Evaluation of Hydration State around Glycerol as a Humectant Using Microwave Dielectric Spectroscopy

Water in intermediate-moisture foods can be broadly classified into bound water, which is strongly bound to components, and free water, which can easily move or evaporate due to changes in surrounding environmental conditions. Free water is essential for the growth of microorganisms that can cause spoilage of foods. Glycerol, a humectant with high water retention capacity, has been used as an additive to reduce the free water content in processed foods because it is easily hydrated to form stable hydrogen bonds with water and has been thought to reduce the free water percentage in foods. Our research group used microwave dielectric spectroscopy and differential scanning calorimetry (DSC) to evaluate the hydration state around glycerol in comparison with several polyols with different water-holding capacity. Microwave dielectric spectroscopy showed that the dielectric spectra of the polyol aqueous solutions exhibited different molecular dynamics. Quantitative estimation of free water content for 10 mol% concentration aqueous solutions based on DSC results indicated that glycerol had significantly higher free water content. These results were consistent, suggesting that humectants with high water retention properties may not exhibit strong hydration. It was also suggested that the hydration state around the solute, with low hydration water content, may be a unique property of glycerol. In any case, the results overturn the conventional interpretation that hydration plays an important role in the water-holding capacity of humectants.