Water activity, definition

In addition to equilibrium between the liquid-phase water in the sample and the vapor phase, the internal moisture equilibrium of the sample is important. If a system is not at internal moisture equilibrium, one might measure a steady vapor pressure (over the period of measurement) that is not the true water activity of the system. An example of this might be a baked good or a multicomponent food. Initially out of the oven, a baked good is not at internal equilibrium the outer surface is at a lower water activity than the center of the baked good. One must wait a period of time in order for the water to migrate and the system to come to internal equilibrium. It is therefore important to remember the restriction of the definition of water activity to equilibrium. 

The concept of a chemical potential is germane to a discussion of water activity (aw), which is technologically defined as the ratio of the equilibrium water vapor pressure over a solution or dispersion (p0) and the water vapor pressure over pure water (jb ). Also by definition, the chemical potential of a solvent ( jl0) or a solute (p ) is the rate of change in energy of either with a change only in the molal content of that component in solution. 

Water is added instead of amylase in control experiments. Our enzymatic activity definition is 1 unit of amylase hydrolyses 1.0 mg of starch in 3 min at pH 6.6 at 20°C. 

One intriguing aspect of enzymatic catalysis in low water environments (this includes 85% dioxane where the water activity is substantially less than unity) is the effect of reaction pH. In aqueous solutions, the ionogenic functional groups of an enzyme respond to the pH of the solution. Definitive pH optima exist for all enzymes. In organic solvents, however, the lack of water as a bulk solvent makes pH an unmeasurable variable although there will be a small concentration of protons in the vicinity of... 

H atom addition, protonation state, active site definition, His tautomers, structural water, pharmacophore definition etc. 

If the sponge is left to dry in the sun, this adsorbed water will evaporate, leaving only a small proportion of water bound chemically to the salts and to the cellulose of the sponge fibers. Like water in sponge, water is held in food by various physical and chemical mechanisms (Table 3.1). It is a convenient oversimplification to distinguish between free and bound water. The definition of bound water in such a classification poses problems. Fennema (1985) reports seven different definitions of bound water. Some of these definitions are based on the freezability of the bound component, and others rely on its availability as a solvent. He prefers a definition in which bound water is that which exists in the vicinity of solutes and other non-aqueous constituents, exhibits reduced molecular activity and other significantly altered properties as compared with bulk water in the same system, and does not freeze at -40"C."... 

Indeed, water activity is a more important factor than water content for fungi growth and metabolism [29, 33-37]. Water activity gives the amount of unbounded water available in the immediate surroundings of the microorganism. Water activity is closely related, but not equal to, the water content. The definition of the water activity is ... 

Adopting the more realistic definition of the internal water activity as being the ratio of the number of moles of free water... 

Water activity has been shown to have a definite influence on the rate of degradation of chlorophyll in freeze-dried, blanched spinach puree [73]. At 37°C and an higher than 0.32, the most important mechanism of chlorophyll degradation was conversion to pheophytin. At lower than 0.32, the rate of pheophytin formation in spinach was low. The rate of chlorophyll-a transformation was 2.5 times faster than chlorophyll-. The study of the degradation of chlorophyll as a function of a . pH, and temperature in a spinach system... 

It should be remarked that the pATa is often defined for a dilute solution, typically 0.1 mol L , with water as the proton solvent. In concentrated solutions, the water activity should be taken into account. When definitions refer to the p a scale in aqueous solutions it has the exact meaning only within the water autoprotolysis range (0 < pA"a < 14). This range can be extended by using solvents more acidic or more basic than water however, such p Ta values are in general less accurate the farther from the water range they are [20]. As to be discussed below, in solvent-free ionic hquid, ionic solid as well as polymeric acid-base systems, the p Ta values are informative only in a qualitative way. 

Microemulsions [512, 513] are special types of emulsions that form spontaneously and have very small particles. Microemulsions are optically clear, thermodynamically stable dispersions of two immiscible liquids obtained by the use of carefully adjusted surface-active molecules (surfactants). Both liquids in a microemulsion will be present in regions of the same order of magnitude, with the dispersed phase on the order of 10-100 nm. Aggregates of surface-active molecules, or micelles, form into colloidal-sized clusters in such a way that hydrophilic groups are directed toward the water. These definitions [514] are general in nature, but they suffice for the current purpose the interested reader is directed to texts on this... 

Taking into account the definition of water activity, namely. 

In a ternaty hqiiid-hqiiid system, such as the acetic acid-water-MIBK system, all three components are present in both liquid phases. At equilibrium the activity A° of any component is the same in both phases by definition [Eq. (15-6)]. 

The general definition of a condensation reaction is a one that involves product formation by expulsion of water (or other small molecule) as a by-product. By this definition, activation and methylolation are also condensations. In more precise terms the chain-building process should be described as a condensation polymerization, however, in the jargon of the phenolics industry, the term condensation is usually reserved for the chain-building process. This terminology is not necessarily observed in the literature [88]. Many literature reports correctly refer to methylolation as a condensation reaction. The molecular weight development of the phenol alcohol adducts may also be classified as a step-polymerization. 

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