Particle sizing in milk by combined differential dynamic microscopy and cryo-FIB-SEM tomography

Milk is a suspension with a multimodal size distribution of fat droplets and protein micelles, which most sizing methods do not distinguish. We demonstrate the use of differential dynamic microscopy (DDM) and cryo-FIB-SEM tomography to size both fat globules and casein micelles in homogenised milk without the need for prior physical separation. The two techniques are complimentary: cryo FIB-SEM tomography can directly identify the 2 distinct constituents and reveal their overlapping size distributions. DDM reliably detects a bi-modal size distribution for whole milk samples, providing a fast high-throughput method to estimate volume-averaged mean sizes. Our results highlight that different sizing techniques seldom, if ever, yield the same answer. Instead, they can provide complementary information and further insights not obtainable from using each technique in isolation.