Preparation of Casein from Skim Milk

Background on Casein

Casein is a mixture of phosphoproteins found in milk to the extent of about 3%. It contains all of the common amino acids an is high in the essential ones. Casein is often used as a dietary base for use in evaluation of vitamins, since it is easy to prepare casein as a pure protein. 
Casein is usually prepared by acid precipitation. It exists in milk as the water soluble calcium salt of a phosphoprotein. Acid treatment removes the calcium cation, leaving a water insoluble phosphoprotein.

Generally acetic acid is used because it is less harsh than hydrochloric acid. Most people extracting casein in the form of a school experiment do so with the solution cold. Such practice leads to the formation of casein with poor physical properties. Softness and rather incomplete separation of the casein are problems encountered.

However, if the precipitation is performed warm, the casein separates as a single large colloid, with complete separation, leaving a yellow solution of whey behind. As a side note, whey primarily consists of milk-sugar, lactose.

Decent casein precipitation is delicate. If too much acid is added, or if the acid is added too quickly, or if the acid is too strong, part of the casein will redissolve. Casein is amphoteric, that is, it forms salts with both acids and alkalines.

Casein may be extracted from whole milk. Disadvantages are that whole milk contains a relatively large concentration of fat. This fat will be physically included in the casein curd as it separates. Thus, if at all possible, use skim milk.


Prepare a solution of 7.0cc glacial acetic acid in 50.0cc water. This is the solution to be used for precipitation. 
Place 600.0cc skim milk in a 1 liter beaker. Warm to 110° F. Dropwise and with stirring add the acetic acid solution prepared above. At a certain point a mass of casein will separate. Gather this from the beaker with a glass stirring rod.

Do not add any more acid than necessary to obtain a good curd. A few minutes must be given between acid additions to allow for stabilization.

Press the mass of casein between tissue to remove as much water as possible. Place the mass of casein in a food blender with 500cc water. Suspend the casein in the water by blending as much as possible.

Separate the casein from the water with a filter and wash with isopropanol. If isopropanol is not available use ethanol or some low molecular weight alcohol. Use of ketones is not recommended due to increased polymerization.

Let the casein powder air dry.

Yield casein: 13.8g 

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