Enthalpy of vaporization for liquid foods using the data of vapor pressure

The vapor pressures of three kinds of liquid food (milk, dry milk solution, skim milk) of concentrations between 6 and 40% (w/w) were measured at temperatures between 10-60°C by a static method. The results measured were fitted to four vapor pressure equations (Clausius-Clapeyron, Antoine, Kirchhoff and Riedel equation), respectively. As a result, Antoine's equation was the most suitable one to relate temperature with the vapor pressure data of sample solutions. The enthalpies of vaporization in sample solutions were calculated directly from the Clapeyron's equation utilizing Antoine's equation without any assumptions in the Clausius-Clapeyron's equation and the Othmer plot principle. Moreover, effects of concentration on the vapor pressure of sample solutions were also investigated. The results suggested a linear relationship between the logarithm of vapor pressure and concentration. Therefore, a new empirical equation with five parameters relating temperature and concentration to vapor pressure, which was derived from adding concentration dependency to Antoine's equation, was developed. Using this equation, the enthalpies of vaporization in sample solutions were calculated from Clapeyron's equation. Furthermore, an empirical equation for the enthalpies of vaporization for sample solutions was proposed as a function of temperature and concentration to be convenient to the practical use.