Benedict's test is an option for determining if reducing sugars are present in milk. The test is performed using Benedict's reagent, which is a solution of sodium citrate and sodium carbonate mixed with a solution of copper sulfate.
Investigators start the test by mixing 2 milliliters of Benedict's reagent with 1 milliliter of milk in a test tube. The tube containing the combined solution is then heated in a boiling water bath for three minutes. If reducing sugars are present, the copper iron oxidizes them. This forms a carboxylic acid and a reddish precipitate of copper oxide that turns the solution orange, red or brown.
Milk produced from mammals tests positive because it contains the reducing sugar lactose whether it's skim milk, whole milk or cream. Typically, soy milk tests negative because the amount of reducing sugar is so small that it is undetectable.
Stanley Rossiter Benedict developed the procedure for using an alkali to fragment sugars when he was Professor of Chemistry at Cornell University Medical College in the early 1900s, and it was the method of choice to test for diabetes for over 50 years. However, his test only indicates the presence of reducing sugars and does not identify the type of reducing sugars that are present. It was eventually replaced by enzymatic methods, including glucose oxides, that are capable of identifying specific reducing sugars.