CAJ | 학술논문

该文应用菲克第二定律、采用低场核磁共振及磁共振成像系统(MRI,magnetic resonance imaging)、差示量热扫描、扫描电子显微镜等技术,分析不同预干燥温度对苹果片脉动压差闪蒸干燥过程中水分扩散和微观结构的变化.试验结果显示:随预干燥温度升高,水分扩散速率加快,有效水分扩散系数随温度升高而变大,整体范围在 9.84×10-9~7.24×10-8 m2/s;干燥作用引起水分状态由高自由度向低自由度迁移,自由水含量在干燥初期迅速降低,不易流动水含量先增加后降低;MRI结果表明:苹果鲜样水分集中于中心部位,随干燥进行,水分向外扩散并均匀分布于样品中,含水率的降低,导致图像亮度呈现降低趋势,同时样品表现出向中心收缩的现象;脉动压差闪蒸干燥过程中,含水率与玻璃化转变(Tg)和水分活度(aw)之间表现出极显著相关性(R>0.90,p<0.01),含水率的降低引起Tg升高、aw降低,且Tg与aw之间呈线性相关(R2>0.81);水分的散失导致微观结构发生变化,细胞破裂形成空腔,脉动瞬间的真空作用促使苹果片多孔海绵状结构进一步形成,赋予其酥脆口感.该试验可以为苹果片在脉动压差闪蒸干燥方面提供理论依据和技术参考.
해문응용비극제이정률、채용저장핵자공진급자공진성상계통(MRI,magnetic resonance imaging)、차시량열소묘、소묘전자현미경등기술,분석불동예간조온도대평과편맥동압차섬증간조과정중수분확산화미관결구적변화.시험결과현시:수예간조온도승고,수분확산속솔가쾌,유효수분확산계수수온도승고이변대,정체범위재 9.84×10-9~7.24×10-8 m2/s;간조작용인기수분상태유고자유도향저자유도천이,자유수함량재간조초기신속강저,불역류동수함량선증가후강저;MRI결과표명:평과선양수분집중우중심부위,수간조진행,수분향외확산병균균분포우양품중,함수솔적강저,도치도상량도정현강저추세,동시양품표현출향중심수축적현상;맥동압차섬증간조과정중,함수솔여파리화전변(Tg)화수분활도(aw)지간표현출겁현저상관성(R>0.90,p<0.01),함수솔적강저인기Tg승고、aw강저,차Tg여aw지간정선성상관(R2>0.81);수분적산실도치미관결구발생변화,세포파렬형성공강,맥동순간적진공작용촉사평과편다공해면상결구진일보형성,부여기소취구감.해시험가이위평과편재맥동압차섬증간조방면제공이론의거화기술삼고.
Food processing is designed to meet the supply for better shelf life for food and its materials, where pulsed sudden decompression flashing drying (PSDFD) is a kind of new type, environmental protection and not fried fruit and vegetable drying technology that give better quality to products. In this experiment, drying technology of hot air joint PSDFD was adopted. Moreover, on the basis of calculating moisture diffusion coefficient, with the combination technology of low-field nuclear magnetic resonance (LF-NMR), magnetic resonance imaging system (MRI), differential scanning calorimetry (DSC) and scanning electron microscope (SEM), the effect of different pre-drying temperature on change of water diffusion, water state, glass transition temperature and microstructure was analyzed in the paper. Further analysis of water diffusion characteristic and microstructure under different pre-drying temperature PSDFD was expounded as well. NMR and MRI as a non-destructive, non-invasive, promising technique has been used in detecting the water state, moisture diffusion and movement by the trace of1H2O proton activities. DSC is another tool to quantify water migration in food and control the shelf life. The correlation between the results of moisture content, water state, water activity (aw) and glass transition temperature (Tg) were also analyzed. In the study, fresh Fuji apple was peeled, pitted and cut into sector slice with 5 mm thickness and then placed evenly on the tray under different temperature of 50℃, 70℃, 90℃ until the water content reached 30%, finally went into PSDFD stage with flashing temperature of 95℃and vacuum temperature of 70℃. Experiment results showed that, moisture diffusion rate was accelerated along with the pre-drying temperature increasing. As expected, the drying time was effectively reduced by high temperature, and the average drying rate proportion of 50℃, 70℃, 90℃ was 1:1.3:2. High temperature at 90℃ could directly shorten drying time, but reduce nutrition quality greatly. Thus 70℃ was selected as the best pre-drying temperature with its suitable color and crisp taste. Calculated by Fick's second law, the effective moisture diffusion coefficient up with increasing temperature, and ranged from 9.84×10-9 to 7.24×10-8 m2/s. All these explained that higher temperature facilitated moisture migration rate from interior to surface and evaporation rate from surface to air. Moreover, drying effect caused water state changes and moisture with high degree of freedom moved to the moisture with low degree of freedom. Free water existed in plant catheter and vacuole firstly diffused due to its low degree of bondage. Mutual transformation occurred between different states of moisture during PSDFD. Carbohydrate concentration increase and degradation of nutrients in cytoplasm resulted in the proton exchange action, which usually occurred on hydroxyl between water molecules and polysaccharide matrix. MRI showed that water in the fresh samples focused on its center. Then, moisture diffused outward and distributed uniformly in the sample during drying. A trend of decline was shown in brightness of MRI images with the water content decreasing, while the sample presented shrinkage to the center. A significant correlation were obtained from water content andTg (r>0.90,p<0.01) that moisture decreased leading to glass transition temperature rising. As well, water content strongly effectedaw (r>0.93,p<0.01) during PSDFD and water activity reduced with water content declining. In addition, a linear correlation were represented betweenTg andaw (R2>0.81) under Origin analysis whereTg decreased withaw increasing. Fresh apple is composed of a large number of swelling cells with compact connection and regular shape. Clear edge structure was showed in the micrograph for it untreated by SEM. Water loss resulted in the changes of microstructure and cell rupture into a cavity. Porous structure was created by vacuum effect under pulsing, which gave product crisp taste. This test could provide the theoretical basis and technical reference for apple slices in PSDFD.