Appearance Quality Control Of Jujube Slices During Drying Based On Glass Transition Theory

Appearance quality deterioration including texture collapse and browning in the drying process of jujube slices were the main problems that restricting the development of the industry.In the present study,correlation between galss transition and macroscopic volume shrinkage and color difference were evaluated by state diagram.A solid matrix simulation system was constructed to reveal the influence mechanism of amorphous small molecule sugars and organic acids on the quality of jujube slices during the drying process,and the quality control technology of jujube slices was further studied by adding macromolecular polymers such as maltodextrin and inulin.The main conclusions were as follows:(1)The heat pump drying(HPD)method can significantly improve the quality of jujube slices.Compared with hot air drying(HAD),jujube slices dried by HPD was significantly reduced color difference(?E)from 3.9 to 2.95.The shrinkage ratio of jujube slices dried by HPD was reduced from13.56%to 10.86%.During drying process,the difference(?T=T-T_g)in temperature between the sample temperature(T)and the glass transition temperature(T_g)had significant effects on jujube colors and shrinkage ratio.T_g increased significantly with the decrease of immobilized water.The macroscopic state diagram was established to visualize the quality change of jujube slices during the drying process.According to the state diagram,the critical moisture content of jujube slices dried by HPD that glass transition occurred at 25°C was 0.061 g/g.(2)The solid model system of jujube slice skeleton(JSS)was constructed to study the effects of single amorphous sugars and organic acids on their glass transition and crystallization.The results showed that the removal ratio of sugar and organic acid in jujube slices was higher than 99%after the ultrasonic assisted water extraction.Fructose(F),glucose(G),citric acid(C)and malic acid(M)solutions of different concentrations(0?32 g/100g)were used to permeate JSS,and the water contents of all HP-dried samples were about 5 g/100g(dry basis).Sugars will reduce the T_g in the system, structure of JSS-G/M blends was changed from "amorphous glassy" to?amorphous rubbery?by increasing the osmotic solute concentration.T_g was decreased from 50.8 to 14.0°C when JSS was osmosed in a 4 g/100g fructose solution.Organic acids induced their crystallization in JSS.The crystallinity of JSS-M immersed in 32 g/100g osmotic solution concentration was increased from 2%to75%.Fructose presented greater influence on the adverse quality of jujube slices.(3)The effects of Maltodextrin(M)and Inulin(J)on galss transition and crystallization of jujube slices were studied.M and J solutions of different concentrations(10%,20%and 30%)were added to jujube slices by spraying,and the obtained samples were marked as M1,M2,M3 and J1,J2 and J3respectively.Adding M and J can significantly improve the T_g(>45°C)in jujube slices,which was about 20°C higher than that of neat jujube slices.M1 had the largest shrinkage ratio,while J2 was the most effective to decrese the shrinkage ratio and obtain the best color.The addition of maltodextrin and inulin delayed and reduced the hygroscopicity from 65%to 35%of jujube slices.With the increase of concentration of Inulin,the crystallization of jujube slices treated with inulin solution was obvious,while the samples treated with maltodextrin and JSS were amorphous.According to the appearance quality and economic cost,jujube slice with 20%inulin solution spraying was a better way to improve its quality.