Dynamics And Quality Control Of Convection-drying Of Xinjiang Red-fleshed Apple

Red-flesh apples,for its long history of cultivation,are widely planted in Xin-Jiang.Red-flesh apple trees boast red leaves,branches,flowers and fruits,which make red-apple trees equipped with sightseeing value.Besides,red-flesh apples are rich in anthocyanins and polyphenols,with high healthy function to human body.However,red-flesh apples are mainly preserved as dried slices because of their inferior storage characteristics.Consequently,it is important to develop innovative product of red-flesh apples and to extend deep-manufacturing industrial chins for improving the market competitiveness.Due to the great retention of apple qualities and the convenience to transport,apple crisps and powders have been increasingly popular in the public.After dehydration,red-flesh apples can be comprehensively utilized,fitting the health,environment-friendly concept,with promising market potential.As a traditional drying technique,hot-air drying has many advantages,including the convenience,high drying efficiency,etc.However,the loss of nutrients and the browning reaction are inevitable problems during the drying process.As a result,how to retain the original qualities of red-flesh apples and reduce the browning has become the restraint of industrializing drying product of red-flesh apples.Taken the hot-air convection-drying equipment and the author investigated comprehensively the drying techniques of Xinjiang red-flesh apples,optimized the color-protecting formula against browning reaction,established the drying mathematic model of hot-air drying and the degradation model of polyphenol and anthocyanins,and finally explored the effect of different drying temperature on qualities of dried apple slices.Results as follows:1.The author chose calcium chloride,citric acid,sodium chloride as color-protecting regent,explored the inhibitory effect against PPO of different concentrations and optimized the formula of color-protecting regent.Results suggested that the capabilities of the three color-protecting reagent against PPO were as follows:citric acid>sodium chloride>calcium chloride.Orthogonal experiment results showed that the mixture of 0.4%calcium chloride,0.5%citric acid and 0.5%sodium chloride was observed the optimum color-protecting effect,reducing PPO ability to 37.5%and inhibiting enzymatic browning.2.The author explored the effects of hot-air temperature,slice thickness and hot-air velocity on drying characteristics of red-flesh apple slices,and obtained the optimal drying model through fitting drying models.Results showed that the drying process of red-flesh apple slices features falling-rate drying in hot-air convection drying equipment.Hot-air temperature,slice thickness and hot-air velocity all had effects on the drying characteristics of red-flesh slices,and hot-air temperature had the most significant effect.With the increase of hot-air temperature and velocity and the decrease of slice thickness,the effective moisture diffusion coefficient increased,and drying activation energy was 21.1433 kJ/mol.Page model was the optimum model.3.The author investigated the degradation regularity of red-flesh apple polyphenols and anthocyanins during hot-air drying process,and established dynamic model to predict polyphenol and anthocyanins content,respectively.Results showed that polyphenol and anthocyanins content decreased as the increase of drying time,and the dynamic models complied with the first order reaction.The half-period time of polyphenols was 119.51-252.05 min,frequency constant 1.6514 min-1 and thermal degradation activation energy 16.8816 kJ/mol in the temperature range of 40?80 ?.In a similar way,frequency constant of anthocyanins was 1.4198 min-1,the half-period time was 91.08?179.11 min and thermal degradation activation energy 15.5505 kJ/mol in the temperature range of 40?80 ?.Both polyphenols and anthocyanins are thermally sensitive substances,heating is prone to degradation,raising the temperature will accelerate the degradation reaction.4.To ascertain the optimum drying temperature,the author investigated the effects of different hot-air temperature(40?80 ?)on the physiochemical characteristics,sensory qualities,nutrition compositions changes and antioxidant activities of apple powder.Results showed that hot-air temperature has significant effects on red-flesh apple qualities:low temperature but long time(40 ?,210 min)and high temperature but short time(80 ?,90 min)all damaged the nutrition compositions,sensory qualities and antioxidant activities of red-flesh apple powder,although physical characteristic including powder parameters had not been significant affected.Taken all factors above into consideration,the mixture of 0.4%calcium chloride,0.5%citric acid and 0.5%sodium chloride can reduce PPO ability to 37.5%and inhibiting enzymatic browning;Page model was the optimum model to describe the hot-air convection drying process of red-flesh apple slices;polyphenol and anthocyanins content decreased as the increase of drying time,and the dynamic models complied with the first order reaction;the temperature of 50?60 ? was the optimum drying temperature.