超声预处理对杏片微波冻干过程中水分迁移的影响

Effect of ultrasonic pretreatment on water migration of apricot slices during microwave freeze-drying

为探究超声前处理对杏片微波冷冻干燥(MDF)过程干燥特性以及水分迁移扩散特性,以鲜杏为原料,采用微波冷冻干燥技术,使用低场核磁共振技术,测定不同超声前处理条件下杏片MFD干燥过程中的横向弛豫时间T 2反演谱。结果表明:超声前处理对杏片MFD前期干燥速率影响较大。扫描电镜观察发现,内部结构最佳的超声处理条件为350 W,35℃,15 min,有利于水分迁移。新鲜杏中所含自由水约占总水分的83%。通过对不同超声处理条件的杏片在干燥过程中内部各状态水分的迁移规律分析发现:随着干燥的进行,部分自由水先向结合水的方向迁移,同时弱结合水逐渐向结合水迁移,结合水向弱结合水的迁移则贯穿整个干燥过程。不同超声前处理条件下杏片干燥过程中有效水分扩散系数变化范围是8.44×10^-11~15.00×10^-11 m^ 2/s,超声处理使杏片的有效水分扩散系数提高了82.29%~223.97%。

To explore the drying characteristics and moisture migration and diffusion characteristics of apricot slices in the microwave freeze-drying process of ultrasonic pretreatment,microwave vacuum-freeze drying technology was selected,and the transverse relaxation time T 2 inversion spectra of apricot slices during MFD drying under different ultrasonic pretreatment conditions were measured by low field nuclear magnetic resonance.The results showed that ultrasonic pretreatment had a great influence on the drying rate of apricot slices in the early stage of MFD.Scanning electron microscopy showed that the best ultrasonic treatment condition for internal structure was 350 W,35℃,15 min,and it was beneficial to the transfer of water.Free water in fresh apricots was found accounting for about 83%of the total water.Based on the analysis of the law of water transfer in different flow states during MFD drying of fresh apricot slices,it was concluded that,with the drying process,part of the free water first migrated to the direction of bound water,meanwhile,weakly bound water gradually migrated to bound water,and the migration of combined water to non-mobile water ran through the whole drying process.The effective water diffusion coefficient of apricot slices during drying under different ultrasonic pretreatment conditions ranged from 8.44×10^-11 to 15.00×10^-11 m^ 2/s.Ultrasonic treatment increased the effective water diffusion coefficient of apricot tablets by 82.29%~223.97%.