Surface composition of industrial spray-dried milk powders. 3. Changes in the surface composition during long-term storage

Abstract

The changes in the surface composition of industrial spray-dried milk powders (skim milk powder, whole milk powder and cream powder) during long-term storage were investigated. The powders were stored in the conditions, which are more likely for commercial circumstances, for 6 months and the surface composition of the powders was analyzed before and after storage using electron spectroscopy for chemical analysis (ESCA). For the fat-containing powders (whole milk powder and cream powder), the various milk fat fractions (surface free-fat, inner free-fat and encapsulated fat) were also extracted before and after storage, and analyzed for their fatty acid and triglyceride compositions to evaluate whether there was a release of fat onto the powder surface during storage and if this causes changes in the melting characteristics of the fat present on the powder surface. The structure of the powders and the physical state of the lactose present in the powder were studied before and after storage by scanning electron microscopy (SEM) and X-ray diffractometer (XRD), respectively. The results showed that the lactose present in the powders remained in the amorphous state over the storage, and therefore no significant changes in structure and surface composition of the powders occurred. However, a release of encapsulated low-melting triglycerides towards the surface of powder during storage was observed in the fat-containing powders, thereby the melting points of the surface free-fat and the inner free-fat were lowered. This phenomenon was observed in both whole milk powder and cream powder, but more significant changes were observed in CP.