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MCAT Sample Questions : Biological Sciences
Milk, one of the most nutritionally complete food sources in nature, contains water, vitamins, minerals, proteins, carbohydrates, and lipids.
An important milk protein is casein, which typically exists in milk as its calcium salt, calcium caseinate. Calcium caseinate is soluble at the normal pH of milk (6.6), but it becomes an insoluble form of casein in solutions that have a pH of 4.6 (the isoelectric point of calcium caseinate).
Milk contains two other important types of proteins, the lactalbumins and the lactoglobulins, both of which can be denatured and precipitated by heat.
The main carbohydrate in milk is the disaccharide lactose (shown below), which can be hydrolyzed with acid to two monosaccharides, galactose and glucose. In the body, lactose is digested to galactose and glucose by the enzyme lactase.
The first step in isolating several of the components of milk is to add a dilute solution of acetic acid to nonfat milk. The solid material that precipitates (Solid 1) is removed. Care must be taken to avoid adding an excess of acetic acid.
The remaining acidic solution is treated with powdered calcium carbonate and heated to boiling, after which time another solid (Solid 2) forms. Solid 2 is removed by filtration, and hot ethanol is added to the aqueous filtrate. The solution is allowed to cool overnight, during which time lactose crystallizes in the flask.
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