On the effect of freeze-drying on paper-borne microorganisms

Today there is an ever-increasing interest in recovering flooded archival materials. Freeze-drying stands out as a valuable method as the water is frozen and the ice is then removed through sublimation. The present work is focused on the effect of freeze-drying on flooded paper contaminated with biodeteriogenic microorganisms (i.e. a bacterium Staphylococcus epidermidis, a yeast Rhodotorula mucilaginosa and a filamentous fungus Alternaria alternata). The effect of freezing and drying on microorganism survival and growth has been separately evaluated. In particular, different operative conditions for freezing (in a domestic freezer at about -20°C, at -40°C with a freezing rate of -1°C/min) and drying (200 μbar at 0°C and 20°C) have been tested. All the tested strains were able to survive after freezing and drying phases, nevertheless a different behaviour was observed: the growth of S. epidermidis was the same as that of the control, R. mucilaginosa showed a slight growth and carotenoid production increase, while A. alternata was inhibited, in particular after the freezing at -40°C, followed by a low temperature drying rate step.