Formation and structure of insoluble particles in reconstituted model infant formula powders

Phase separation of insoluble particles, so called “white flecks”, is a physical defect in reconstituted milk obtained from spray-dried, fat-containing dairy powders. Compared with bulk powder particles, white flecks have quite different chemical and morphological characteristics. They consist mainly of proteins and fat, forming a dense network. Here, two concurrent formation mechanisms are suggested based on the structures revealed by confocal microscopy imaging. Fused spherical particles with free fat on the surface were the most abundant and likely formed during the manufacturing process. Sharp-edged particles containing shapes typical of lactose crystals formed during storage. Fat globule stability was thus determined to be critical in preventing white fleck formation. Stability was increased with (a) the presence of lecithin in the oil phase; (b) systems with high protein-to-fat ratio and (c) storage in dry conditions since humidity leads to lactose crystallisation and disruption of the fat globules in powders.