Abstract:In this study, low-field nuclear magnetic resonance (LF-NMR) technique combined with drying curve was used to investigate the influence of heat pump drying and variable temperature differential pressure puffing drying (EPD) on the moisture migration of silkworm pupa, to reveal the state, distribution, and content of moisture inside silkworm pupa during two different drying processes. The results showed that EPD led to a more rapid moisture decrease during the drying process, and its drying rate was also higher than that of the heat pump drying (HPD). The moisture of fresh silkworm pupa could be divided into three parts: bound water T21 (0~2 ms), non-flowing water T22 (2~100 ms) and free water T23 (100~1000 ms), with their corresponding peak areas of the three peaks as 49.531, 661.651, and 1730.075, respectively, and peak area percentages as 2.03%, 27.10%, and 70.87%, respectively. With the extension of the drying time for the silkworm pupa, the T2 relaxation time of the EPD and HPD showed the same changing trend. On one hand, the peak position shifted to the left, and on the other hand, the total peak area A kept decreasing. The main state of moisture gradually became non-flowing water and bound water. The relationship equations between the moisture content on dry basis (x) and the total peak area A under the two drying methods, EPD and HPD, were A=702.01x+173.92 (R2=0.9997) and A=695.94x+229.24 (R2=0.9968), respectively. Both showed a significant linear relationship (p<0.05), with EPD showing a better fit, but HPD having a relatively uniform distribution of hydrogen proton density. Comprehensive evaluation revealed that EPD can describe better the water migration law during the drying process of silkworm pupa.
