The age-thickening of sweetened condensed milk: I. Rheological properties

Summary

The viscosity and flow properties of fresh and age-thickened separated sweetened condensed milk have been investigated using a rotating cylinder viscometer. The rheological properties, and the changes therein brought about by stirring, were studied by means of flow curves obtained by plotting stress against the corresponding rate of shear. Condensed milk during age-thickening is shown to develop thixotropic properties, with a high initial viscosity or ‘gel strength’ which, however, is easily reduced by stirring. On prolonged stirring the viscosity reaches a minimum (the ‘permanent’ viscosity), which becomes progressively higher as the age-thickening process continues. On storage subsequent to stirring there is only an insignificant recovery of the lost viscosity at room temperature, but a substantial recovery at higher temperature.

Sweetened condensed whey shows no age-thickening, and the casein-free supernatant liquids obtained by centrifuging dilutions of both fresh and age-thickened condensed milk have the same nitrogen contents and the same viscosities. This is interpreted to mean that the whey proteins play no significant part, directly or in association with casein, in the age-thickening process. Also the sedimentation rate of the casein during centrifuging is greater with age-thickened than with fresh condensed milk, which indicates an increase in casein particle size during agethickening.

The following working hypothesis is suggested to account for this behaviour. During age-thickening a slow irreversible change occurs in the size or shape of the casein micelles, probably by aggregation, which produces the rise in ‘permanent’ viscosity. These micelles orient themselves in such a way as to form a loose network enclosing some of the dispersion medium, thereby producing the large increase in the initial viscosity. On stirring, this network is broken down, and although it can reform in time at high temperatures, such recovery is greatly retarded at room temperature by the high viscosity of the aqueous phase containing the soluble constituents of the condensed milk.