Shrinkage of cellular food during air drying

Shrinkage of potato and carrot (cylinders and slabs) was determined during convective drying at air temperature of 25, 55 or 65 °C with air speed of 1.6 or 0.5 m/s. Initial moisture content of slab samples at different positions within the carrot root and potato tuber was determined gravimetrically and by the vacuum method at 60°C. Structure of potato slabs at exterior and center of tuber was analysed by Scanning Electron Microscope (SEM).Glass transition temperature (Tg) was approximately 50 °C for dry potato (0.021 kg/kg dm), and 40 °C for dry carrot (0.005 kg/kg dm) indicating that potato and carrot would be in rubbery state throughout drying at 65 °C. Glass transition of the samples could be observed at the end of the drying process with air temperature of 25 °C. The initial moisture content of carrot at exterior and center of root were identical. However, the center potato layers had initial moisture content higher than the exterior potato layers. SEM images showed that the cells of the exterior potato samples had smaller volume with higher starch content than the center sample, thus indicating a correlation between structure characteristics and initial solids content.For all drying conditions used in this study, shrinkage of carrot samples (cylinders and slabs) had a linear relationship with moisture content during drying. For potato, however, a change in the slope of the shrinkage curve as a function of moisture content during drying was observed for cylinder samples. For potato slabs, only those having initial moisture contents lower than 5.5 ± 0.5 kg water/kg dm showed a similar change in slope, otherwise slabs presented a straight line for the whole water content range. The air temperature and air speed had no effect on the shrinkage behaviour of both carrot and potato samples under the operation conditions used in this study. No significant impact of glass transition on shrinkage of carrot and potato samples has been found, and if any this impact would be considered minor. Differences in starch content, in structure of the cellular material and its relationship with mechanical properties of material may explain the different shrinkage behaviour observed in this work.