Biophysical mechanisms of gravity separation and creaming in milk:

• Statement (I) is true: Milk fat has a lower density ($\rho_p \approx 0.93\text{ g/ml}$) than skim milk serum ($\rho_f \approx 1.036\text{ g/ml}$). Under gravity, fat globules rise towards the surface at a velocity governed by Stokes' Law: $v = \frac{2 r^2 (\rho_f - \rho_p) g}{9 \eta}$.
• Statement (II) is true: Individual fat globules are too small ($r \approx 1.5\text{ }\mu\text{m}$) to rise rapidly. At cold temperatures ($\lt 10^\circ\text{C}$), high-molecular-weight cryoglobulins (specifically pentameric IgM immunoglobulins) adsorb onto fat globule membranes, cross-linking individual globules into large flocculated clusters. This phenomenon is known as 'cold agglutination'. Because Stokes' rising velocity increases with the square of the particle radius ($r^2$), clustering accelerates creaming velocity by several hundred-fold.