Variation of the viscosity of a concentrated, sterically stabilized, colloid: Effect of ethanol on casein micelles of bovine milk

Rheology of Swiss Cheese Fondue

Cheese fondue is a popular Swiss dish prepared by melting cheese under the addition of wine, starch, and seasoning. The flow behavior or rheology of fondue is crucial for mouthfeel, flavor release, and the tendency of fondue to cling to the bread. Fondue is a complex multiphase system whose rheology is determined by the interactions of its colloidal ingredients. We establish cheese fondue as a water-continuous system with dispersed fat droplets, charged caseins, and starch granules. Irreversible phase separation, a common issue in fondue preparation, may be prevented by addition of a critical minimum starch concentration. Fondue was found to be a shear-thinning yield stress fluid, which is desirable for mouthfeel and facilitates fondue to cling to the bread for consumption. Fondue showed a viscoelastic stress response around the gel point (G' ≈ G″), which is proposed as crucial for the balance of orally perceived gumminess (G') and liquidity (G″). Ethanol addition and lowering pH toward the isoelectric point of casein, as associated with wine addition, decrease fondue viscosity due to a decrease in casein micelle size. Below the isoelectric point of casein, fondue is unstable and phase separates, potentially impeding fondue digestion. Thus, fondue rheology is governed by the complex colloidal interactions within its ingredients, and ultimately determines fondue eating experience.

Changes in milk on heating: viscosity measurements

Skim milk was heated at 85 °C for different holding times. As a result of such heating, whey proteins, in particular β-lactoglobulin, denatured and associated with casein micelles. This led to an increase in size of the casein micelles but also to a different interaction between them. Both these changes could be described by using a quantitative model which was developed for the viscosity of so-called adhesive hard spheres. We applied the model successfully to skim milk and were able to describe on a quantitative basis the changes due to the heat treatment of milk. It was shown that after heating the casein micelles became larger and acquired a mutual attraction. The unfolding of the whey proteins and their subsequent association with the casein micelles appeared to be responsible for these changes. How this reaction influences the fouling of heat exchangers is discussed.