Water Activity and Osmotic Pressure

Solutes in an aqueous solution decrease the activity of water ( w). As a first approximation, the decrease in aw when increasing amounts of solutes are added is a dilution effect. In other words, the mole fraction of water decreases when solutes are added. As its activity thus decreases, the chemical potential of water is lowered (consider the RT In a term in fij). The presence of solutes also leads to an osmotic pressure (II) in the solution. An increase in the concentration of solutes raises the osmotic pressure, indicating that II and aw change in opposite directions. In fact, the osmotic pressure and the water activity are related  

Unfortunately, the use of the terms osmotic pressure and osmotic potential, as well as their algebraic sign, varies in the literature. Osmotic pressures have been measured using an osmometer (Fig. 2-8), a device having a membrane that ideally is permeable to water but not to the solutes present. When pure water is placed on one side of the membrane and some solution on the other, a net diffusion of water occurs toward the side with the solutes. To counteract this tendency and establish equilibrium, a hydrostatic pressure is necessary on the solution side. This pressure is often called the osmotic 

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Moulds are able to grow on a wide range of organic substrates the species which occur and the extent of their growth are determined, not only by the substrate, but also by physical and environmental parameters such as temperature, pH, water activity and osmotic pressure. Many foods, and the raw materials that go into their manufacture, are transported over long distance from producer to consumer and are stored for extended periods. If storage conditions are not controlled there are thus many opportunities for mould... 

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