Influence of cosolvent systems on the thermostability and heat-induced gelation mechanism of globular proteins
The objective of this study was to investigate the impact of low molecular weight cosolvent systems on the gelling mechanism of globular proteins, including bovine serum albumin (BSA), β-lactoglobulin, (β-Lg), and whey protein isolate (WPI). The influence of sucrose, glycerol, sorbitol, and cosolvent mixtures (0 to 40 wt%) on the thermal denaturation and gelation of bovine serum albumin (BSA) in aqueous solution has been studied. The effect of these cosolvent systems on heat denaturation of 0.5 wt% BSA solutions (pH 6.9) was measured using ultrasensitive differential scanning calorimetry. The unfolding process was irreversible and could be characterized by a denaturation temperature (Tm). As the cosolvent concentration increased from 0 to 40 wt%, Tm also increased depending on the cosolvent system surrounding the protein molecule. The rise in Tm was attributed to the increased thermal stability of the globular state of BSA relative to its native state because of differences in their preferential interactions with the cosolvent system. The change in preferential interaction coefficient (ΔΓ3,2) associated with the native-to-denatured transition was estimated for each cosolvent system. The dynamic shear rheology of 2, 4, and 5 wt% BSA solutions (pH 6.9, 0–200 mM NaCl) was monitored as they were heated from 30 to 90°C, held at 90°C for either 15 or 120 minutes, and then cooled back to 30°C. The impact of each cosolvent system on the gelation temperature was characterized. Depending of the cosolvent system surrounding the protein molecules, the complex shear modulus (G*) of cooled gels either increased or remained constant with cosolvent concentration when they were held at 90°C for 15, 70, or 120 minutes. The turbidity of the same solutions was monitored as they were heated from 30 to 90°C at 1.5°C min−1 or held isothermally at 90°C for 10 minutes. As the cosolvent concentration increased the protein gelation temperature increased, but the isothermal gelation rate at 90°C decreased. The impact of various cosolvent systems on gel characteristics was interpreted in terms of its ability to decrease the protein-protein collision frequency, increase the attractive forces between proteins (at low temperatures) and increase protein thermal stability.