Research On Key Technology Of Processing Clarified Snow Pear Juice

Snow pear is soft,juicy and nutritious,however it is easy to rot and causing serious waste.Therefore,processing snow pear into juice can effectively reduce the loss and increase the added value of its products,which is of great significance to promote the development of snow pear industrial chain.The following problems exist during the processing and storage of pear juice:discoloration,turbid precipitation,flavor change and nutrient loss,can seriously affect the quality of pear juice products and reduce the market competitiveness of pear juice products.In this paper,snow pear was used as the experimental material,the key technology of processing snow pear juice is studied,which provides a certain reference for the deep processing of snow pear.The main research is as follows:1.In order to inhibit the enzymatic browning of snow pear juice,the enzymatic characteristics of polyphenol oxidase（PPO）and the effect of different color protectors on PPO activity were studied by Spectrophotometry with catechol as the substrate;Three additives,L-cysteine,D-isoascorbic acid and ascorbic acid,were selected to protect the color of snow pear juice,and the optimal combination of color protection was determined by response surface optimization test.The results showed that the optimal pH and temperature of snow pear PPO were 4.5 and 30℃;snow pear PPO had certain thermal stability,and the time needed to inhibit PPO activity decreased with the increase of temperature;the kinetics of catechol catalyzed by snow pear PPO was in accordance with the Michaelis equation,the maximum reaction rate Vmax=217.39 U/min and Michaelis constant Km=0.152 mol/L.The optimal combination of color fixative for processing snow pear juice is:6.56mmol/L L-cysteine,4.58mmol/L D-isoascorbic acid and 6.18mmol/L ascorbic acid.Under this condition,the inhibition rate of browning of snow pear juice can reach 90.82%.2.On the basis of the best technology of inhibition of enzymatic browning,the clarification technology of snow pear juice was studied.Snow pear juice was clarified by adding gelatin,chitosan and pectinase in juice,respectively.The results showed that adding chitosan was the best method for clarification of pear juice.The optimum conditions were chitosan usage 0.80g/L,under 35℃and treated for 40min.The light transmission of juice measured was 99.3%.3.Based on clarified snow pear,two sterilization methods,high temperature sterilization and ultra-high pressure（UHP）sterilization,were used.The commercial sterility and juice quality were used as the inspection indicators to compare the two sterilization methods and explore the best sterilization conditions.The results show that the optimal conditions of UHP treatment was 300mpa for 10min,and the optimal conditions of high temperature treatment was 121℃for 1min,which can meet the requirements of commercial sterility.According to the effects of different sterilization methods on the physical and chemical properties,sensory evaluation and volatile aroma components of pear juice,it is concluded that the physical and chemical properties,color and flavor of pear juice after ultra-high pressure treatment（300mpa,10min）are more similar to that of fresh pear juice,indicating that ultra-high pressure sterilization can better maintain the inherent nutrition and color of pear juice.4.The influencing factors and optimum storage conditions of clarified pear juice were studied with browning degree as the index.The results showed that temperature had a significant effect on the browning of stored pear juice.The higher the temperature was,the higher the browning degree of pear juice was;the higher the pH was,the higher the browning degree of pear juice was;different metal ions had different promoting effects on non-enzymatic browning,among which Fe³⁺>Fe²⁺>Cu²⁺;oxygen and light could promote non-enzymatic browning By controlling these factors,non-enzymatic browning of pear juice can be effectively inhibited.The physical and chemical indexes of snow pear were analyzed by path analysis at different storage conditions to investigate the influence of each index to the browning degree of pear juice.The results showed that,at 4℃,the content of 5-HMF in snow pear juice affected the browning degree most;the second is the interaction between 5-HMF and pH;and the third is the interaction between 5-HMF and amino nitrogen.Under 25℃,the interaction between reducing sugar and amino nitrogen had the greatest influence on browning degree,followed by amino nitrogen,and the last was the interaction between soluble solids and amino nitrogen.Therefore,the best storage conditions for clarified pear juice are as follows:4℃,natural pH,vacuum seal,avoiding air and light,keeping away from metal ions.Under such conditions,the maximum acceptable browning degree of its sensory color was 0.317,and the corresponding time for storage was 60 days.The product had a delicate fragrance and a moderate acid sweet ratio.That is,the shelf life of clarified snow pear juice can be up to 2months at 4℃.