Abstract #M103

# M103
Size distribution of casein micelles in raw skim milk from individual cows as studied using cryo-TEM.
Maneesha S. Mohan*1, Federico M. Harte1, 1Food Science Department, Pennsylvania State University, University Park, PA.

Casein micelles have a complex structure comprising of 4 caseins: αS1, αS2, β, and κ caseins, assembling as many as 20,000 casein proteins. Polydispersity is the degree of disparity or variation in size of particles. Traditionally, casein micelles have been considered highly polydisperse with relative polydispersity between 0.27 to 0.5 as reported by different studies using dynamic light scattering techniques. A recent study has defined casein micelles to be more monodisperse. Hence, we decided to get a visual confirmation of the polydispersity of casein micelles by studying the casein micelle size distribution in milk from 4 cows by cryo-transmission electron microscopy (cryo-TEM). The raw milk from 4 cows of first parity, mid lactation and low somatic cell count, was skimmed by centrifugation (6414 × g for 20 min). The diluted skim milk (10 µL in 500 µL of protein-free serum obtained by ultrafiltration of skim milk using a 3-kDa filter) was subjected to cryo-TEM at 200kV. The diameter of casein micelles was measured manually using ImageJ software. The relative polydispersity (relative standard deviation of size distribution) of casein micelles was a Gaussian distribution (x-axis = number of micelles as a fraction of the total micelles measured, y-axis = diameter of casein micelles) spanning between 10 to 341 nm (1545 micelles), 24 to 693 nm (1073 66 micelles), 11 to 514 nm (2621 micelles) and 11 to 532 nm (1891 micelles) for milk from 4 cows, respectively. The positively skewed distribution curve had a tapering tail of large casein micelles of diameter greater than 350 nm. The maximum number of casein micelles existed in the size range of 90 to 110 nm for milk from all the 4 cows. The relative polydispersity ranged between 0.39 and 0.47 for casein micelles with mean diameters (D10) between 112 to 134 nm for the milk from the 4 cows. The results indicate high level of polydispersity in the casein micelles in milk of every cow. Hence it was possible to effectively utilize cryo-TEM to elucidate the size distribution of casein micelles and visually reconfirm the polydispersity of casein micelles in bovine milk from individual cows.

Key Words: polydispersity, electron microscopy, Gaussian