Experimental study on drying characteristics of wheat by low-field nuclear magnetic resonance

摘要

The characteristics of transverse relaxation time (T-2) of water in wheat were studied by measuring the relaxation time of low-field nuclear magnetic resonance. Analysis of the exponential distribution of T-2 revealed that wheat contains five water components. The T-2 relaxation time and distribution significantly changed during drying. The dynamic characteristics of five water components during wheat drying were determined using the signal quantity of their characteristic peaks, which showed different features. Weakly chemically bound water (T-22) and water ascribed to cell wall (T-23) were the main source of water loss. Moreover, most T-23 and extracellular water (T-24) were removed during drying. Water migration between strongly chemically bound water (T-21) and the other water components was bidirectional. This process was not only affected by temperature but also by wheat moisture content and proportion of the five water components. The start time of water migration advanced and growth rate of T-21 at the end of drying to that before drying increased at 60, 70, and 80 degrees C. Drying at varied temperatures should be applied according to the characteristics of five water components during the drying process. In addition, high initial temperature was found to be necessary to achieve high drying rate of T-23, T-24, and free water (T-25). The use of drying temperature of 80 degrees C at the early stage and then changing to 70 degrees C reduced the heat consumption by 4.81% and increased the drying time by 9.61%.