Relationship with water activity

Now we turn our attention to the water and the solids that compose the myriad of fresh and processed foods we consume. When a component is added to water (or coexists with water, as in a fresh food), the overall mobility of the water decreases, compared to that of pure water. The magnitude of the decrease depends on the number, amount, and nature of the component(s) added, as well as the effect of any processing methods used. In the past, researchers focused their attention on the relationship between water (activity, availability, mobility) and food stability. Based on the introduction of the polymer science approach to food stability by Slade and Levine (1985, 1988, 1991), the focus has shifted to the relationship... 

Labuza (1971) has described the complex relationship between water activity (uw) and lipid oxidation, with a minimum observed at intermediate aw ( 0.4)... 

Figure 1-23 Relationship Between Water Activity aw) and Glass Transition Temperature (Tg) of Some Plant Materials and Biopolymers. Source Reprinted with permission from J. Cherife and M. del Pinar Buera, Water Activity, Water Glass Dynamics and the Control of Microbiological Growth in Foods, Critical Review Food Sci. Nutr., Vol. 36, No. 5, p. 490, 1996. Copyright CRC Press, Boca Raton, Florida.

Figure 1-26 Relationship Between Water Activity and a Number of Reaction Rates. Source Reprinted with permission from D.P. Bone, Practical Applications of Water Activity and Moisture Relations in Foods, in Water Activity Theory and Application to Food, L.B. Rockland and L.R. Beuchat, eds., p. 387, 1987, by courtesy of Marcel Dekker, Inc.

Figure 8-46 Relationship Between Water Activity and Crispness of Popcorn. Source. Reprinted with permission from E.E. Katz and T.P. Labuza, Effect of Water Activity on the Sensory Crispness and Mechanical Deformation of Snack Food Properties, J. Food Sci., Vol. 46, p. 403, 1981, Institute of Food Technologists.

Dehydrogenase is an enzyme present in all living microbiological material. The presence of this enzyme can be determined using 2,3,5-triphenyltetrazolium chloride. This colourless water-soluble compound is reduced to a red compound called formasan in the presence of dihydrogenase enzyme. This compound can be extracted in ethanol, the content determined photometrically, and a relationship with microbiological activity can then be estabhshed. 

The two-step charge transfer [cf. Eqs. (7) and (8)] with formation of a significant amount of monovalent aluminum ion is indicated by experimental evidence. As early as 1857, Wholer and Buff discovered that aluminum dissolves with a current efficiency larger than 100% if calculated on the basis of three electrons per atom.22 The anomalous overall valency (between 1 and 3) is likely to result from some monovalent ions going away from the M/O interface, before they are further oxidized electrochemically, and reacting chemically with water further away in the oxide or at the O/S interface.23,24 If such a mechanism was operative with activation-controlled kinetics,25 the current-potential relationship should be given by the Butler-Volmer equation... 

In summary, it is clear that water absorbs into amorphous polymers to a significant extent. Interaction of water molecules with available sorption sites likely occurs via hydrogen bonding such that the mobility of the sorbed water is reduced and the thermodynamic state of this water is significantly altered relative to bulk water. Yet accessibility of the water to all potential sorption sites appears to be dependent on the previous history and physical-chemical properties of the solid. In this regard, the water-solid interaction in amorphous polymer systems is a dynamic relationship depending quite strongly on water activity and temperature. 

Table 124 Water-gas shift activity at 120 °C for treated 3% or 5%Au/a-Fe203 catalysts using a feed containing 57.5 cm3/h CO and 921.5 cm3/h using 0.5 g catalyst after 3 hours onstream. Relationship with catalyst reducibility523... 

The octanol-water partition coefficient, Kow, is the most widely used descriptor of hydrophobicity in quantitative structure activity relationships (QSAR), which are used to describe sorption to organic matter, soil, and sediments [15], bioaccumulation [104], and toxicity [105 107J. Octanol is an amphiphilic bulk solvent with a molar volume of 0.12 dm3 mol when saturated with water. In the octanol-water system, octanol contains 2.3 mol dm 3 of water (one molecule of water per four molecules of octanol) and water is saturated with 4.5 x 10-3 mol dm 3 octanol. Octanol is more suitable than any other solvent system (for) mimicking biological membranes and organic matter properties, because it contains an aliphatic alkyl chain for pure van der Waals interactions plus the alcohol group, which can act as a hydrogen donor and acceptor. 

Figure 8. Specific activity of Radium in ordinary plants and algae (solid line) compared to Characeae algae (dotted line), showing roughly a linear relationship with the Radium activity concentration in the water.

Eppler et al. [103] viewed these results as having a potential relationship to salt-activated enzyme preparations, particularly in relation to the mobility of enzyme-bound water. Specifically, the authors examined both water mobility [as measured by T2-derived correlation times, (tc)D20] and NaF-activated enzyme activity and observed a linear relationship. This suggests that the salt-activated enzymes contain a more mobile water population than salt-free enzymes, which facilitates a more aqueous-like local environment and dramatically increases enzyme activity through increased flexibility. Therefore, enzyme activation appears to correlate with the properties of enzyme-associated water. Once again, the physicochemical properties of water dictate enzyme structure, function, and dynamics. Hence, salt activation has proven to be a useful technique in activating enzymes for use in organic solvents and has provided a quantitative tool to better understand the role of water in enzymatic catalysis in dehydrated media. 

Effect of Water Activity. A preliminary study was done to determine the a at which encapsulated orange peel oil was the most stable to oxidation. Figure 1 summarizes the results of this study. The formation of the limonene oxidation product, limonene oxide, was the slowest for the powder adjusted over Mg(NO3)2 (a 0.536). While the levels of oxidation product do not follow in exact order of a, it is evident that better storage stability correlates with a higher a of the powder. This relationship was not anticipated. Literature on lipid oxidation (2, 2) indicates that there is an optimum a for product... 

Frequently enzymes act in concert with small molecules, coenzymes or cofactors, which are essential to the function of the amino acid side chains of the enzyme. Coenzymes or cofactors are distinguished from substrates by the fact that they function as catalysts. They are also distinguishable from inhibitors or activators in that they participate directly in the catalyzed reaction. Chapter 10, Vitamins and Coenzymes, starts with a description of the relationship of water-soluble vitamins to their coenzymes. Next, the functions and mechanisms of action of coenzymes are explained. In the concluding sections of this chapter, the roles of metal cofactors and lipid-soluble vitamins in enzymatic catalysis are briefly discussed. 

Table 15.5 lists concentrations of the major photooxidants in surface waters, diurnally averaged over 24 hours. Note that, even if kox(i) values are measured or estimated accurately (within a factor two or three), oxidant concentrations in the environment vary widely, and averaged values have a variance of five- to tenfold for any given location. In extreme locations, such as pristine marine waters, or heavily polluted surface waters, oxidant concentrations may be 100 times smaller or larger than the values Table 15.5 lists. Table 15.6 lists rate constants (kox) for various photooxidants in their reaction with major classes of organic compounds. To estimate the rate of an indirect photoreaction for chemical C (Equation (18)), either a measured or estimated value of kox is required, specific for each oxidant and for each class of organic compounds. Methods for estimating kox from molecular structure with structure-activity relationships (SARs) have been developed for many photooxidants and are discussed below. 

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