Characterization of β-lactoglobulin fibrillar assemblies

The overall goal of this research was to study the mechanism of heat-induced whey proteins fibrils and to characterise their properties for possible food applications. Experimental parameters such as protein concentration, pH, ionic strength, time and temperature of heating were varied to optimize the process of fibril formation of β- lactoglobulin (β-lg). Fibrillar structures could only be formed at acidic pH; the optimal conditions were pH 2, low ionic strength and 2% (w/w) protein concentration. A two-stage mechanism of β-lg fibril formation was proposed: 1) denaturation, partial unfolding and increase in β-sheets content, with hydrolysis of monomers, followed by 2) the linear aggregation of polypeptide fragments into fibrils via noncovalent interactions, accompanied by hydrolysis and a decrease in β-sheets content and overall secondary structure. In this study, β-lg and whey protein isolate (WPI) – derived fibrils were observed by atomic force microscopy (AFM). Their height was ca. 2 - 3 nm, while their length and periodicity was up to 15 μm and ~ 30 nm, respectively. Various process treatments such as high pressure, heating, shearing and acidification can induce changes in the physicochemical properties of whey proteins and their fibrillar aggregates. The studies on pH stability of β-lg fibrillar structures morphology revealed that the longer fibrils break up in the isoelectric range (pH 4.6 - 6) but were stable starting with pH 7 to 12. High dynamic pressure treatment (microfluidization) produced changes in the general physical dimensions of whey protein fibrils and in the secondary structures of their constituent units. Long fibrils (15 μm) were fractured under high-pressure treatment, the end result being shorter length fibrils (< 350 nm) with the same thickness of 2-3 nm. The study on the foaming properties of whey protein fibrils revealed that foam capacity and stability were related to the protein concentration, pH, whipping time, thermal and/or high pressure treatment of the protein and their assemblies. Results indicated the fibrillization of whey proteins increased foam capacity and stability compared to non-fibrillar whey proteins.