pH-stability and thermal properties of microbial transglutaminase-treated whey protein isolate

Whey protein isolate (WPI) was treated to various extents using microbial transglutaminase (MTGase) and changes in pH-stability and thermal stability of its protein components were investigated. The MTGase treatment significantly increased the denaturation temperature (T(d)) of beta-lactoglobulin in WPI, from 71.84 degrees C in the untreated sample to 78.50 degrees C after 30 h of incubation with MTGase. The enthalpy change of denaturation of WPI did not change upon cross-linking, indicating that the increase in T(d) was primarily due to covalent cross-linking and not due to an increase in nonpolar interactions within the protein. The surface hydrophobicity (S(o)) of the protein decreased upon cross-linking; however, this decrease was not due to burial of the surface hydrophobic cavities in the protein interior, but due to occlusion of the hydrophobic cavities to the fluorescent probes. Fluorescence emission and circular dichroism spectroscopic analyses revealed no major changes in the secondary and tertiary conformations as a result of cross-linking. However, unlike native WPI, the cross-linked protein exhibited a U-shaped pH-stability profile with maximum turbidity at pH 4.0-4.5. The study revealed that even though enzymatic cross-linking significantly improved the T(d) of beta-lactoglobulin in WPI without causing major structural changes, a reduction in the hydrophilic-hydrophobic balance of the protein surface as a result of elimination of the positive charge on lysyl residues caused precipitation at pH 4.0-4.5.