Probing early-stage casein micelle aggregation during milk acidification with differential dynamic microscopy

The sol-gel transition of casein micelles during milk acidification is relevant to many dairy manufacturing processes. However, the early microscopic events that precede visible gelation remain poorly resolved due to milk's intrinsic turbidity, which hinders conventional light-scattering methods. Here, we show that Differential Dynamic Microscopy (DDM) enables direct, in situ quantification of casein micelle dynamics and aggregation throughout acid-induced gelation of skim milk, without dilution or tracer particles. Time-resolved DDM measurements reveal a progressive reduction in the micellar diffusion coefficient, accompanied by the emergence and growth of fractal aggregates that ultimately form a continuous gel network. Comparison of heated and unheated milk shows that heat treatment, which is known to shift the onset of aggregation to higher pH, subtly alters the coupling between cluster size and mobility. Besides providing multi-scale quantitative insight into the microstructural aspects of acid milk gelation, these results establish DDM as an accessible yet powerful tool for probing gelation and network formation in moderately turbid colloidal systems.