values of the artificial neural network model were more accurate,with a relative average error of 10.61%.In the analysis of dried product quality,the rehydration ratio of the dried sweet potato slices was less than 0.5,indicating that irreversible changes occurred inside the sweet potato slices after hot air drying.Color change is an important indicator for evaluating the quality of dried products.To further evaluate the effect of hot air temperature,the experimental dimension range was increased to100 ℃.When the hot air temperature was increased from 60 ℃ to 80 ℃,the color difference increased by 12.93;when the hot air temperature was increased from 80 ℃ to 100 ℃,the color difference significantly increased by 40.27.This indicates that hot air temperatures above 80 ℃will cause serious damage to the quality of sweet potato dried products.Therefore,the hot air temperature should be lower than 80 ℃ during actual production processes.(2)In order to further explore the drying characteristics of sweet potato hot air drying process,a numerical prediction model for sweet potato slice hot air drying process was established in this study.Compared with the theoretical model,this model can consider the nonlinear shrinkage deformation of sweet potato slices during the drying process.Two effective moisture diffusion coefficients based on temperature and shrinkage were used to predict the moisture content and temperature changes of sweet potato slices.The results showed that the determination coefficients of the simulation values based on shrinkage were in the range of0.977 to 0.985,while those based on temperature were in the range of 0.93 to 0.955.This is because using effective moisture diffusion coefficients based on shrinkage can relate the shrinkage phenomenon to the moisture content of sweet potato slices,which can better predict the changes in moisture and temperature compared to those based solely on temperature.(3)In order to improve energy utilization and further explore the application of solar collectors in the hot air drying process of sweet potato slices,a numerical simulation method was used to establish a model for a flat-plate solar collector and a phase change thermal storage unit.Optimization schemes for the solar collector were proposed,as well as solutions to the problem of overheating of the hot air outlet temperature in the collector during summer.The results showed that the optimal efficiency of the solar collector was achieved when there were11 baffles and the thickness of the insulation layer was 10 mm.For the new type of solar collector(with an added transparent insulation layer),the drying temperature requirement for general agricultural products could be achieved without additional heat sources in winter.However,in summer,the high intensity of solar radiation led to an excessively high temperature at the hot air outlet,so a phase change thermal storage unit was used to absorb heat and prevent irreversible damage to the quality of dried products.After calculating the addition of the thermal storage unit,the outlet temperature could be reduced to below 80 ℃,and the air could be reheated during periods of low solar radiation which were unable to provide effective heat.By11:00 pm,the temperature could reach above 50 ℃,meeting the hot air drying temperature requirement for sweet potatoes.