Comparative Evaluation of Total Solids, Freezing Point and Specific Gravity of Raw Milk Using an Ultrasonic Milk Analyzer Versus Standard Methods

Abstract

Forty five composite raw milk samples were each subdivided into five portions and then combined with 8, 4 or 0% water, and 2 or 5% whole milk powder (WMP). Ten samples randomly selected from each subsample were comparatively analyzed for their solid content using an ultrasonic milk analyzer (UMA) versus the standard sand-pan oven dry method. Similarly, fifteen samples from each subsample were analyzed for freezing point using the UMA versus a cryoscope. All milk subsamples were also analyzed for specific gravity using the UMA and a lactometer versus the standard gravimetric method. The solid content of the respective milk subsamples averaged 12.61, 12.93, 13.84, 15.31 and 17.52 % while those produced by the UMA averaged 12.81, 13.41, 13.69, 16.02 and 18.82 %. On average, the UMA readings provided a difference of 0.64 % higher (P<0.05) milk solids than the standard method. Mean freezing points of the milk subsamples analyzed by the two methods were -0.512, -0.531, -0.542, -0.651 and -0.799°C for the UMA and -0.493, -0.508, -0.549, -0.662 and -0.854°C for the cryoscope, respectively. The mean specific gravity determined by the three analysis methods increased (P<0.05) from 1.0265 to 1.0278, 1.0292, 1.0340 and 1.0407 for the five respective milk subsamples. The UMA as well as the lactometer consistently provided higher (P<0.05) specific gravity values than the standard oven dry method for all milk subsamples. However, on average, a difference of only 0.0014 specific gravity was observed between the UMA and the standard method. Contrary to this, a greater difference (P<0.05) of 0.0043 specific gravity was evident between the lactometer and the standard method. It was therefore considered advisable that the UMA instruments be regularly calibrated with the local raw milk standard. It was also recommended that the accuracy of lactometers be evaluated and their specific gravity readings be adjusted accordingly.