Ruminations on heat and mass transfer in freeze drying: a personal reflection.

Examples are given to clarify the difference between heat and temperature, the sequence involving the passage of heat to and within a sample undergoing freeze-drying at reduced pressure is scrutinised and the beneficial and counterproductive effects of "modifying the vacuum" are examined together with the mechanism involved. The commentator also glances at possibly undesirable contamination of the product and the system as well as questioning the various forms of measurement of vacuum and temperature and their significance in validation procedures. An assertion is made that if the process is correctly conducted. The sublimation phase will be a square law function of the product depth and not a linear one. A proposal is made that heat input should be based on a variable energy profile (with appropriate safeguards) rather than on the basis of shelf temperature.