Abstract
Microorganisms require water for their metabolic activities. Only a fraction of water in foodstuffs, the so-called free water, is available for this purpose. The amounts of free water previously estimated by two different methods (Frosch et al. (2010), Frosch et al. (2011), and Low (1969)) are compared for aqueous solutions of four electrolytes, NaCl, NH4Cl, Na2SO4, (NH4)2SO4: (i) vapour pressure measurements of the solutions relative to that of pure water (water activities) and (ii) low-wavenumber Raman spectra in the R(ν)-representation. For each electrolyte deviations were found between results from the two methods. All water molecules in the illuminated volume contribute to the Raman data. The vapor pressure measurements refer to water molecules at the water/atmosphere interface where surface tension is important. Differences in surface tension for the four electrolytes qualitatively explain deviations between the amounts of “free water” observed by the two methods.
Highlights
The concept of free and bound water is essential in an understanding of the role of water in all aspects of life sciences
The slopes increase in the sequence NH4Cl < NaCl < (NH4)2SO4 < Na2SO4 for the four electrolytes, the influence of the surface tension on the vapour pressure is expected to increase in this sequence
The water activity described by the vapour pressure of a water-containing substance relative to pure water is considered to show the amount of “free” water available to microorganisms
Summary
The concept of free and bound water is essential in an understanding of the role of water in all aspects of life sciences. Water/protein interactions in human and animal skin were previously investigated by low-wavenumber Raman spectroscopy in the R(ν)representation, and a distinction between protein-bound water and “free” water was performed [6, 9]. Aw, is an important factor in preservation of foodstuffs, and aw can be expressed as the vapour pressure of a water-containing substance, p, divided by that of pure water, p0 at the same temperature [17]: aw
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