Imaging food freezing using X-ray microtomography

X-ray micro-computed tomography (X-ray micro-CT) has been applied to visualise ice crystal structures formed during freezing of a number of foods. Materials were frozen unidirectionally at -5 degrees C and then freeze-dried to remove the ice crystal structure and leave voids that can be measured by the X-ray. The system reconstructs the 3-D image based on a set of 2-D images, and is capable of micrometre-scale visualisation. This study demonstrates the capability of the technique to characterise the internal ice crystal microstructure of a range of frozen materials; meat, fish, chicken, potato, cheese and carrot. Results show the voids corresponding to the ice crystals formed within these materials at different directions to the heat flux and various axial positions. Electron microscopy of the same materials, both fresh and frozen at -5 degrees C, indicates the same shape of voids seen by the tomographic technique. Ice crystal parameters such as size, area and width can be quantified by the technique. Ice crystals in carrot were larger than in the other materials, while cheese and potato had the lowest values. The ice crystal distribution of all the experimental materials varied with axial distance from cooling surface; the closer the measurement was to the cooling surface, the smaller the crystal size. The results demonstrate that X-ray micro-CT might be useful in the analysis of frozen foods.