Impact of the freezing method on the microstructure of the freeze-dried food products

This study examines how the pre-freezing method affects the microstructure of freeze-dried food products. Cylindrical potato and apple samples were pre-frozen either by convective freezing or vacuum freezing and then identically freeze-dried. High-resolution X-ray micro-computed tomography with a voxel size of 6.17 µm quantified air volume fraction, pore diameter distributions, sphericity and tortuosity. Segmentation consisted of image smoothing, artefacts removal, image binarisation, object separation and labelling. A one-way analysis of variance (α=0.05) assessed the effect of pre-freezing within each product. Apples exhibited higher air fractions than potatoes (0.78 versus 0.75), and also a greater share of mid-range pores but lower sphericity (0.11 versus 0.28), consistent with the intrinsic tissue architecture differences. Between convective and vacuum freezing of the same samples, the group means did not differ significantly for any property, except sphericity for apples. Thus, under the conditions studied, product type dominated pore formation, while the pre-freezing method induced only minor, non-significant changes. Nevertheless, the trends observed under vacuum freezing imply prospective advantages for mass-transfer efficiency and rehydration behaviour that merit further exploration. These findings support a view in which freezing sets the pore network later conserved by sublimation.