The effect of minor components on milk fat crystallization behavior, microstructure and mechanical properties
This thesis is an investigation of how the minor lipids in milk fat affect the physical properties of the fat. This thesis was undertaken to determine what effect, if any, minor components have on the crystallization behavior, microstructure and mechanical properties of milk fat triacylglycerols (MF-TAG). Minor components were removed from anhydrous milk fat (AMF) and the behaviors of the native AMF and purified MF-TAG compared. The effect of milk fat and standard diacylglycerol (DAG) isomers on milk fat physical properties were also studied. Minor components were found to exert a kinetic, rather than thermodynamic effect. Primarily, minor components serve to inhibit milk fat nucleation at low degrees of supercooling ('i.e'. above 20°C). When milk fat DAGs were added to MF-TAG at the 0.1wt% level, crystallization was also delayed, although some batch to batch variability was observed. In the case where a high degree of molecular complementarity was found between the DAGs and MF-TAG, the delay in crystallization was more pronounced. Racemic purity was shown to be an important determinant of a DAG's influence on TAG crystallization. Only 'sn'-1,2 isomers of palmitic and oleic acid DAGs increased the energy barrier to MF-TAG nucleation and delayed subsequent crystallization. Evidence for the inhibition of solidification by minor components at both the nucleation and crystal growth levels was found. Structural and chemical complementarity between the minor component and triacylglycerols (TAGs) allows a minor component to co-crystallize with the TAGs. The subsequent incorporation of TAGs into the crystal lattice is then inhibited because of steric factors. Although crystallization kinetics were affected, the properties of the resulting network structures remained virtually unchanged by milk fat minor components. The microstructures and mechanical properties of samples with and without minor components were similar after 24h. Fractal dimension which is an indicator of network microstructure was significantly correlated with three crystallization parameters ([tau]SFC, 'k' and 'n') confirming the link between fat crystallization behavior and crystal network structure.