EFFECT OF ULTRAFILTRATION OF SKIM MILK ON CASEIN MICELLE SIZE DISTRIBUTION IN RETENTATE

Casein micelles obtained from skim milk concentrated three or five times by UF were mixed with agar and embedded in resin, and their thin sections were examined by transmission electron microscopy. The examination revealed roughly spherical particles with a wide range of sizes similar to the range of sizes found in unaltered skim milk. However, there was a significant change in casein micelle size distribution when the milk was ultrafiltered to higher concentrations. The highest proportion of casein micelles was changed from the 80- to 100-nm range in skim milk to 60 to 80-nm in milk concentrations, the numbers of casein micelles with diameters less than 80 nm increased, but those with diameters greater than 100 nm decreased. Consequently, both the volume distribution and the average diameter of the casein micelles were altered. There was a decrease in casein micelle diameter from 118 nm in normal skim milk to 92 and 87 nm in milk concentrated three or five times, respectively. The change in the composition of casein and minerals, particularly Ca and P, as a result of UF may be responsible for the change in casein micelle size distribution, volume distribution, and average diameter of the casein micelles.