Optimization Of Vacuum Freeze Combined Drying Process For Peach Crisps Based On Different Pretreatments

In this paper,the experiment materials were white peach(meicui)and yellow peach(5th jintong).It was realized that high-humidity hot air impingement blanching(HHAIB)could improve the quality of hot air dried(75?)and vacuum freeze-dried peach chips.The best HHAIB pretreatment times were difinited.Then different ultrasonic assisted osmotic pretreatments were carried out and the feasibility that isomaltose oligo-saccharide replace maltitol during the pretreatment for hot air dried(75?)and vacuum freeze dried yellow peach chips were realized.On the basis of above experiments,the changes of color,sample temperature,environment temperature,the glass transition temperature(Tg),total polyphenols and ascorbic acid(VC)of yellow peach slices during hot air drying(75?)and vacuum freeze drying were explored and the diagrams were distructed in oder to selected the water transitions of combined drying methods.Then experiments of combined drying methods were carried out and the best combined drying method was realized.The main conclusions were as follows:(1)After HHAIB pretreatment,the browning of hot air dried peach chipss was significantly reduced while the fading of vacuum freeze dried peach chips were more serious.Compared with untreated hot air dried and vacuum freeze dried samples,HHAIB could increase the contents of total polyphenols,ascorbic acid and antioxidant capacity for both hot air dried and vacuum freeze dried white peach chips and yellow peach chips.Furthermore,the increase of the contents of total polyphenols,ascorbic acid and antioxidant capacity of vacuum freeze dried peach chips were more significant.Based on the results,60 s or 90 s are proposed as the optimum blanching times before drying of peach chips.(2)The water loss of US+IMO treatment was the highest which reached to 23.36%while the solid increment reached to 2.25%,which met the addition requirements.After different USOD treatments,the relaxation time and reltive contents of free water and immobilized water of raw yellow peach slices were decreased and increased respectively.After different USOD treatments,the microstructure were changed which caused the reduction of total phenolic content,ascorbic acid content,ABTS free radical scavenging ability and the reduction of Fe2+reducing ability of hot air dried and vacuum freeze dried yellow peach chips.However,the total carotenoid content was increased and the content of yellow peach chips under US+IMO pretreatment was the highest.In addition,the results of the electronic tongue showed that the taste of three different USOD treatments of hot air dried and vacuum freeze dried yellow peach chips were similar.Based on the above findings,oligo-isomaltose could be used to replace the traditional sugar liquid(such as maltitol)to osmotic dehydration which could improve the taste of yellow peach chips.(3)The changes of relative indexes during hot air drying and vacuum freeze drying were determined.The state diagrams were constructed.Based on the state diagrams,moisture transition points of the different combined drying methods were choosed.For the combined drying methods of hot air drying follwed by vacuum freeze drying(AD-FD),3.61 g/g d.b.and 2.79 g/g d.b.were selected as the moisture conversion points which corresponding to the combination 1-1 and the combination 1-2.For the combined drying methods of vacuum freeze drying follwed by hot air drying(FD-AD),0.20 g/g d.b.,0.12 g/g d.b.,and 0.05 g/g d.b.were selected as the water transition points,which corresponding to the combination 2-1,the combination 2-2 and the combination 2-3.The combined drying experiments were carried out and the relative indicators were determined.The results showed that drying times of the combination 1-1 and combination 1-2 were too long to dry ao that the combination 1-1 and combination 1-2 were not feasible.Based on a comprehensive view of multiple indicators such as texture,nutritional quality and drying time,vacuum freeze drying followed by hot air drying(FD-AD)was the most feasible while the best drying parameters was vacuum freeze drying for 15 h follwed by hot air drying for 5 h.