Question

Did the Benedict’s test for reducing sugars net a positive or negative result for milk? Research the sugar content of milk and the definition of a reducing sugar to explain if your result is what you would expect.

Background info: Milk started as white. We added 40 drops of Benedict’s reagent and let sit in boiling water for 10 minutes. The milk turned a burnt orange color, indicating that it had reduced sugars.

Answer 1

Benedict's test is performed to estimate the amount of reducing sugar in a given sample. It exploits the ability of reducing sugars to reduce ions to ions in alkaline condition. Benedict's reagent contains following components

1. Anhydrous sodium carbonate - provides alkaline environment to carry out reduction reaction
2. Copper (II) sulfate pentahydrate - Molecule which undergoes reduction reaction after dissociating into ions when dissolved in solution
3. Sodium citrate - It complexes with copper (II) ions and prevents reduction reaction during storage

Milk contains a reducing disaccharide called lactose which consists of galactose and glucose molecules linked via  β-1,4 glycosidic bond. Thereby, the carbonyl carbon in glucose molecule remains free for carrying out reduction reaction. Any colour change after addition of Benedict's reagent to the sample indicates the presence of reducing sugar to some extent. Since, lactose is present in the milk sample it turns orange colour after addition of Benedict's reagent indicating presence of reducing sugar at relatively higher concentration in the milk sample. Therefore, result of Benedict's test is positive for the given milk sample.