Effict Of Heat Pump Drying Mode And Packaging Method On Longan Drying And Storage Characteristics

With fresh longan as raw material,drying characteristics were studied through industrial large-scalefixed bed heat pump drying(model ?)and laboratory fixed bed heat pump drying(model ?),by measuring the weightlessness rate,reducing sugar content,colour and lustre,peroxidase(POD)and polyphenol oxidase(PPO)et al of dried longan in the process of drying.What's more,Drying effect between model ?fixed bed heat pump drying andmodel ? fixed bed heat pump drying were compared.Storage characteristic of semi-dry dried longan for different water content and packaging type were compared.This study provided certain theoretical and practical basis for application ofindustrial large-scalefixed bed heat pump drying and laboratory fixed bed heat pump drying.The main results were presented as following:(1)Effect of longan drying by model ?-fixed bed heat pump on the drying behaviours such as air velocity,temperature of longan fruit,the weightlessness rate and moisture content.The results showed that different wind speed and temperature distribution of fixed bed,resulted in the efficiency differences of longan.Wind speed at the 3rd fixed bed was obviously lower than others after they were loaded.3 In the early stage of the drying process,more far distance from fixed bed dry outlet,the efficiency of the drying was lower.After about 20 h,longan weightlessness rate on the position of the 3rdbegan to lag behind the others.The drying efficiency of the 3rd fixed bed was minimum for44 h drying time,whosewet basis moisture content of longanwas about 10% lower thanthe 1st fixed bed,the highest.That is the phenomenon of drying efficiency from different fixed beds on industrial large-scale fixed bed heat pump was obviously uneven,and the drying efficiency remains to be further improved.It suggests that the temperature plays a main roleon the drying efficiency,during the increasing of temperature on industrial large-scale fixed bed heat pump drying.After the temperature is relatively stable,the influence of wind speed on drying efficiency plays a main role.The enzyme activity of peroxidase(POD)of dried longan passivated after 24 h,while polyphenol oxidase(PPO)passivated after 16 h.The results also showed that the color of dried longanchanged significantly after 16 h.(2)Model ?-fixed bed heat pump drying was used to dry longan fruit,with wind speed,the surface temperatureof longan,the weightlessness rate and moisture content compared and analyzedon the process of drying.Results found that in process ofmodel?-fixed bed heat pump drying,temperature was the main reason of the efficiency difference of longan drying.Wind speed among the fixed beds inthe laboratory small fixed bed heat pump drying process is relatively uniform while full loaded,it has no influence on drying efficiency.But the fruit surface temperature of longan in the 4t h fixed bed was significantly lower thanlongan in other fixed beds in the process of heat pump drying.Between the 4t h fixed bedis more away from heat source outlet,whose wind temperature is minimum,resulting the weightlessness rate of longan in the 4t his the lowest.The laboratory small fixed bed heat pump drying oven is small in size,heating rapidly,so the drying temperature can reach more than 60 ? in 4 h.Thereforethe enzyme activity of POD and PPO passivated respectively after 12 h and 8 h.The results also showed that the color of dried longanchanged significantly after 16 h.Color of dried longan changed obviouslyin 28 hin the process of drying.(3)Semi-dry dried longanwith water content of respectively 25%,35% and 45% wererespectively processed by ordinary packaging,vacuum packaging,deoxidizer packaging,vacuum&deoxidizer packaging and nitrogen filled packing,conducting accelerated test with 37 ?and RH90 %.The results showed that the changes of weight of dried longan were not obvious for 60 d storage.theweight of dried longan with moisture content of 25% and 45%respectively showed a trend of rising or falling,while the weight of dried longan with moisture content of 35% fluctuatly changed,with no obvious regularity.Storage effect of semi-dry dried longanwithdeoxidizer packaging and vacuum&deoxidizer packaging dry were more obvious than other packing ways for 60 d,obviously delayed the brown stain of dried longan,to be antibacterial.As for vacuum packing and nitrogen filled packing,semi-dry dried longan did not achieve the target,such as effective in inhibiting effect and brown staindelaying.Because the vacuum degree is of packing was difficult to control in fruit shell of dried longan.The results also showed that with higher moisture content of dried longan,the storage performance of ordinary packaging,vacuum packaging,deoxidizer packaging,vacuum&deoxidizer packaging and nitrogen-filled packaging weakened.