SUMMARY: The changes in morphology of Penicillium expansum during freezing and thawing were observed under a light microscope fitted with a temperature controlled stage. At slow rates of cooling (< 15°C min-1) extensive shrinkage of the hyphae was observed. Cooling at rates greater than 50°C min-1 induced intracellular ice formation in all hyphae. At rates intermediate between 15 and 50°C min-1 shrinkage and intracellular nucleation were observed, occasionally within the same hyphae. These data are examined in terms of the osmotic stresses that cells are exposed to at different temperatures during freezing and the biophysics of injury at different rates of cooling is discussed.
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