Interactions between modified milk fat globule membrane and casein micelles in rennet-induced milk gels
This research project investigates the interactions taking place between modified milk fat globule membrane and casein micelles and their impact on the light-scattering, rheological, and microstructural properties of rennet-induced milk gels. Rennet coagulation behaviour of gels containing model milk fat globules (sodium caseinate (NaCas) and whey protein isolate (WPI) stabilized fat globules) were investigated using diffusing wave spectroscopy, small deformation rheology, high performance liquid chromatography, confocal microscopy, and scanning electron microscopy. This work is the first attempt to investigate in more details the interactions occurring between the fat globules and the casein micelles during the pre-gelation stages of rennet coagulation using diffusing wave spectroscopy. It was found that in addition to the interfacial properties of fat globules, their colloidal state also affect the aggregation behaviour of casein micelles. Mildly flocculated, in contrast to stable, WPI-stabilized fat globules promoted the aggregation behaviour of casein micelles and enhanced the mechanical characteristics of rennet gels highlighting the importance of the flocculation state of the emulsion droplets in affecting the structure formation of the gel. Significantly impaired aggregation of the casein micelles in the presence of NaCas-stabilized fat globules was also clearly demonstrated. The results suggest that the inhibition of the secondary stage of rennet coagulation may result from the migration of individual caseins (i.e., after emulsification small portion of NaCas remained non-adsorbed to the oil interface) present in the milk serum into/onto the micellar surface enhancing the steric stabilization of these protein particles. Increasing the concentration of soluble Ca 2+ and enzyme did not recover the impaired rennetability of the recombined milk containing NaCas-stabilized fat globules suggesting that the inhibition of rennet coagulation was not caused by the reduced availability of soluble Ca2+ critical for the aggregation to proceed.