Microbial growth is to water activity as Bob Marley is to reggae. Microbial growth is the reason water activity began to be used in the food industry. Before the discovery that water activity controls microbial growth, food producers never knew why some batches molded and others didn't. They suspected it had something to do with water, but everyone knows that high-moisture salt pork doesn't mold, while low moisture nuts still do.
In 1953, W J Scott established that water activity activity is the key to determining if microorganisms will grow. Since yeast, molds and bacteria require a certain amount of available water to support growth, designing a product with an aw below 0.6 provides an effective control. Water activity is defined as the equilibrium relative humidity (ERH) divided by 100. Some common spoilage organisms and their aw limits are listed in Table 1 (click for a larger version).
Water activity better predicts the growth of microorganisms because microorganisms can only use "available" water, which differs considerably depending on the solute. On average, ions bind the most water, whereas polymers bind the least water; sugars and peptides fall into an intermediate position. At the same molecular concentration, salt lowers the water activity more than sugar.
When a substance is added to lower water activity, the result can be complicated. Ideally, an inert material could be added which would decrease water activity without any other effects such as increased ionic strength and decreased surface tension. In reality, the choice of substance can have a profound effect. For instance, salt could be added to one reaction mixture and sugar to another. The amounts can be controlled so that the resulting compounds have identical water activity. But the results of the reaction will differ because of the differing influences of salt and sugar on biological reactions. Salt and sugar form different additional hurdles.