Casein Micelles and their Properties: Polydispersity, Association with Vitamin A and Effect of Ultra-High Pressure Homogenization

The internal structure of casein micelles is not fully understood. In the present work, we explore some of the basic questions about casein micelles in bovine milk including its size distribution, native binding properties and effect of ultra-high pressure homogenization (ultra-HPH) on technological properties of casein micelles. The size distribution of casein micelles was studied by cryo-TEM using skim milk from four cows. The degree of variation in casein micelle sizes (polydispersity) ranged between 0.39 and 0.47 for a size distribution ranging from 10 to 693 nm. These results indicated high polydispersity of casein micelles in milk from single cows. Casein micelles associated with vitamin A in four pasteurized skim milks (1.6 - 2.5 micro gm/mL of milk; 14 - 40% of the initial quantity detected in milks), while other protein fractions contained negligible vitamin A. Thus, casein micelles can inherently associate with hydrophobic probes like vitamin A in milk. This association of vitamin A to casein micelles in milk provided protection from degradation on exposure to ultraviolet light when compared to apple juice. Further, the effect of ultra-HPH up to 500 MPa on the physicochemical (apparent casein micelle size by dynamic light scattering) and technological characteristics (rennet coagulation - firmness [ 90 min, 1Hz, 0.01% strain]; acid gelation using 3%w/v glucono delta lactone – firmness [22°C, 110 min, 1Hz, 0.01% strain], SDS-PAGE) of casein micelles were studied. Casein micelle size increased from ca. 180 nm at 100 MPa to ca. 280 nm at 500 MPa HPH pressure. With increase in HPH pressure, renneting ability decreased until no coagulation was obtained for 500 MPa HPH milk. The firmness of HPH milk acid gels increased from about 76 Pa to 108 Pa when pressure was increased above 100 MPa (up to 400 MPa) HPH as compared to acid gels made from non-homogenized milk. Overall we elucidated the size distribution, binding ability to vitamin A and changes occurring on ultra-HPH in casein micelles. This information can be utilized by the industry to modify and utilize casein micelles as an ingredient for different end uses.