Comparison of Vapor and Liquid Isotherms for Casein and Casein‐Sucrose Mixture

Hydration by vapor sorption and liquid water addition for the effect on sorption isotherms of casein and a casein‐sucrose mixture was compared. Both isotherm data and NMR relaxation rates were obtained from the same samples. Data showed biphasic linear plots with breaks at the same water activity (a_w). At a given moisture content, liquid hydration of casein showed a lower a_w and a lower NMR mobility than the vapor sorbed samples. The casein‐sucrose mixture showed the opposite. As a_w was increased above 0.90, casein alone showed decreasing difference between vapor and liquid hydration while the mixture showed rapidly diverging lines. These isotherm differences between vapor and liquid hydration were corroborated by NMR.