Particle Structure in Spray-Dried Whole Milk and in Instant Skim Milk Powder as Related to Lactose Crystallization

The structure of instant skim milk and whole milk powders was studied by scanning electron microscopy with special respect to lactose crystallization during storage. X-ray diffraction analysis was used to confirm the crystallization. Some samples were ashed in a plasma asher and the residues were also examined by scanning electron microscopy. The structures of instant skim milk and whole milk powder particles were well-preserved after the ashing procedure. The crystallization of ~X-Iactose hydrate occurred in instant skim milk and whole milk powders according to moisture uptake. In instant skim milk powder, ~X-Iactose hydrate crystals were observed on the particle surface. In whole milk powder, numerous droplets of free fat appeared on the surface of particles stored under conditions favoring the crystallization of ~XIactose hydrate except few lactose crystals. Only ,6-lactose was in the state of crystal in whole milk powder stored at 3rC for 5 months at a relative humidity of less than 20%. In fresh powders of skim milk and whole milk , prismatic crystals of lactose formed in Heinz fluid , whereas in stored powders, they did not