Evidence for Fibril-Like Structure in Bovine Casein Micelles

Baseline absorbance (arbitrary units) of diluted raw skim milk: —— 25×; —— 50×; — 75×. Diluent: solid curves = simulated milk ultrafiltrate, dashed curves = deionized water.

Absorption spectrum (arbitrary units) for raw skim milk diluted 50×: —— = raw milk + simulated milk ultrafiltrate (SMUF); - - - - = Congo red + SMUF; — — = raw milk + Congo red + SMUF; — - — = difference curve.

Congo red difference spectra for 50× dilution: —— = skim milk (SM); —— = whey; — = casein micelles (CM). Diluents: solid curve = simulated milk ultrafiltrate (SMUF); short dashed curve = deionized water (DI); long dashed curve=0.05 M HEPES (pH=6.8).

Thioflavin T (ThT) fluorescence spectra (arbitrary units) for undiluted raw skim milk in a 0.1 cm×1.0cm cuvette: —— = raw milk; - - - - = raw milk + ThT; — — = difference curve.

Thioflavin T (ThT) fluorescence spectra (arbitrary units) for 15× diluted raw skim milk in a 1.0-cm² cuvette: —— = raw milk; - - - - = raw milk + ThT; — — = difference curve.

Thioflavin T (ThT) fluorescence spectra (arbitrary units) for diluted raw skim milk: —— 20×; —— 25× — 30×. Diluents: solid curve = simulated milk ultrafiltrate (SMUF); short dashed curve = deionized water (DI); long dashed curve=0.05 M HEPES (pH=6.8).

Thioflavin T (ThT) fluorescence spectra (arbitrary units) for 20× dilution: —— skim milk (SM)*; - - - ultracentrifugation whey (UW)*; — — casein micelles (CM)*; — - - — acid whey (AW)*; — - — sodium caseinate (SC)†; and — — - — — sodium caseinate + Ca⁺² (SC + Ca²⁺)†. Samples were diluted with simulated milk ultrafiltrate (*) or 0.5 M HEPES, pH=6.8 (†).

Congo red difference spectra (arbitrary units) for 0.5g/L casein solutions in 0.5 M HEPES (pH=6.8): —— = α_S-casein; - - - = β-casein; and — — = κ-casein.

Thioflavin T (ThT) fluorescence spectra (arbitrary units) for casein solutions in 0.5 M HEPES (pH=6.8): —— = α_S-casein; —— = β-casein; — = κ-casein. Concentrations: solid curve=1.0g/L; short dashed curve=0.5g/L; and long dashed curve=0.1g/L.

Effect of 25mM CaCl₂ addition on thioflavin T (ThT) fluorescence spectra (arbitrary units) for casein solutions in 0.5 M HEPES (pH=6.8): —— = α_S-casein; —— = β-casein; — = κ-casein. Solid curve = without Ca²⁺; short dashed curve = with Ca²⁺.

Transmission electron microscopy images of: (a) a casein micelle; (b) sodium caseinate. White bar=50nm; black bar=25nm.