[关键词]
[摘要]
为了研究香芋片微波真空干燥过程中水分扩散特性,采用微波真空干燥箱在1.5、2.0和2.5 W/g 3个微波强度下对香芋片进行了干燥试验,利用低场核磁共振技术测定了香芋片微波真空干燥过程中水分迁移及分布情况。结果表明,香芋片微波真空干燥过程主要为降速阶段,3种微波强度下平均干燥速率为0.149、0.185和0.224 g/(g·min);有效水分扩散系数为7.30×10-9、9.46×10-9、1.14×10-8 m2/s。NMR T2谱显示,香芋中存在结合水、不易流动水和自由水3种状态,其横向弛豫时间分别为T21 (2.31~9.32 ms),T22 (10.72~49.77 ms)和T23 (57.22~705.48 ms)。干燥过程中,MVD香芋内自由水含量显著降低(p<0.05),不易流动水所占比例呈现先增加后下降的趋势,结合水含量变化不显著(p>0.05),所占比例逐步上升。MRI检测表明,MVD香芋内外同时失水,且微波强度越高,弛豫信号消失的越快。该研究揭示了微波真空干燥中香芋内水分扩散规律,即微波强度越高,样品内水分扩散速率及不同组分水分之间的转化越迅速。该研究为香芋的微波真空干燥产业化生产提供理论依据。
[Key word]
[Abstract]
Low field-nuclear magnetic resonance (LF-NMR) was employed in this work to evaluate water mobility and distribution in taro samples during the processing of microwave vacuum drying (MVD) under different microwave intensities of 1.5, 2.0 and 2.5 W/g. Results showed that there was no constant drying rate period, and the entire microwave vacuum drying process occurred in the falling rate period. The average drying rates under microwave intensities of 1.5, 2.0 and 2.5 W/g were 0.149, 0.185 and 0.224 g/(g·min), respectively. The effective moisture diffusivity (Deff) values for MVD taro slices at different microwave intensities were 7.30×10-9、9.46×10-9、1.14×10-8 m2/s, respectively. Among the transverse relaxation time (T2), T21 (2.31~9.32 ms), T22 (10.72~49.77 ms) and T23 (57.22~705.48 ms) were corresponding to bound water, immobilized water and free water in fresh taro samples, respectively. The dramatically decreased content of free water for MVD taro was accompanied (p<0.05) by the increase in the proportion of bound water, although the content of bound water did not changed significantly (p>0.05). The proportion of immobilized water in taro samples increased, and then decreased with the drying time. The MRI results directly showed that the water-desorbing rate of outer layer in taro is equivalent to inner layer during microwave vacuum drying processing. The higher the microwave intensity, the faster relaxation signals disappear. This work obtaineded the laws of moisture diffusion in taro samples during microwave vacuum drying processing that the water diffusion and the conversion between the different components of moisture in MVD taro samples increased by the microwave intensity . It can provide a theoretical basis for industrial production of taro.
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[基金项目]
国家自然科学基金资助项目(31701663);安徽省自然科学基金资助项目(1608085QC55);安徽师范大学博士科研启动基金项目(2014bsqdjj52);国家级大学生创新基金训练项目(2017103370164)