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

Abstract

Mixtures of /c-casein and /Mactoglobulin (/?-lg) were heated at 70 °C in 20 mM-imidazole buffer, pH 6-8 containing 20 mM-EGTA. Aggregation of the *-casein//?-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 heated /c-casein//?-lg mixture followed by a return (> 350 s) to a spectrum comparable to the native state, indicating molecular rearrangement. Apparent hydrophobicity of /c-casein decreased 80 % within 250 s compared to a 38% decrease for /?-lg. Under similar conditions, the /c-casein//?-lg mixture (1:1) showed a faster (2 -5 times) decrease in apparent hydrophobicity than /c-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 cooled /c-casein//?-lg to aggregate reached a maximum after heating at 70 °C for 720 s.

Influence of Cation Sequestering and pH on Quiescent Thermal Association of .BETA.-Lacto-globulin NB from Fresh Cheddar Whey: An Insight into Gelation Mechanism.

“…Hydrophobic interactions, the main driving force for protein folding, are made up of chain-free energy as well as hydration-free energy (Oobatake & Ooi, 1993). These interactions and resulting association causes an overall decrease in surface hydrophobicity of proteins due to overlapping (Haque, 1989;Haque & Kinsella, 1988). The ␤-Lg molecules had already interacted to different degrees based on pH at the pre-gelation stage (<70˚C) (Fig.…”

“…Exact nature/role of calcium/␤-lactoglobulin interaction is not clear though significant interaction between calcium and free sulfhydryl groups have been reported in whey proteins (Schmidt et al, 1984). Haque and Kinsella (1988) around 70˚C; limited exposure started around 50˚C. ␤-Lactoglobulin has one free sulfhydryl group at position 121, which lies buried at the sheet-helix interface (Papiz et al, 1986).…”

Quality attributes of the set-style skimmed yoghurt containing enzymatic cross-linked or thermal polymerized whey protein isolate

“…It is known that thermal treatment of whey protein induces protein denaturation and more importantly polymerization of whey protein (Fitzsimons, Mulvihill, & Morris, 2008), which brings about modified properties to whey protein. During thermal treatment of yoghurt milk, whey protein (primarily β-lactoglobulin) is denatured; after then, β-lactoglobulin interacts with к-casein through the disulfide bridgings and hydrophobic interactions (Haque & Kinsella, 1988), resulting in polymerization of milk proteins. This polymerization is very important and desired in yoghurt production; for example, it will lead to short fermentation time (Labropoulos, Collins, & Stone, 1984;Thomopoulos, Tzia, & Milkas, 1993) and decrease yoghurt syneresis (Şanlı et al, 2011).…”

Influence of preliminary treatments on structural properties of casein micelles affecting the rennetability

“…Mild heat treatments, like thermization (64-68 "C for lOs), have been reported to adequately preserve the renneting properties of raw milk. More severe procedures cause irreversible molecular changes which are related mainly to complex formation via sulphhydryl-disulphide interchange and hydrophobie interactions between~-Iactoglobulin (~-Lg) and x-casein (K-Cn) (Zittle et al, 1962;Sawyer, 1969;Haque and Kinsella, 1988). It seems to be the most important factor affecting the rennet coagulability of heated milk (Van Hooydonk et al, 1987;Dalgleish, 1990).…”