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Rennet-induced casein gels made from skim milk were studied rheologically. A scaling model or framework for describing the rheological behavior of gels is discussed and used for classification of the structure of casein gels. There are two main parameters in the model that describe the number of deformable links in a strand and the bendability of the links. In the model at least five types of gel structure can be distinguished. Application of the model to experimental data on rennet-induced casein gels at pH of 6.0–6.6 and 25 °C shows that they contain straight strands with a large number of deformable links. Analysis of the experimental data of the storage modulus, maximum linear strain and yield stress as a function of the volume fraction results in the same information about the gel structure.

The properties of air/water adsorbed and spread monolayers of native and dephosphorylated β-casein were monitored using surface pressure (Langmuir trough) and surface rheology (ring trough) techniques. Two stages of rearrangement are observed for native β-casein at surface areas of about 1.0−1.3 and 0.7 m2 mg-1. The first accounts for distinct surface elasticity changes in the film, which are probably due to the expulsion of the most hydrophilic segments of the protein chain. The second accounts for the collapse of the monolayer. The experiments on dephosphorylated β-casein monolayers show that dephosphorylation changes the surface elasticity behavior of the monolayer, in particular between 1 and 1.3 m2 mg-1. We calculated a two-dimensional Flory exponent, ν, for both proteins. This exponent is constant over a (semi-) dilute range of surface tensions, maximally up to a surface area of around 1.3 m2 mg-1. The adsorption of native β-casein is shown to be diffusion-limited up to a surface area of around 1 m2 mg-1. Experiments at high ionic strength show the importance of charge on the typical surface elasticity behavior of β-casein. Experiments with enzymatically treated β-casein show the importance of the presence of a hydrophilic section in the molecule on the surface elasticity behavior. It is assumed and shown that, at surface concentrations below monolayer collapse and at given solvent conditions, native β-casein and dephosphorylated β-casein show irreversible (air/water) adsorption behavior. Furthermore, the proteins in the monolayer are very flexible (i.e. quick relaxations).