Effect of Moisture Activity on Germination 



Russell J. Beers 



Department of Bacteriology, Iowa State University, Ames, Iowa 



IT IS well known not only that water is necessary for biological activity, 

 but that too low a level of availability may restrict or completely prevent 

 such activity. For example, spore germination and vegetative growth 

 of micro-organisms will not take place in a completely dry environment; 

 furthermore, it is possible to restrict the water content in either moist solid 

 materials or in solution to such an extent that germination and growth are 

 completely inhibited. In porous solids control of moisture is a simple matter, 

 and in solution a high concentration of some inert substance will limit the 

 water availability. 



Water availability, however, is not precisely measured by percentage of 

 moisture, since biological materials vary widely in their chemical affinity 

 for water; thus, the amount of so-called "bound water" is not the same foi 

 all materials at a given moisture percentage. The water availability would 

 be more truly expressed by a quantity which represents the net capacity of 

 the moisture present to act as liquid water. Such a quantity would be the 

 relative humidity of an atmosphere which would be in equilibrium with the 

 material, and can be easily determined by measuring the equilibrium vapor 

 pressure as compared to that of pure water. This quantity, expressed either 

 as a percentage or as a decimal fraction, is called the water or moisture 

 activity. Thus spores of a given species of mold may show different minimum 

 percentages of moisture required for germination on different foods, but the 

 calculated moisture activities will be the same. Unfortunately this important 

 quantity is often neglected in studies of moisture requirements. 



The critical moisture activities for spore germination have been rathei 

 w^ell explored for fungi (Gottlieb, 1950). Most species require from 70% 

 to 100% moisture activity for normal germination, with a few species ex- 

 hibiting feeble signs of germination at 62% (Snow, 1949). Most of the 

 common molds fall into the 75%-100% range, 75% being the commonly 

 accepted value for safe storage of agricultural products. These values are 

 arrived at by microscopic observation of spores planted on solid materials or 

 glass slides incubated in atmospheres of controlled relative humidity. Some 

 of tlie powdery mildews, however, have been found to germinate at moisture 

 activities of nearly zero (Semeniuk and Gilman, 1944), probably by virtue 

 of a high internal osmotic pressure and therefore presumably a high imbibi- 



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