Question

You are given a drink that you are told is sweetened with either maltose or lactose. Using the following resources explain how you would test if the drink was sweetened with either lactose or maltose? Explain the carbohydrate chemistry behind the test to justify your answer. The only (!) resources available to you include: - the drink - the enzyme lactase - the Somogyi-Nelson reagents for the determination of reducing sugars Questions: i) concisely describe the tests that you would use: ii) describe the observations that would be made if the drink contained maltose: iii) describe the observations that would be made if the drink contained lactose:

Answer 1

Answer:

Based on the given information:

• Drink contains either Maltose or Lactose.
• Enzyme available: Lactase
• Somogyi-Nelson reagent for the determination of reducing sugars.

Test that can be used: Determination of reducing sugars by Nelso-Somogyi method.

Observations if drink contained Maltose:

• Since Lactase enzyme will not act on Maltose, therefore Maltose sugar will remain intact. BaSO₄ to remove non-glucose reducing substance.

Observations if drink contained Lactose:

• Lactose will be broken down to glucose and galactose in the presence of Lactase enzyme. Glucose will give a positive Nelson-Somogyi test result. The solution will appear blue and the absorbance can be determined at 620nm.

Explain the carbohydrate chemistry behind the test to justify your answer:

• Lactose and Maltose are Reducing disaccharides sugars. Maltose is a disaccharide formed from two units of glucose joined with an alpha(1-->4) bond, whereas, Lactose  is a disaccharide composed of galactose and glucose. Lactase enzyme catalyzes the breakdown of lactose into the simple sugars glucose and galactose.
• In this test, the reducing sugars (example glucose, lactose) when heated with alkaline copper tartrate solution, reduces the copper from the cupric to cuprous state leading to the formation of cuprous oxide. Cuprous oxide then reacts with arsenomolybdic acid causing the reduction of molybdic acid to molybdenum blue. The blue colour developed is compared with a set of standards in a colorimeter at 620 nm. Therefore, reducing sugars changes the color of the reagent to blue color that can be measured using a spectrophotometer.