| Longan(Dimocarpus longan Lour.)is knowed as evergreen fruit trees for the sub-division into Sapindaceae and Dimocarpus Lour.It is produced in Fujian,Guangdong,Guangxi,Hainan and other provinces(regions)mainly.Longan pulp is nutritious and protein-rich,it contains varieties of amino acids,carbohydrates,minerals and so on,so is regarded as a valuable natural food since ancient times.But the flesh fruit is difficult to be storaged and don't have a long shelf life,because color change,deteriorate and decay rapidly after harvest since it mature in the hot season(from August to September).Processing longan or longan pulp into dried products is an important way to to solve the flesh longan waste problem,and extending the period of the longan commodity supply.At present,the traditional methods as sun,baking or hot-air dehydration,are usually used to dry products.But there are some problems,such as the enterprises are small,the dried technologies are lagging behind,the browning reaction is bad for storaging,the products quality is difficult to be guaranteed and having an adverse effect on human health,the nutrients and flavor compounds of products loss seriously,edible inconvenience and so on.In addition,It's difficult to control the temperature during the drying process,the drying time is long and the energy consumption is large.Combined hot-air and microwave vacuum drying is a rapid and efficient dehydration method with low energy consumption,the sensory quality and nutrient of products can be retained better after combined drying.At the same time,reducing the drying time and improving energy efficiency of the dehydration.The objective of this thesis was to produce new-dehydrated longan pulp using combined hot-air and microwave vacuum drying technology.The blanching and sulphur-free color-protection treatment of longan pulp was carried out to reduce the shrink and browning reaction during drying process.The hot-air drying characteristics and the drying mathematical model of fresh longan pulp were explored as well as the microwave vacuum drying.The optimum conditions for hot-air and vacuum microwave combination to dry longan pulp were found by the response surface methodology(RSM),then the packing storage technology of dehydrated longan pulp was investigated.In order to reduce the shrink and browning reaction during drying process,the three factors with three levels for L9(34)orthogonal experiments were carried out to investigate the effects of temperature,time and colded ways on the activities of polyphenol oxidase and peroxidase in blanched longan pulp,the effects on the water content,color and sensory evaluation of dehydrated longan pulp were compared with different combinations and concentrations of sulphur-free color-protection liquid,then the optimized parameters were determined by comprehensive optimization methods.The result showed that the optimum blanching temperature was 100 ?,blanching time was 3 min,and cool at room temperature,and 0.10%vitamin C+0.20%citric acid+0.10%cysteine +0.10%CaCl2 can provide the optimum quality for drying longan pulp.The optimal parameters can inhibit the browning reaction effectively to maintain the natural color and flavor of drying longan pulp.Therefore solving the serious shrink,browning reaction,poor color,such as sulfur dioxide residue problem,and extend the shelf life of drying longan pulp.The hot-air drying characteristics and the drying mathematical model of fresh longan pulp were explored as well as the microwave vacuum drying.Effects of the longan pulp size,hot-air temperature and air velocity on hot-air drying rate were studied,and the drying kinetic model was established as Page model,it was satisfied for hot-air drying.Effects of the longan pulp size,microwave power,vacuum pressure and loading capacity on dehydration characteristics of longan pulp were analyzed,and the moisture variation regular pattern during microwave vacuum drying could be described accurately by the Modified Henderson and Pabis model,then to achieve the purpose as online testing by predicting the moisture content at any time.Taking total sugar content,color,energy consumption and sensory quality as evaluation indexes,the response surface methodology(RSM)was carried out to investigate the effect of hot-air temperature,conversed moisture content and microwave intensity on the quality of dehydrated longan pulp.The optimum conditions for drying longan pulp by hot-air and vacuum microwave combination were found that hot-air temperature was 73 ?,conversed moisture content was 36%,and microwave intensity was 8 W/g.Compared with hot-air drying,vacuum drying and microwave vacuum drying,the optimum conditions for drying longan pulp could greatly improve the quality of dehydrated longan pulp,short the drying time and reduce the energy consumption.During storage,the dehydrated products' stability was affected by packaging materials,packaging methods and storage conditions directly.In order to investigate the storage stability of dehydrated longan pulp,the adsorption isotherms of dehydrated longan pulp were determined and simulated.The results show that the adsorption isotherms of dehydrated longan pulp is J-shaped,and the Peleg model can be used to describe the adsorption characteristics of dehydrated longan pulp.In addition,the effects on the quality of dehydrated longan pulp by different packaging materials,packaging methods and storage temperatures were studied forusix months,including water content,water activity,color and browning index.On the basis of the scientific and optimized parameters of hot-air combined microwave vacuum drying,the dried longan pulp was Obtained.HACCP system was applied for possible hazard analysis during processing,inclduing each step.Then established aix critical control points:acceptance of the raw materials,blanching-color protection,hot air-microwave vacuum drying,product grade,packaging and storage.In response to these critical control points,the HACCP plan table was developed,the critical limits were established for each corresponding critical control points,and corresponding operational corrective measures were proposed to ensure that dried longan pulp's quality is stable,controllable and safety. |
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