Research On The Combination Drying Technology Of Apple Slices

In an increasing puffed food market,apple chips occupy big market shares due to the characteristics of crispy,containing high nutrition,easily eaten and carried.However,in the process of producing apple chips,the pre-drying(dry the fresh apple to apple slices)has crucial effects on ensuring the energy consumption and quality of final products,so the premise of producing high quality apple chips is to develop high efficiency,energy saving,and high quality drying technology to promote the quality of apple slices.In this study,single drying techniques(air jet impingement drying,microwave vacuum drying,hot air drying,hot air assisted radio frequency drying and freeze drying)and combined drying techniques(air jet impingement combined microwave vacuum drying and air jet impingement combined hot air assisted radio frequency drying)were performed on apple slices.After researching the drying technology of apple slices,the apple slices were processed into final products apple chips,and the difference of final products quality which treated by different drying methods would be compared.The major contents and results of the study are as the followings:(1)The fresh apple slices were dried by air jet impingement drying,microwave vacuum drying,hot air drying,hot air assisted radio frequency drying,and freeze drying,respectively,the drying process terminated when the samples reached the designated moisture content(less than 15%).The drying kinetics,and the properties(drying time,energy consumption,color,Vc,total phenolic)of the dried apple slices were investigated.The results showed that Page model was suitable to simulate the drying dynamics of apple slices under microwave vacuum drying,air jet impingement drying,and hot air drying,Logarithmic model was suitable to mulate the drying dynamics of apple slices under hot air-assisted radio frequency drying.To reach the designated moisture content(12-15%),it took the longest time for freeze drying,secondly were hot air-assisted radio frequency drying,hot air drying,air jet impingement drying in turn,and the shortest for microwave vacuum drying.Compared with the apple slices dried by freeze drying,microwave vacuum drying,air jet impingement drying,hot air-assisted radio frequency drying,and hot air drying turned apple slices darker and more yellow.Among them,product from hot air-assisted radio frequency drying showed the greatest change,secondly was microwave vacuum drying and hot air drying,the change of product dried by air jet impingement drying was not significant.In the aspects of nutrition,compared with fresh apple,the apple slices dried by freeze drying had lowest loss ratio of Vc,but had the highest degradation in total phenolic,the apple slices dried by air jet impingement drying and microwave vacuum drying had high retention in Vc and total phenolic.Considering the drying efficiency and product quality,the air jet impingement drying and vacuum microwave drying are superior to other techniques,but which also had some disadvantages,for examples,compared with microwave vacuum drying,air jet impingement had better quality,but had longer drying time,so combined drying methods could be considered to further investigation.Hot air-assisted radio frequency drying was not a suitable single drying method because of high energy consumption and low quality.In order to exploit its drying advantages,we could combine air jet impingement drying with it.(2)Combined air jet impingement and micro wave-vacuum drying.The sample was first dried by air jet impingement drying,the drying processes were optimized by response surface methodology.Air temperature(55,60 and 65 ℃),air velocity(1.3,1.8 and 2.3 m/s),and sample thickness(4,6 and 8 mm)were set as the treatment factors,and drying rate and color were evaluated as the responding quality parameters.After determining the optimal drying condition,combined air jet impingement and microwave vacuum drying to dry apple slices,energy consumption,color and Vc were measured to determine combination method.Finally,compared energy consumption,color and Vc in apple slices dried by combined drying method with single air jet impingement drying,microwave vacuum drying,and traditional hot air drying in energy consumption and quality.The results showed that optimal conditions for air jet impingement drying of apple slices were 65℃ of drying temperature,2.3 m/s of air velocity,and 6 mm of thickness.After determine the optimal conditions for air jet impingement drying,combined air jet impingement drying and microwave vacuum drying,considering the color,energy consumption,and loss ratio of Vc,the suitable combination was that dried by air jet impingement drying 60min first,then dried by microwave vacuum drying until the sample achieving the target moisture content(12-15%).Finally,the drying efficiency and quality of apple slices dried by four different drying methods were compared.The results showed that the combined drying method shorted the drying time when compared with air jet impingement drying method,and improved the product quality when compared with microwave vacuum drying,so the combined air jet impingement and microwave vacuum drying was an alternative drying method.(3)Combined air jet impingement and hot air assisted radio frequency drying.The combined drying conditions were optimized from three aspects,electrode gaps of radio frequency system,temperature of hot air,and drying time of air jet impingement drying.After determining the optimal combined drying condition,the drying time and sample quality among three drying methods,hot air-assisted radio frequency,air jet impingement combined hot air-assisted radio frequency,and air jet impingement combined hot air would be compared.The results showed that the optimal drying conditions of combined air jet impingement drying and hot air assisted radio frequency drying were 12 cm,60 ℃,and AJID drying 100 min for electrode gap,hot air temperature,and air jet impingement drying time,respectively.The combined air jet impingement drying and hot air assisted radio frequency drying was one method which can be used in apple slices drying because of the advantages of low energy and high quality.(4)Firstly,the fresh apple slices were dried by air jet impingement drying,microwave vacuum drying,hot air drying,combined air jet impingement and hot air assisted radio frequency drying,and combined air jet impingement and microwave vacuum drying,respectively,the drying process terminated when the samples reached the designated moisture content(12-15%).Then dried the apple slices to apple chips through moisture equilibrium,puffing,and vacuum drying.The results showed that the quality of apple chips dried by two combined drying methods were not better than which dried by air jet impingement drying,but were significantly better than microwave vacuum drying and hot air drying.