Comparison between structural changes of heat-treated and transglutaminase cross-linked beta-lactoglobulin and their effects on foaming properties

The main whey protein, β-lactoglobulin, was enzymatically modified by transglutaminase and analyzed for structural and conformational changes and their impact in protein foaming properties: foamability and foam stability. Solutions containing 25 mg mL−1 of β-lactoglobulin, 0.07 M cysteine in 20 mM sodium phosphate buffer pH 8.0 were incubated with transglutaminase, at a level of 1 U g−1 substrate, for different periods of time: 30, 60, 120 and 180 min. Protein structural characterization was discussed based on electrophoresis, fluorescence and viscosity studies. Comparison between the effects on foaming properties of this enzymatic treatment with those produced by heating, assayed in a previous work, was made. While 3 min was pointed as the critical time in heating treatment, 60 min was identified as the corresponding crucial time for transglutaminase treatment. The most significant conformational change, the greatest amount of dimers and trimers, and approximately the same proportion among protein species were verified at these times. Foamabilities were similar regardless of the treatment, but foam stability for heated β-lactoglobulin, measured through the change in foam volume with time is ∼250% higher than the same property for the protein enzymatically treated. Heating produces a higher degree of unfolding and index of surface hydrophobicity; less compact and more asymmetrical structures, with higher flexibility, which implies a greater capacity of rearrangement in the interface, producing a stiffer viscoelastic film, which slows down disproportionation through mechanisms that involve resisting compression and reducing gas transport. This improved film can be responsible for the higher foam volume stability.