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SUMMARY: A mixture of Pseudomonas jluorescens and coliphage T2 was dried at -20, 0 and +ZOO in solutions of each of the 1G combinations of mannitol and sucrose at 0, 0.25, 0.50, and 0.75~. For P. cfluorescens there was no significant average effect of the drying temperature on the numbers which survived drying, but for coliphage T2 average survival increased as the drying temperature was changed from -20 to 20°. For both organisms, however, there were a number of very highly significant interactions between solutions and drying temperatures, In other words, the effects of the temperature depended on the solutes present and vice uersa. There were also some substantial differences between the responses of the two organisms. It is concluded that survival during drying is likely to be determined by factors additional to those studied in these experiments. The most suitable procedures for drying micro-organisms without loss of viability have not yet been defined. There is, however, a widespread opinion that drying from the frozen state gives the best results especially for organ- isms easily destroyed by drying. For example, Fry (1954) in reviewing results with bacteria stated that 'all the evidence shows that drying from the frozen state is desirable for all organisms and is apparently essential for the more sensitive ones'. Similarly, Harris (1954) suggested that labile viruses should be freeze-dried ' at the lowest convenient temperature '. On the other hand, very many types of micro-organisms show at least a small fraction which survives after a great variety of natural and artificial drying procedures, in many of which drying has undoubtedly occurred at temperatures substantially above 0". In addition, there have been some comparisons which show that drying above Oo gives greater survival than freeze-drying. Stamp (1947) found that slow drying of Serratia murcescens over P,O, at room temperature gave approximately three times as many survivors as either of two methods of freeze-drying. For two types of fungal spores Mazur & Weston (1956) found that spray-drying caused little or no mortality, whereas freeze-drying killed more than 90 yo of the spores. Greiff, Blumenthal, Chiga & Pinkerton (1954) compared freeze-drying of influenza virus when the ampoules were suspended in baths at 0, -30, -60 and -80". At -SOo and at 0' survival was nearly 100 yo, but was much less at the two intermediate temperatures. In these experiments of Greiff et at. the actual temperatures of the virus suspension were not measured during drying and, as the system was evacuated, the tem- perature of the product might have been much lower than the temperature

The Death of Micro-Organisms During Drying in Relation to Solute Concentration and Drying Temperature