Similar size distributions of casein micelles determined by cryo-electron microscopy and nanoparticle tracking analysis

Nanoparticle tracking analysis (NTA) and cryo-transmission electron microscopy (cryo-TEM) were used to determine histograms of the number and volume fractions of different sizes of casein micelles in unprocessed cow milk samples. Using a composite of measurements on five individual raw milk samples, cryo-TEM data gave a unimodal histogram of volume fractions with a modal radius at 85 nm and full width at half height of 50 nm. With a sample of unpasteurized bulk milk, the corresponding figures from the NTA were a modal radius of 85 nm and full width at half height of 40 nm. Both histograms were skewed to larger sizes by a tail extending to a radius of 250–300 nm. Histograms of number fractions showed fewer smaller micelles by NTA than cryo-TEM. Even though different unprocessed milk samples were used, the results demonstrated that the rapid NTA method could give a very similar histogram of volume fractions to cryo-TEM. Histograms of the unprocessed milks could be fitted approximately by either the log-normal density distribution function or a discrete distribution of sizes derived from the multivalent-binding model of the casein micelle. The potential applications of the NTA method in dairy science and technology were demonstrated by the facile and rapid measurement of size histograms of commercial pasteurized or sterilized milk samples, which were distinctly different in shape from the unimodal histograms of unprocessed milk.