Rheological characterisation of age thickening with special reference to milk concentrates.

Age thickening is a well known phenomenon within the dairy industry, where the apparent viscosity of concentrated dairy solutions increases with storage time under low shear. The objective of the paper is to develop new tools to analyse several aspects of age thickening, including the increases in shear thinning, the degree of thixotropy and the rate of structural build up with time, as well as irreversible structural changes and gelation. The build up of the dispersed phase structure during storage, was captured by a snap shot technique, where concentrated milk samples were stored, and aliquots were taken at periodic intervals and subjected to shear sweep tests to collect flow curves. Rheological modelling using the Herschel-Bulkley model showed an increase in yield stress and consistency, indicating a build up in structure during age thickening. The trend in apparent viscosity with storage time showed two-phases, a slow and steady increase followed by a sharp rise, which could be used to identify the onset of firm gelation. The extrapolated viscosity at infinite shear rate increased with storage time, indicating the build up in irreversible structural components during age thickening.

Effect of total solids content and temperature on the rheological behaviour of reconstituted whole milk concentrates.

This work investigated the combined effect of solids content, heating and storage temperatures on the rheological behaviour of reconstituted whole milk concentrates. The powder was reconstituted at 35 degrees C in a custom built recombination rig to various total solids content (TS) from 10-48% TS. The concentrates were then heated to 45-85 degrees C and stored at the heating temperature. The rheological behaviour shifter from Newtonian behaviour (below 30% TS) to power law (below 40% TS), with the yield stress observed from 40% TS upwards and time-dependent behaviour was noticed above 44% TS. Higher heating temperatures tend to promote non-Newtonian behaviour at lower solids content. The viscosity-solid content curve showed an exponential relationship, while the viscosity-temperature curve exhibited a minimum at 65-75 degrees C above 46% TS. During age thickening, the yield stress and the consistency coefficient increased, while the flow behaviour index decreased with storage time. This indicated that the milk concentrates deviated away from Newtonian behaviour during age thickening.