Revisiting the interpretation of casein micelle SAXS data

Area of Science:

• Biophysical chemistry
• Food science
• Materials science

Background:

• Small-angle X-ray scattering (SAXS) is widely used to study milk proteins.
• Previous models often misattribute features in SAXS data to colloidal calcium phosphate (CCP).
• A critical review highlighted limitations in existing model-dependent fitting of SAXS data.

Purpose of the Study:

• To develop a new mathematical model for interpreting SAXS data of bovine skim milk.
• To accurately assign structural features observed in SAXS data to specific milk components.
• To clarify the origin of scattering peaks in SAXS profiles of milk.

Main Methods:

• Critical review of existing model-dependent fitting methods for SAXS data.
• Development of a novel mathematical model for SAXS data interpretation.
• Utilizing calcium-edge resonant soft X-ray scattering data for validation.
• Analysis of SAXS data under varying conditions (hydration, EDTA, urea, pH).

Main Results:

• A new mathematical model was developed for SAXS data analysis.
• Calcium-edge resonant soft X-ray scattering confirmed the scattering signature of colloidal calcium phosphate at q = 0.035 Å(-1).
• Major SAXS features were attributed to casein micelle structure (20 nm regions, water channels) and nanoscale protein inhomogeneity (1-3 nm).

Conclusions:

• The primary features in SAXS data of bovine skim milk originate from protein arrangements within casein micelles.
• The proposed model provides a more accurate interpretation of SAXS data compared to previous models.
• The findings are consistent with the behavior of milk proteins under different environmental conditions.