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Surface Features of Foam-Dried Milk Powder Granules from Krypton Adsorption Measurements

Dairy Products Laboratory, Eastern Utilization Research and Development Division, USDA, Washington, D. C.

ABSTRACT

Krypton adsorption data were used to calculate the surface areas of various dried milk powders utilizing the standard Brunauer, Emmett, and Teller (BET) treatment. Measured areas were all very low (1 mg²/g); however, small changes in powder structure and porosity as affected by processing techniques were observed. Materials studied included conventional spray-dried powders, spray-dried milk foams, vacuum-dried foam, and instantized powders. Evidence for marked differences in structure of spray-dried foams as affected by the nature and volume of gas injected during the foaming process was obtained by comparing surface area values for these materials. The magnitude of the surface area was directly proportional to the volume of N₂ injected into the liquid milk concentrate prior to the spray drying. When soluble gases, CO₂ or N₂O, were used, a nonlinear relationship between surface area and injected gas volume was observed. Differences in surface areas are attributed to porosity effects, as corroborated with evidence from density measurements and microscopic observations.