Cold-stage scanning electron microscope measurement of ice morphology in apple tissue as a function of freezing rate

A cold-stage SEM was used to document the morphology of ice in apple tissue and a sucrose solution frozen at rates ranging from 450 to 0.03 degrees K/min. Freezing rates of 3-mm-thick apple discs were measured with a differential thermocouple technique, which gave measurement of the growth velocity and the temperature gradient through the solidified specimen as well as the cooling rate during solidification. Cold-stage SEM micrographs were used to measure the dendritic spacing of the ice structures, and these data were found to correlate linearly with the square root of the cooling rate during solidification as would be predicted by a theoretical analysis of mass transfer in the formation of dendrites. Comparison of freeze-substituted and freeze-dried apple-tissue micrographs with those from a cold-stage SEM showed that the cold-stage SEM technique was the only one which correctly represented ice morphology in apple tissue.