It has been widely used in the food industry as well as basic food science. The Aw is a thermodynamic value and is defined as the ratio of the equilibrated vapor pressure ( P 1) of the sample to the saturation of vapor pressure ( P 0) of pure water at the same temperature, Aw = P 1/ P 0. The concept of water activity (Aw) was introduced over 60 year ago ( Scott, 1953) and has been used as a practical and reliable indication of the microbial growth, enzymatic activity, preservation, and quality of food ( Roos, 1993 Sablani et al., 2007). The interactions between water molecules and biomaterials and ingredients in food are crucial in life and food sciences. We discuss the implications of Aw in the preservation of food at the molecular level. More importantly, Aw values of glycerol–water mixtures can be explained by the hydrogen bonding network and molecular dynamics of water in the solution. Translational diffusive and/or rotational motions in time and space analyzed by IQENS appeared when Aw > ≈ 0.7, and are correlated with hydrogen bonding related local vibrational dynamics in the glycerol–water mixtures. Hydrogen bonding related molecular vibrations were analyzed by ATR-IR, which indicated that the OH stretching in water molecules is significantly different for Aw > ≈ 0.7. DSC showed that bulk-like water existed at Aw > ≈ 0.7 at 27☌. Three regions were identified and classified based on DSC results.
In this study, the molecular basis of Aw was examined by using differential scanning calorimetry (DSC), attenuated total reflection Fourier-transform infrared spectroscopy (ATR-IR), and incoherent quasi-elastic neutron scattering (IQENS) based on moisture sorption isotherms of glycerol–water mixtures.
Glycerol–water mixtures can provide a variation of Aws by controlling the ratio of glycerol and water. However, a molecular basis of Aw is still under debate in multiple related disciplines. Water activity (Aw) is a reliable indication of the microbial growth, enzymatic activity, preservation, and quality of foods.
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