when mixtures of whey protein and lactose are spray-dried. This is consistent with earlier investigations made on other proteins, such as sodium caseinate and bovine albumin, which have also been spray-dried together with lactose in solution (28). These proteins are also present in relatively high concentrations at powder surfaces when present in low concentration in solution
It has been suggested that the composition of the air-water interface of the droplets formed during the spray-drying process are reflected in the powder surface composition after spray-drying. During the drying process there will be an accumulation of surface-active protein at the air-water interface of droplets, which in turn will appear on the dry powder surface (28)
Figure 1 also shows a decrease in surface tension at the air-water interface as whey protein is added to a lactose solution. This means that the surface-active whey protein accumulates at the air-water interface and lowers the surface tension.
This is a different description of the course during the spray-drying process than the conventional, where low molecular weight solutes are described as migrating toward the interior of the droplets because of concentration gradients occurring during drying (30,31). However, these investigations were performed on solutions without any surface-active material (such as salt solutions), on relatively large droplets giving long drying times without the possibility to estimate the surface composition of the dry powders. Furthermore, the importance of surface activity on the formation of powder surfaces is emphasized by the fact that the domination of surface-active protein and absence of carbohydrate on spray-dried powder surfaces also occurs when lactose is changed for monosaccharides as well as highmolecular weight polysaccharides (P.Faldt and B. Bergenstahl, unpublished results).