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Tablets & Capsules September 2020 29 motion and exert pressure on the inside of the container's sides and top. The higher the concentration of water molecules in the headspace and the faster the molecules are moving, the higher the vapor pressure. The vapor pressure of water in a material (p) is defined in the same way. When a solid material is placed in a closed container, water molecules can escape from the material into the headspace, resulting in a vapor pressure, as shown at the top of Figure 1. The vapor pressure for a solid material will always be less than it is for pure water, due to the association of water to the material. The less the degree of association of water to the material, the higher the water activity, up to a defined maximum of 1.0. When a material only contains water molecules that are so tightly bound to the material that they cannot escape into the gas phase (as with crystal water), the material's water activity is 0.0. Water activity values between 0.0 and 1.0 reflect the difference in fugacity of "free" water in the material. The water activity depends on the total amount of water in the material, as well as on the material's ability to bind water in its structure—often referred to as hygroscopicity. A material's hygroscopicity depends on its chemical composition, physical-chemical state, and physical structure. Capillary forces, solutes, sur- face, and colligative effects can all impact hygroscopicity. Water activity and product quality To understand the difference between water activity and moisture content for product stability, one should understand that not all water in a material is available for microbial growth or enzymatic and chemical reactions. Completely bound water, like water of crystallization, is not available for microbial growth or reactions [7]. Available water is only the water that is loosely bound to the material. Water activity is the qualitative measure of the energy state of free water in a sample and, therefore, determines the availability of water for microbial growth or enzymatic and chemical reactions. Figure 2 shows the relative reaction rate of different types of reactions as a function of water activity. Each type of reaction (or organism growth) has an optimal water activity range at which it proceeds at the highest rate. Non-enzymatic browning reactions, for example, have an optimal water activity of 0.75, and the reaction rate of degradation reactions is exponentially related to water activity [8, 9]. Microorganisms have critical water activity values below which they cannot grow. At water activity values below 0.6, all growth of microorganisms is inhibited [10]. Water activity and moisture migration Differences in water activity, or fugacity, between two discrete materials or one material and the environment are the driving force for moisture migration. Water will always migrate from high water activity to low water activity until reaching equilibrium, even if the material with lower water activity has higher moisture content. Traditionally, discussions about water in products or ingredients focus on moisture content, which is a quan- titative measure of moisture in a product [2]. In food sci- ences, however, the qualitative parameter, water activity, is often considered the critical moisture parameter [3]. Nowadays, the pharmaceutical industry is also acknowl- edging the importance of measuring water activity, as evidenced by a US Pharmacopeia chapter <1112> Appli- cation of Water Activity Determination to Nonsterile Pharmaceutical Products and a proposed chapter <922> Water Activity [4]. For example, water activity is mea- sured to reduce the degree of microbial testing required to control the product or to define packaging criteria to ensure stability over its entire shelf life. Definition of water activity Water activity (A w ) is a measure of the energy status of water in a material. It is defined as the ratio of the fugacity of the water (f w ) to the fugacity of pure liquid water under reference conditions (f w 0 ). Fugacity is the escaping tendency of a substance and can be replaced by the vapor pressure (p), provided the vapor behaves as an ideal gas [5]. Water activity is therefore often defined as the vapor pressure of water in a material (p) relative to the vapor pressure of pure water (p 0 ), at the same tem- perature (T) and pressure: A w = f w ≈ p f w 0 T p 0 T (1) The vapor pressure of pure water (p 0 ) can be under- stood as follows. When pure water is placed in a closed container, some water molecules will escape from the liquid water into the container's headspace, as shown at the bottom of Figure 1. The water molecules in the gas phase are in equilibrium with the liquid water; there are constantly vapor molecules returning to the liquid state and liquid water molecules escaping to the gas phase. The molecules in the gas phase are in constant random Figure 1 Water activity of a material (Ratio of the vapor pressure of water in the material to the vapor pressure of pure water [6]) A w = P water above product P pure water mm Hg mm Hg =