Tempering governs the milk fat crystallisation and viscoelastic behaviour of unprocessed and homogenised creams

Abstract

The crystallisation behaviour of milk fat plays an important role in the functionality and sensory properties of fat-rich dairy products. In this study, we investigated the impact of tempering to 25 °C on the viscoelastic properties, particle size and thermal behaviour of 20% w/w unprocessed and homogenised creams prepared from bovine milk. The crystallisation properties were examined by synchrotron X-ray diffraction (XRD) at small (SAXS) and wide angle (WAXS) and differential scanning calorimetry (DSC). Oscillation rheology was performed to characterise the cream's viscoelastic properties. Homogenisation (35 MPa) reduced the average droplet size from 4.4 to 1.3 µm. After 24 h storage at 4 °C, milk fat structures showed triacylglycerol (TAG) 2L and 3L_{(001, 002, 003, 005)} lamellar stacking orders associated predominantly with the α and β' polymorphic forms. Tempering to 25 °C induced the complete melting of the 3L crystals and led to an irreversible loss in the elastic modulus (G') and a reduction in the viscous modulus (G'') once returned to refrigerated conditions, due to changes in the particle-particle interactions and structure of the reformed milk fat crystals. The results demonstrate that crystallisation behaviour of milk fat is influenced by droplet size and the rearrangement of triacylglycerol (TAG) upon tempering, and lead to changes in the viscoelastic behaviour of dairy products containing a high level of milk fat.

Keywords: Crystallisation; Droplet size; Homogenisation; Lamellar stacking; Oscillation rheology; Polymorph; Synchrotron SAXS/WAXS; TAG; Thermal behaviour; X-ray diffraction.