pH determination : New applications of mid-infra-red spectrometry for the analysis of milk and milk products

The pH determination in raw milk by automated mid-infra-red spectrometry was recently introduced into routine analysis. This provides new options for monitoring the quality of samples within the quality-assurance system of routine laboratories. The need for such a quality-control parameter is shown by the influence of pH changes - caused by bacterial souring - on the infra-red determination of parameters like fat and protein content. Whereas the parameters fat, lactose, urea and freezing-point equivalence show a decrease with lower pH values, the protein read-out shows an increase. Principal technical aspects of IR determination are mentioned and critical success factors for the pH measurements by mid-infra-red FT-IR-spectrometry are discussed. Although based on only few available data, performance characteristics are shown to be promising compared to traditional methods. Indicative values for repeatability are in the range from 0.02 to 0.05, for intralaboratory reproducibility at about 0.05 and for interlaboratory reproducibility in the range from 0.05 to 0.065 pH units. Accuracy data, obtained by using the potentiometric pH measurement as a reference method, show a standard error of prediction of 0.064 pH units. It is concluded that the pH option with IR instruments is a very promising tool to improve the quality of analytical data obtained from raw milk samples in quality-testing and payment schemes as well as in dairy herd improvement schemes. More attention should be paid to the analytical quality of data extracted from raw milk samples with abnormal pH values.