Determination of water content in powdered milk

The determination of the water content in foodstuff is one of the most frequently performed analyses in the food industry (Isengard, 2001). In dried milk IDF method 26A in which the sample is dried at 102.0 ± 2.0 °C in a standard air drying oven has been used for many years as reference method. The method exhibits excellent repeatability but reproducibility is insufficient. Therefore the IDF working group on water developed an alternative drying method which involves drying the sample in a column at 87.0 ± 1.0 °C under a stream of dried air passing trough the column. The drying conditions were chosen to obtain equivalent results to the standard method. As the physico-chemical properties of the food might influence the outcome it is doubtful that any of the drying techniques allow a well-defined cut between adsorbed and bound water. On the contrary it is possible to define the total water content in milk powder as all water molecules present in whatever form. This is usually measured by the Karl–Fischer (KF) method (Rüegg & Moor, 1987). In this study we measured samples the moisture or water content of a large number of milk powders by several techniques (IDF 26A, desiccation by P2O5, KF) and used water activity and near infrared spectroscopy to further understanding. We show how the measurement techniques relate to each other and demonstrate that the KF technique seems to be the most precise and best-defined method. This results, when the KF method is applied properly, in the best reproducibility of the tested methods.