Water activity aw

A higher temperature may cause an increase in fluidity of cell membranes, allowing more rapid diffusion of preservatives into the cytoplasm (Chikthimmah, LaBorde, and Beelman, 2003). For example, sodium lactate is primarily used in meat products to extend cold storage and its effects are 

The presence of organic acids can lead to intracellular production of H202 and is toxic for a bacteria cell. Resistance to H202 can be induced by ethanol and could explain antagonistic effects that may result when using ethanol in combination with organic acids (Barker and Park, 2001). 

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Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

The most fundamental manner of demonstrating the relationship between sorbed water vapor and a solid is the water sorption-desorption isotherm. The water sorption-desorption isotherm describes the relationship between the equilibrium amount of water vapor sorbed to a solid (usually expressed as amount per unit mass or per unit surface area of solid) and the thermodynamic quantity, water activity (aw), at constant temperature and pressure. At equilibrium the chemical potential of water sorbed to the solid must equal the chemical potential of water in the vapor phase. Water activity in the vapor phase is related to chemical potential by... 

Fig. 8.5. Water activity aw versus stoichiometric ionic strength 7s of NaCl solutions at 25 °C and 300 °C, according to the activity model of Helgeson (1969). Dashed line shows 3 molal limit to the model parameterization values to right of this line are extrapolations of the original data.

Fig. 24.8. Evolution of fluid chemistry during the simulated evaporation of seawater as an equilibrium system at 25 °C, calculated using the Harvie-Mpller-Weare activity model. Upper figures show variation in salinity, water activity (aw), and ionic strength (/) over the reaction path in Figure 24.7 bottom figure shows how the fluid s bulk composition varies.

Ukhun, M.E. and E.N. Dibie. 1989. Cyanide content of cassava mash and gari flour and influence of water activity (aw) during storage. Bull. Environ. Contam. Toxicol. 42 548-552. 

The stability of moisture-sensitive drugs has been studied by Vromans and Schalks [1.143] using amorphous vecuronium bromide. Its decomposition in a formulation depends more on the water activity aw (see Chapter 4, point 2) than on the water content. Glass-forming excipients may not only be cryoprotective but also stabilizing. 

The storability of the dried product depends to a large extend on the selected type, e. g. strawberries, carrots and green beans [4.7]. For meat, the fat content can be important. Karel [4.8] studied the influence of the water content in stored dried food, and found that not only was the amount of water of influence, but also the kind of bond to the solids. This link can be described by adsorption isotherms, as shown in Fig. 4.1. In food technology, the bond of water is often given by the term water activity, aw ... 

Mauritz et al., motivated by these experimental results, developed a statistical mechanical, water content and cation-dependent model for the counterion dissociation equilibrium as pictured in Figure 12. This model was then utilized in a molecular based theory of thermodynamic water activity, aw, for the hydrated clusters, which were treated as microsolutions. determines osmotic pressure, which, in turn, controls membrane swelling subject to the counteractive forces posed by the deformed polymer chains. The reader is directed to the original paper for the concepts and theoretical ingredients. 

Figure 11.5 Influence of thermodynamic water activity, aw, on the hydrolytic ( ), alcoholytic ( ) and total (A) activity for the Candida antarctica lipase -catalyzed acylation of 2-pentanol with methyl propanoate as acyl donor.

Figure 8.11 Elastic modulus versus interfacial water activity aws of protein layers at the air-water interface ( ) adsorbed p-casein ( ) adsorbed a-lact-albumin ( ) adsorbed p-lactoglobulin ( ) spread p-lactoglobulin. Reproduced from Damodaran (2004) with permission.

Figure 7.9 Nomograph for direct estimation of water activity (aw) of unripe cheeses from % H20 and % NaCI. Examples If % H20 = 57.0, and % Nad = 1.5, then aw = 0.985 if % H20 = 44, % NaCi = 2.0, then aw = 0.974 (from Marcos, 1993).

Basic Protocol Isopiestic Determination of Water Activity (aw) by... 

ISOPIESTIC DETERMINATION OF WATER ACTIVITY (aw) BY EQUILIBRATION WITH SATURATED SALTS... 

Table A2.3.1 Water Activities (aw) of Selected Saturated Salt Solutions at Various Temperatures0 3...

Figure A2.3.2 An apparatus with circulated air chamber and in situ weighting system for measurement of water activity (aw). Adapted from with permission Igbeka and Blaisdell (1982).

ISOPIESTIC DETERMINATION WATER ACTIVITY BY EQUILIBRATION WITH STANDARDS OF KNOWN WATER ACTIVITY (aw)... 

Water activity (aw) is the ratio of the partial vapor pressure of water above a solution to that of pure water at the same specific temperature. It plays an important role in evaluating the microbial, chemical, and physical stability of foods during storage and processing. The vapor pressure in the headspace of a food sample can be measured directly by a manometer. A manometer has one or two transparent tubes and two liquid surfaces where pressure applied to the surface of one tube causes an elevation of the liquid surface in the other tube. The amount of elevation is read from a scale that is usually calibrated to read directly in pressure units. Makower and Myers (1943) were the first to use this method to measure vapor pressure exerted by food. Later, the method was improved, in terms of design features of the apparatus, by various scientists (Taylor, 1961 Labuza et al., 1972 Lewicki, 1987). Trailer (1983), Lewicki (1989), and Zanoni et al. (1999) used a capacitance manometer instead of a U-tube manometer for the measurement of vapor pressure. Lewicki et al. (1978) showed that the precision and reproducibility of the method can be improved by the simultaneous measurement of the water vapor pressure and temperature of the food sample. The method is reviewed in detail by Rizvi (1995) and Rahman (1995). 

Calculate the water activity, aw, of the sample from ht and h2 values using Equation A2.4.1. 

Summarizing the findings above, control of water activity aw during enzyme reactions in organic solvents is extremely important, as water activity exerts a crucial influence, and enzyme reactivity crucially depends on it. 

Thermal inactivation of Bacillus cereus spores was confirmed to be affected by the solutes used to control water activity (Aw) of the heating medium. For the spores of B. cereus ATCC 7004 subjected to a heating inter-valbetween 92 and 106°C, Mazas et al. (17) determined the following parameters (1) z-value of 8.3°C for spores in water (Aw = 1.0) at pH 7.60, as a medium control (2) z-value of 10.3°C for spores in 5.6 M NaCl solution at pH 8.35 and Aw = 0.75. For both extreme systems, the spores of B. cereus exhibited lower thermal stability than even the system of GFPuv in phosphate solution (z-value minimum of 13.75°C). Among the environmental conditions that affect spore thermoresistance, Vessoni Penna et al. (18)... 

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 Activity (aw) of a Selection of Hawaiian Starchy Foods at Two Different Temperatures0... 

Water activity, aw, is defined as the ratio of the vapor pressure of a solution to that of pure water at a specified temperature. This is expressed as aw = P/P0, where P is the vapor pressure of a solution and P0 is the vapor pressure of pure water. 

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