Interaction between heated κ-casein and β-lactoglobulin: predominance of hydrophobic interactions in the initial stages of complex formation

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Abstract
Summary: Mixtures ofκ-casein and β-lactoglobulin (β-lg) were heated at 70 °C in 20 mM-imidazole buffer, pH 6·8 containing 20 mM-EGTA. Aggregation of theκ-casein/β-lg mixture occurred within 90 s and susceptibility to hydrolysis by chymosin decreased significantly within 180 s even though covalent interaction was not detected until after 4000 s. UV-absorbance indicated initial structural destabilization of the heatedκ-casein/β-lg mixture followed by a return ( > 350 s) to a spectrum comparable to the native state, indicating molecular rearrangement. Apparent hydrophobicity ofκ-casein decreased 80% within 250 s compared to a 38% decrease forβ-lg. Under similar conditions, theκ-casein/β-lg mixture (1:1) showed a faster (2·5 times) decrease in apparent hydrophobicity thanκ-casein alone with concomitant exposure of acidic (hydrophilic) groups. The results suggested that the driving force for the rearrangement was mainly hydrophobic, i.e. entropic in origin. The tendency of heated and subsequently cooledκ-casein/β-lg to aggregate reached a maximum after heating at 70 °C for 720 s.