Smart NMR Method of Measurement of Moisture Content of Vegetables During Microwave Vacuum Drying

Microwave drying is usually combined with vacuum environment in conjunction with hot air flow to draw the moisture rapidly. The moisture content of the vegetables undergoing drying is hard to measure online. This research designed a microwave vacuum drying (MVD)-low-field nuclear magnetic resonance (NMR) smart device and investigated the feasibility of NMR method for online measurement of state of moisture during MVD. The relation between the signal amplitude (A 2) and the true moisture content (M 1) of six kinds of vegetables (mushroom, carrot, potato, lotus, edamame, vegetable corn) was fitted to estimate if NMR can measure the M 1 of vegetables directly. Results showed that A 2 and M 1 of different fresh vegetables had no single empirical mathematical model to fit. However, for each kind of these vegetables, the A 2 and corresponding M 1 in different MVD stages showed a significant linear relationship. The predicted moisture content (M 2) of mushroom: M 2 = 5.25351 × 10−4 A 2 − 0.34042, R = 0.996; carrot: M 2 = 5.78756 × 10−4 A 2 − 0.14108, R = 0.998; potato: M 2 = 3.10019 × 10−4 A 2 − 0.10612, R = 0.991; lotus: M 2 = 2.32415 × 10−4 A 2 − 0.01573, R = 0.998; edamame: M 2 = 3.13310 × 10−4 A 2 − 0.4198, R = 0.996; vegetable corn: M 2 = 1.69461 × 10−4 A 2 − 0.09063, R = 0.995. The linear models between M 2 and A 2 were able to estimate the end point (M 1 0.950).