| Peanut is China’s most internationally competitive export-oriented bulk crop,but also an important source of vegetable protein and edible oil.In recent years,quality issues have become the main factors affecting the international competitiveness of China’s peanut products.If peanuts are not dried in a timely manner after harvest,they are very susceptible to mildew and even toxins,which can reduce the quality of peanuts.Therefore,the drying of peanuts after harvesting is particularly important for the maintenance of peanut quality.Drying process parameters are important factors affecting the quality of peanuts after drying,in addition,they are also affected by the variety of materials,physical and chemical properties,internal structure and external shape.Therefore,this topic uses a customized digital cave test device to dry peanut pods in a thin layer,analyzes the influence of different drying process parameters on the hot air drying characteristics of peanut pods and fits to establish a kinetic model,and on this basis uses the response surface method to optimize the hot air drying process of peanut pods to obtain the best drying process parameters;takes high oleic acid,high sugar and common peanut varieties as the research object,and takes the best drying process To investigate the differences in the comprehensive quality and volatile components of peanuts of different varieties after hot air drying as the test conditions;to investigate the relationship between the geometric characteristics and thermal physical parameters of peanut pods and the moisture content and fit them as a function of moisture content,so as to lay the foundation for the determination of subsequent simulation parameters;a three-dimensional two-component structural model of peanut pods was constructed using COMSOL Multiphysics to simulate the drying of peanut seeds under the optimal drying process test conditions and reveal the thermal mass migration mechanism during the drying process of peanut pods.The model was validated by thin-layer drying tests,and the changes of internal moisture distribution state and microstructure during the drying process of peanut pods were further analyzed by combining low-field NMR technology and electron scanning microscopy.The main research contents and conclusions are as follows:(1)Study on hot air drying characteristics and optimization of process parameters of peanut podsThe drying characteristics of peanut pods under different drying process conditions were studied by using a custom-made digital cave test device with high oleic peanut as the research object and air temperature,air speed and material layer thickness as the test factors,and six common drying mathematical models were selected to fit the single-factor test data respectively.The drying rate,color and crude fat content were used as the evaluation indexes,and the response surface test was conducted on the basis of the single-factor test,and the subordinate degree integrated scoring method was used to optimize the drying process of peanut pods comprehensively.The results showed that the drying process of peanut pods was mainly reduced-rate drying,and among the six different thin-layer drying models selected,the Two-term model had the best fit with the experimental data and could be used to describe the hot-air drying process of peanut pods.The effects of air temperature and air speed on drying rate and crude fat content were highly significant(p<0.001),and the effect of material layer thickness on color was highly significant(p<0.01).The optimum process parameters for hot air drying of peanut pods were obtained by the subordinate degree integrated scoring method,which were 42℃ air temperature,0.75 m/s air speed and 2 cm layer thickness.It was verified that the error between the theoretical and experimental values was small,and the regression model had some reliability.(2)Study on the quality of different varieties of peanuts after drying based on the optimized hot air drying processThe optimum drying process parameters were used as the test conditions to study the comprehensive quality(hardness,protein,fat,amino acids,fatty acids,tocopherols and volatile components)of peanuts after hot air drying and to analyze and compare the differences among different varieties(high oleic acid type,high sugar type and common type).The results showed that the hardness of peanut shells all showed a trend of decreasing and then increasing with the decrease of dry basis moisture content during the drying process,and the hardness of peanut kernels all showed a trend of increasing-decreasing-increasing.the protein content of the three varieties of peanuts ranged from 18.10% to 20.90%,and the crude fat content ranged from 40.93% to 42.51%.there was no significant difference in the crude protein content of high oleic acid and high sugar peanuts,but There was a significant difference in crude fat content(p<0.05),and the crude protein and crude fat content of high sugar peanuts were the highest;amino acids and fatty acid composition were consistent,with a range of 36.91-39.76 g/100 g of total amino acid content,and the essential amino acid content of high oleic peanuts was the highest,followed by common peanuts,and the lowest in high sugar peanuts;there was no significant difference in the percentage content of linolenic acid,and palmitic acid,stearic acid There was no significant difference in the percentage content of linolenic acid,palmitic acid,stearic acid,oleic acid,linoleic acid,arachidonic acid and behenic acid(p<0.05),the highest oleic acid content in high oleic peanuts and common peanuts,and the highest linoleic acid content in high sugar peanuts,both of which were inversely proportional.Three types of tocopherols,α-tocopherol,γ-tocopherol and δ-tocopherol,were detected in different varieties of peanuts,and the total VE content of high-sugar peanuts was significantly higher than that of common peanuts and high-oleic peanuts(p<0.05).In addition,81 volatile substances were detected in 12 categories,38 in high-sugar peanuts,26 in high-oleic peanuts and 57 in common peanuts.Both high-sugar peanuts and high-oleic peanuts were mainly alcohols and alkanes,and common peanuts were mainly alkanes and olefins.The differences in volatile components and contents were related to the differences in amino acid and fatty acid composition of peanuts.(3)Study on the main physical parameters of peanut pods of different varietiesTo investigate the relationship between geometric properties and thermal physical parameters of peanut pods and water content,triaxial dimensions,peanut shell thickness,relative density,100-grain weight,and thermal conductivity,specific heat capacity,and thermal diffusion coefficient of the main botanical types of peanuts(intermediate,multigrain,and pearl bean types)were determined.The results showed that the triaxial size of peanut pods was less affected by water content,and all of them obeyed normal distribution.Significant differences in peanut shell thickness existed between large and small fruits and were directly proportional to the size of peanut pods.There was no significant difference in the relative density of peanut kernels of different varieties,while the relative density of peanut shells of Kainong 308 variety was significantly higher than that of other varieties(p<0.05).The triaxial size and 100-kernel weight of peanut kernels with different moisture contents increased approximately linearly with the increase of moisture content.The variation in dry basis moisture content was greater from 0.3 to 0.7 and gradually decreased below 0.3.The differences between varieties are determined by their physical properties.For the thermal physical parameters,the thermal conductivity and specific heat capacity showed a single linear relationship with moisture content,and the thermal diffusion coefficient varied relatively insignificantly,and then the above thermal property parameters were fitted as a function about moisture content,and the functional equation was obtained and applied to the subsequent simulation parameters.(4)A heat and mass transfer model of peanut pods hot air drying based on a two-component structureBased on the thermal mass balance principle,a three-dimensional two-component structural thermal mass transport model of single-seeded peanut pods was constructed using COMSOL Multiphysics to simulate the internal moisture distribution and temperature changes of peanut pods during hot air drying to study the moisture transport mechanism,and the model was validated by thin-layer drying tests.At the same time,the changes of internal moisture distribution state and microstructure of peanut pods during drying were further analyzed by combining low-field NMR technique and electron scanning microscopy.The results showed that the thermal mass migration during drying was dominated by the mass transfer process.The three-dimensional two-component structure of peanut pods thermal mass migration model has high accuracy,and the maximum error between the simulated and experimental values of dry basis moisture content of peanut pods in the drying process is less than 10.25%,and the maximum errors between the simulated and experimental values of dry basis moisture content of peanut shells and peanut kernels in the one-component model are9.10% and 7.60%,respectively,which can simulate the thermal mass migration of peanuts in the hot air drying process more accurately.In addition,the internal moisture of peanut kernel during the drying process is greatly influenced by the tissue structure.Free water and part of the weakly bound water are the main moisture dissipated in peanuts during the drying process,the change in oil content is small,and the bound water content is basically unchanged.The peanut kernel structure shrinks and deforms as drying proceeds,and the oil gradually migrates to the surface.Compared with peanut kernel,the change of peanut shell tissue structure was not obvious. |
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