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.

Optics of colloidal light scattering in fluid milk:

• Light scattering requires particles with diameters comparable to or larger than the wavelength of visible light ($400\text{--}700\text{ nm}$).
• Milk fat globules (diameter $1\text{ to }10\text{ }\mu\text{m}$) strongly reflect and scatter visible light (Mie scattering) across all wavelengths, imparting a dense white opacity (Constituent A).
• Casein micelles (diameter $50\text{ to }300\text{ nm}$) form a dense colloidal dispersion that also scatters light efficiently, ensuring that even completely skimmed milk remains opaque whitish-blue (Constituent C).
• Lactose and mineral salts form a true molecular solution, while whey proteins ($2\text{--}5\text{ nm}$) form a transparent molecular solution that does not cause white opacity.

Enzyme renaturation and false-positive pasteurization diagnostics:
(A), (B), (C) do not exhibit significant reversible renaturation producing diagnostic false-positives.
(D) Alkaline phosphatase (ALP) is the statutory index enzyme for verifying proper milk pasteurization. However, when milk (particularly high-fat cream or milk subjected to ultra-pasteurization / HTST treatments $\gt 72^\circ\text{C}$ for short holding times) is held at room temperature ($20\text{--}30^\circ\text{C}$) in the presence of magnesium and zinc ions, denatured monomer subunits can reassemble and spontaneously regain catalytic activity. This phenomenon, known as 'enzyme reactivation', generates a false-positive result in the Aschaffenburg-Mullen or fluorometric ALP test, falsely suggesting under-pasteurization.