EFFECTS OF pH ON WHEY PROTEIN CONCENTRATE GEL PROPERTIES: COMPARISONS BETWEEN SMALL DEFORMATION (DYNAMIC) AND LARGE DEFORMATION (FAILURE) TESTING

The effects of pH on the properties of gels prepared from commercially available whey protein concentrate were investigated by nondestructive dynamic shear testing with a Bohlin Rheometer and by large deformation compression, tension and penetration tests with an Instron Universal Testing Machine. Dynamic storage modulus, G', and the slopes of force-deformation curves from all three types of large deformation test exhibited maxima at or near both pH 4 and pH 7. Electrostatic interactions were the dominant factor in explaining this behaviour. Failure displacements in the large deformation tests generally increased with increase in pH. It is proposed that the formation of disulphide bonds at pH values above 6.5 is the dominant factor in explaining this behaviour. Failure forces in the large deformation tests remained low up to pH 6.5, increased rapidly to a maximum at pH 7–7.5 and then decreased again. The variation in both electrostatic interactions and disulphide bonds with pH is necessary to explain this behaviour. The combination of penetration failure force, penetration failure displacement and penetration rigidity (slope of force-displacement curve) provided as much qualitative understanding of the variation of gel properties with pH as similar information from any other single test.