Heat-induced gelation of whey protein at high pH studied by combined UV spectroscopy and refractive index measurement after size exclusion chromatography and by in-situ dynamic light scattering

The interactions between beta-lactoglobulin and alpha-lactalbumin involved in gelation at 67.5 degrees C at high pH and low salt concentration were st udied by size exclusion chromatography, followed by UV and refractive index measurements, and by in-situ dynamic light scattering. This was achieved b y choosing whey protein samples with different proportions of the two prote ins. The ratio of absorbance at 280 nm to the refractive index was used to demonstrate that alpha-lactalbumin was incorporated in aggregates and gels and drastically changed the properties of the gel, making them much more tu rbid than the transparent gels formed by beta-lactoglobulin alone at the sa me pH and ionic strength. At a ratio of 1:2 for a-lactalbumin relative to b eta-lactoglobulin in the samples, the gel consisted of a 1:1 mixture of the two proteins. The aggregates present after 10 min of heating at 67.5 degre es C had molar mass of about 6.10(6) g/mol and a radius of gyration of abou t 40 nm. After gel formation the field autocorrelation function could be de scribed as a power law over many decades of lag time for all samples, demon strating selfsimilarity of the gel structure. The only exception to this wa s for the gel with high content of alpha-lactalbumin which showed an oscill atory behaviour of the autocorrelation function. Significant amounts of gly cosylated caseino-macro-peptide were observed in many of the samples at the position of beta-lactoglobulin. However it did not affect gelation as it r emains in solution.