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Abstract
The understanding of the mutual migration behavior and water diffusion mechanism of different water components inside wheat granules is crucial for drying process selection and process parameter determination. In this study, the binding forms of water in wheat granules and the migration and diffusion characteristics of different water components during drying processes were studied on the basis of low-field nuclear magnetic resonance and magnetic resonance imaging. In the early drying stage, the internal water migration rate of wheat granules was larger than the external water evaporation rate. The drying rate was mainly dominated by the external water evaporation rate. In the middle drying stage, the internal water migration rate changed from larger than to less than the external water evaporation rate. The internal water migration rate gradually became the main control factor of the drying rate. During the drying process, the relationship among different bound water was a “parallel connection,” and the relationship between bound and free water was a “series connection.” In this study, the relationship between the activation energy and the different water components was obtained, and the Arrhenius-type expression of the effective moisture diffusivity was presented.
Keywords:
Nomenclature
| β | = | volume shrinkage |
| = | concentration gradient (kg/m4) | |
| d.b | = | drying basis (kgwater/kgdry matter) |
| Deff | = | effective moisture diffusivity (m2/s) |
| D0 | = | pre-exponential factor of the Arrhenius equation (m2/s) |
| D | = | moisture diffusivity (m2/s) |
| Dagg | = | aggregate moisture diffusivity (m2/s) |
| D22 | = | moisture diffusivity of T22 (m2/s) |
| D23 | = | moisture diffusivity of T23 (m2/s) |
| D24 | = | moisture diffusivity of T24 (m2/s) |
| Ea | = | activation energy (J/mol) |
| J | = | diffusion flux (kg/(s/m2)) |
| LF-NMR | = | low field nuclear magnetic resonance |
| m0 | = | initial mass (g) |
| me | = | drying mass (g) |
| mi | = | instantaneous mass (g) |
| MRI | = | magnetic resonance imaging |
| Mi | = | instantaneous moisture content (kgwater/kgdry matter) |
| MR | = | moisture ratio |
| Mt | = | moisture content at time t (kgwater/kgdry matter) |
| M0 | = | initial moisture content (kgwater/kgdry matter) |
| Me | = | equilibrium moisture content (kgwater/kgdry matter) |
| R | = | equivalent sphere radius (m) |
| R | = | universal gas constant (J/mol K) |
| R2 | = | decisive coefficient |
| S1, S2, S3 | = | different wheat samples |
| T21 | = | strong chemically bound water |
| T22 | = | strong binding water |
| T23 | = | physically bound water |
| T24 | = | free water |
| t | = | time (s) |
| T | = | absolute temperature (K) |
| γ | = | shrinkage of the surface area |