Kinetics And Process Parameters Optimization Of Combined Infrared And Hot Air Drying Of Cantaloupe (Cucumis Melon L.) Slices

Cantaloupe is a characteristic fruit and vegetable in northwestern China,which is not resistant to storage.Because of its high sugar content and low thermal conductivity,there are problems of long drying time,the poor color of dried products,high hardness and serious nutritional loss in drying processing.As a radiation drying technology with fast heating efficiency and energy-saving,combined infrared and hot air drying（IR-HAD）,has achieved better results in improving the drying rate and product quality of agricultural products and foodstuffs.To improve the drying rate,shorten the drying time and better maintain the quality of the dried products,the drying kinetics,effective moisture diffusion coefficient and activation energy of cantaloupe slices by single hot air drying（HAD）and combined infrared and hot air drying（IR-HAD）under different drying temperatures were comparatively studied.Secondly,to better analyze the IR-HAD engineering of cantaloupe slices,a coupled model of temperature and moisture fields was constructed by COMSOL Multiphysics software to simulate the spatial temperature and moisture distribution characteristics of cantaloupe slices during infrared drying.Finally,based on the Box-Behnken central combined design test,the optimal process parameters for drying cantaloupe slices were studied by the response surface method to control the quality of the dried cantaloupe slices.The main research contents and conclusions are as follows:（1）The changes of drying characteristics and kinetic curves of HAD and IR-HAD of cantaloupe slices at different drying temperatures were investigated.Compared with HAD,IR-HAD can improve the drying rate,shorten the drying time and accelerate the drying process of cantaloupe slices;no constant drying rate cycle was found in the drying process,and the whole drying process was mainly dominated by the decreasing rate phase;the variation ranges of the effective moisture diffusion coefficients of cantaloupe slices during HAD and IR-HAD were 1.06×10^-10～1.52×10^-9m²/s and 1.56×10^-10～2.94×10^-9m²/s,respectively,and increased with the increase of drying temperature;the activation energy required for IR-HAD（28.15 k J/mol）was lower than that required for HAD（33.51 k J/mol）.The six commonly used thin-layer drying models were evaluated by three statistical indicators,and the Page model was most suitable for describing the variation of drying characteristics of cantaloupe slices.（2）The coupled model of temperature and moisture fields was constructed to realize the simulation of cantaloupe slices in the IR-HAD process,The numerical solution results of theD_effmodel considering shrinkage were in better agreement with the measured results of temperature and moisture content of cantaloupe slices in the IR-HAD process,indicating that the established model and simulation solution can characterize the IR-HAD process of cantaloupe slices,and based on the results of numerical simulation,the moisture and temperature distribution characteristics of cantaloupe slices in the IR-HAD process are further analyzed.The moisture gradient between the sample interior and surface gradually decreases and finally tended to be uniform.And the temperature distribution in the beginning stage due to the forced hot air effect,resulting in the center temperature of the sample lower than the surface temperature,with the enhancement of infrared radiation,the temperature of the center rises rapidly,at which time the center temperature of the sample is higher than the surface temperature of the sample,and finally remains in a certain range.（3）The optimal process parameters for IR-HAD of cantaloupe slices were obtained by the response surface method using the infrared hot air temperature,slice thickness and infrared radiation distance as the influencing factors,and the color（L value,Δ）,hardness and vitamin C retention of cantaloupe slices were used as response values.The results of the study show that the infrared radiation temperature and slice thickness had extremely significant effect on L value and color difference（Δ）（p<0.01）,with higher infrared radiation temperature and thin slice thickness leading to a decrease in L value and an increase inΔ.Hardness and vitamin C content were significantly affected by infrared radiation temperature,slice thickness and radiation distance,of which the slice thickness was the most distinct factor affecting the hardness value.Higher infrared radiation temperature and larger slice thickness and radiation distance resulted in higher vitamin C degradation.For the given constraints（maximized vitamin C content and L value,minimizedΔand hardness value）,the optimum drying parameters were infrared radiation temperature 58℃,slice thickness 6 mm and radiation distance 90 mm.Under the optimum drying combination conditions,the experimental values were 65.58（L value）,8.57（Δ）,10.49N（hardness）and 106.58 mg/100g（vitamin C content）,respectively.