Nonfat dry milk is the product resulting from the removal of fat and water from milk and contains the lactose, milk proteins, and milk minerals in the same relative proportions as in the fresh milk from which it was made. It contains not over 5% by weight of moisture. The fat content is not over 1.5% by weight unless otherwise indicated. Nonfat dry milk is utilized in dairy products, bakeries, dry mixes, chemicals, meat processing, and in homes for cooking.
Nonfat dry milk is the manufactured by spray drying of condensed skim milk. Spray drying involves atomizing concentrated milk into a hot air stream between 356 to 392 degrees Fahrenheit. The atomizer may be either a pressure nozzle or a centrifugal disc. By controlling the size of the droplets, the air temperature, and the airflow, it is possible to evaporate almost all the moisture while exposing the solids to relatively low temperatures. Spray drying yields the concentrated and dry milk ingredients with excellent solubility, flavor and color.
The spray drying process is typically a two-stage process that involves the spray dryer at the first stage with a static fluid integrated in the base of the drying chamber. The second stage is an external vibrating fluid bed. The product is moved through the two-stage process quickly to prevent overheating of the powder. The powder leaves the dryer and enters a system of cyclones that simultaneously cools it.
Roller drying is another process which is currently not widely used in the manufacture of most dry milk products. This process involves direct contact of a layer of concentrated milk with the hot surface of rotating rollers. This process causes adverse effects of excessive heat on milk components. In this process heat often causes irreversible changes such as lactose caramelization, Maillard reaction, and protein denaturation. Roller drying typically results in more scorched powder particles and poor powder solubility than spray drying. However, roller-dried milk absorbs more moisture than spray-dried powder and is preferred in some food applications such as bakery products.
All this information can be found in the Wiley-Blackwell Dairy Processing & Quality Assurance book.
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