The Study Of Moisture Diffusivity Characteristics Of Apple Chips During Explosion Puffing Drying

In this study, Fuji apple was chosen as the raw material to study moisture diffusion characteristics during explosion puffing drying process, with the drying mathematical models, low-field nuclear magnetic resonance and imaging technology. The changes of moisture content during drying process affected by process variables, pretreatment methods and initial drying moisture content were examined. By establishing the kinetics model and calculating the moisture effective diffusivity, the effect of drying parameters on the moisture effective diffusivity during the drying process was investigated. Meanwhile, some dried product attributes were measured in order to establish a correlation between the moisture diffusion and crisps quality, which was aimed to provide basic data for improving explosion puffing drying technology and enriching puffed drying theory.Through the research of moisture diffusivity characteristics of apple chips during explosion puffing drying, the following conclusions can be made:1. Apple’s initial moisture contents were90.9±2.1%; the drying process mainly occurred in the falling rate drying process, wihich the entire drying process was dominated by the internal moisture diffusion. The Guggenheim, Anderson and De Boer (GAB) model were the best one in describing the apple chips equilibrium moisture content mathematical model at different temperatures and wide ranges of water activity. The apple crisp under different temperatures showed a good stability.2. Under different process parameters, the values of effective moisture diffusivity (Deff) were determined to be in the range of98.03×10-12～274.17×10-12m2s-1. The moisture diffusion was mainly affected by vacuum drying temperature and slice thickness, and slightly affected by puffing temperature (within the range of70癈～90癈). The values of Deff increased with the rise of puffing temperature and vacuum drying temperature; and decreased with the increment of apple slice thickness.3. Four different pretreatments including freezing, maltose syrup, calcium chloride and standard pasteurized milk immersion at atmospheric pressure were used to treat the apple slices, the results showed frozen and milk immersion treatment accelerated puffing drying rate, and there were no significant difference between the calcium ion solution and malt syrup solution immersion treatment and the controlled group.The values of the effective moisture diffusivity of frozen and milk immersion treatment apple were397.69×10-12m2s-1and302.67×10-12m2s-1, respectively, which were significantly higher than the control samples; value of Deff of the samples immersed with maltose syrup solution’s was154.07×10-12m2s-1, which was lower than the control apple slices; calcium solution immersion processing effective moisture diffusion coefficient was261.48×10-12m2s-1.4. In the study of the effects of different pre-drying moisture content on moisture migration during drying process, it was found that moisture diffusion coefficient was significantly influenced by raw material pre-drying moisture content. The effective moisture diffusivityvalue increased with the decreasing pre-drying moisture content (PDMC); the highest value of Deff was observed in the critical value of PDMC; while the value of De/f declined with PDMC decreasing when PDMC exceeded the critical value.5. By comprehensive consideration, Parabolic model was selected as the best one in simulating the changes of the moisture content during explosion puffing drying process.6. The quality varied from the processing techniques and the quality indicators such as color, water activity, hardness, brittleness and microstructure with Deff values showed regularity changing trends.7. The T21, T22, T23and T24value of apple slices, has the certain regular changes during explosion puffing drying. Fresh apple’s water basically are free water, After samming process, different characteristics of water form a certain proportion in the sample.