Applications of non-invasive measuring techniques of internal changes during drying of food products

The internal information (microstructure and moisture distribution) of food products during drying is useful for designing and optimizing the process control of drying conditions. Conventional measuring techniques like scanning electron microscopy (SEM), light microscopy (LM), confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM) have been widely used to investigate the internal and structural characteristics of dried agricultural products. However, these conventional measuring techniques have some limitations, which include the destruction of the food samples, sample preparations, and the development of artifacts. In order to overcome the limitations of conventional measuring techniques, non-invasive measuring techniques like X-ray microcomputed tomography (XCT), low-field nuclear magnetic resonance (LF-NMR), magnetic resonance imaging (MRI) and neutron imaging have been used to investigate the internal changes of agricultural products during drying in ex-situ and in-situ. However, the use of these non-invasive measuring techniques has yet to gain wide acceptance and utilization in the drying of agricultural products. Therefore, the purpose of this paper is to review the applications, advantages and limitations of XCT, LF-NMR/MRI and neutron imaging in the drying of agricultural products. The information contained in this review would enhance further developments and applications of these non-invasive measuring techniques in the drying of agricultural products. The review also shows that the non-invasive techniques could provide information on the drying properties (drying kinetics, porosity, pore size, wall thickness, moisture profile, local water distribution, shrinkage and moisture distribution) of food products subjected to drying in ex-situ and in some cases in in-situ.