核桃干燥过程的低场核磁共振横向驰豫分析

In order to quickly monitor the changes of moisture and oil in the drying process, low field-nuclear magnetic resonance (LF-NMR) technology was used to determine the free water, weakly and strongly bound water and oil of walnuts under the constant temperature and variable temperature drum catalytic infrared-hot air drying, and compared with the single hot air drying. Moreover, a mathematical model between the proportion of the moisture lateral relaxation peak area and the actual moisture content was established. The results showed that the changing trend of the drying curve under the three drying methods was the same. Compared with the single hot air drying [duration 20 h, maximum drying rate 0.11 g/(g•min)], the variable temperature and constant temperature drum catalytic infrared-hot air drying efficiencies [drying times of 14.38 h and 16.16 h, maximum drying rates of 0.28 g/(g•min) and 0.48 g/(g•min)] were significantly increased, the drying times were shortened by 28.10% and 19.20% respectively, and the maximum drying rates were increased by 2.55 and 4.36 times respectively, indicating that the catalytic infrared drying treatment significantly improved drying effectiveness. The LF-NMR transverse relaxation map showed that the free water peak area decreased significantly during the drying process until it disappeared, the weakly bound water peak area was significantly reduced, and the strongly bound water and oil peak areas remained basically unchanged, indicating that the drying process removed almost all the free water and most of the weakly bound water and had no obvious effects on the strongly bound water and oil contents. A mathematical model between the proportion of the moisture relaxation peak area and the actual moisture content was established. R² was greater than 0.90, and the prediction effect was good. In general, compared with the national standard method, LF-NMR can detect the change of moisture and oil during the drying process very well, and it was a new method for efficient and rapid detection of moisture content.