Quantitation of added water in milk by using vapor pressure osmometry

A vapor pressure osmometer (VPO) was successfully studied in 1975. An improved osmometer with greater range and temperature stability was evaluated. Repeatability on 290 mOs/kg standard solution was 289.5 +/- 1.08. The mean and standard deviation on 36 water-free milk samples was 280.5 +/- 4 compared to 280.1 +/- 3 in the previous study. Instrument data from 2 osmometers and 2 cryoscopes produced acceptable chi-square values, and the correlation coefficients between methods were better than 0.99 for samples containing 1--21% added water. In a survey involving 760 samples, more than 37% of producer samples in one cheese plant contained greater than 3% added water. Additional evidence of added water in market milk samples suggests the immediate application of the VPO to help prevent milk adulteration. The vapor pressure osomometer method for quantitating added water in milk has been adopted as official first action.