•A new non-destructive approach is applied to investigate water status in broccoli.•T2 studies with paramagnetic Mn2 + provide the information of water status and water distribution.•Spatial distribution of water is visualized by proton weighted imaging.•Effects of hot-air drying on water status and structure of broccoli were observed.•Detect limit of moisture content by low-field NMR method was calculated.Many quality attributes of food products are influenced by the water status and the microstructure. Low-field nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) methods are applied to non-destructively monitor the water status and structure of food. The aim of this study is to investigate the water status and distribution inside broccoli tissues and the effects of hot-air drying on the water status by using NMR and MRI methods. Transverse relaxation times (T2) provide the information of water status and water distribution. Results show that three water fractions with different T2 relaxation times were detected inside broccoli, which corresponded to different cell compartments. Proton weighted imaging could monitor the spatial distribution of water. Image analysis indicates that the water distribution inside broccoli was heterogeneous and the water content reduced from the stalk to the buds. During hot-air drying experiments, different drying kinetics were observed in the florets and stalks, which were related to their different structures. In addition, a detection limit of the moisture content was calculated for LF-NMR (about 11.35%). The results of this study show that the low-field NMR and MRI methods can precisely provide the quantitative information of water status inside food materials, and can be used to investigate the effects of food processing on product quality. The method provided in this study can be used to monitor changes of water status and distribution in a sample non-destructively during drying process.Download high-res image (112KB)Download full-size image

•The characteristic transport length (CTL) or the dimensionless CTL (DCTL) models have been estimated analytically.•Two ways to yield the estimates: pre-exponential factors and coefficients related to time-dependent solution.•The estimations are consistent with the parameters reported previously.In process engineering practice, including those in food industry, simple mathematical solutions are more useful. Learned assumptions are necessary to support effective simplifications. Previously it has been suggested that for a conduction and convection coupled system, there is an approximately linear temperature or concentration gradient between the surface and the average temperature, which occurs at a ‘fixed location’ within the conduction domain. The local temperature at the point is also said to be similar as the average temperature. This gradient is thus approximately the same as the temperature gradient at the interface between the conduction domain and the convection medium when thermal properties are considered constants. The distance from the surface to this ‘fixed location’ is marked as the characteristic transport length (CTL), which is a fraction of the size of the conduction medium. The previous findings were based on the agreements between the numerical solutions and compartmental, and then integral solutions in different occasions The argument has been validated among moderate Biot numbers (Bi) of <10 and moderate Fourier numbers (Fo) of >0.3. Similarly, one should find that the diffusional mass transfer process has the same property due to the same mathematical nature involved. Here, the mass diffusion process has been analyzed to yield the CTLs for the cases of infinite Biot number, where the analytical solutions for longer times for semi-infinite slab, infinite cylinder and sphere are available which can be put to great use. When applying these classical solutions for the above purpose, there are still new discoveries, which are interesting to report here.