Solutes and ions tie up water in solution. Solutes lower aw and this reduction in aw depends on the total concentration of dissolved molecules and ions. Since these bind to water molecules, there is reduction in aw. Therefore, an increase in the concentration of dissolved substances such as sugars and salts is in effect a drying of the material. Not only is water tied up by solutes, but also water tends to leave the microbial cells by reverse osmosis to maintain equilibrium between the concentration of solute outside and inside the cells.