Molecular and Microstructural Studies of Thermal Denaturation and Gelation of β-Lactoglobulins A and B

The thermal properties of β-lactoglobulins (β-lg) A and B at pH 3, 5, 7, and 8.6 were studied by differential scanning calorimetry. Fourier transform infrared spectroscopy was used to monitor changes in the secondary structure of the proteins when heated from 25 to 95 °C. The microstructure of β-lg A and B gels made from 10% (w/v) protein solutions by heating at 90 °C for 30 min was studied by scanning and transmission electron microscopy. β-Lg B had greater thermal stability and required more energy to denature than β-lg A; denaturation of β-lg B was also more cooperative. Infrared spectroscopy showed that β-lg B had a higher proportion of β-sheet than the A variant at pH 3 and 5. At pH 7 and 8.6 the secondary structures of the two variants were similar. At all four pH values, aggregation bands (1682 and ∼1622 cm - 1 ) were observed when the proteins were heated. Electron microscopy showed that the gel matrix of β-lg B at both acid and alkaline pH was made up of larger aggregate structures than β-lg A. The aggregates formed by both variants were large (1−2 μm) and globular at acid pH but much smaller (nanometer range) and amorphous at alkaline pH. This information provides a useful model for studying the relationship between protein structure and function.