X-ray microtomography of ice crystal formation and growth in a sponge cake during its freezing and storage

X-ray tomography has been used to characterize microstructural changes taking place into sponge cake upon freezing and subsequent storage at fluctuating temperatures. The CellDyM cooling cell, initially developed for snow, was used to subject a sponge cake sample to freezing and thermal cycling directly inside the X-ray tomograph. 3D images of the same sample have been scanned at different times during 6 days. An original image processing based on Digital Volumetric Correlation (DVC) has been implemented in order to segment the images and then to quantify porosity changes and ice crystal formation and growth. Overall, the obtained results have shown that (i) all the ice crystals are located at the pore surface and no crystal was found inside the sponge cake crumb matrix and (ii) during the thermal cycling, the size of the ice crystals linearly increases while the number of ice crystals tends towards a plateau. These tendencies can be related to a recrystallization phenomenon occurring due to thermal cycling. More generally, the proposed methodology could be applied to other frozen porous products during freezing and frozen storage.