The 'No Added Sugar' Cranberry Drink
A student is asked to check the label claim on a bottle of 'no added sugar' cranberry drink. She sets up three test tubes: A contains 2 cm³ of fresh orange juice, B contains 2 cm³ of the cranberry drink, and C contains 2 cm³ of 1% glucose solution. She adds 2 cm³ of Benedict's solution to each tube and stands all three in a boiling water bath for five minutes. Tube A turns brick-red, tube B stays blue, and tube C turns brick-red. She also tests each sample with iodine solution and sees no blue-black colour anywhere. She concludes that the cranberry drink contains no reducing sugar and that the label is therefore accurate. Her teacher points out that the drink has a pH of about 2.8, contains a strong red colouring, and lists 'fruit juice concentrates' first and 'glucose-fructose syrup' fourth on the ingredients list.
1. Evaluate the student's conclusion that the cranberry drink contains no reducing sugar. Give two reasons why the blue colour may be misleading, and describe the further test needed to check the sweetener named on the label.
Read the worked answer
Benedict's solution gives a brick-red precipitate only with reducing sugars (glucose, fructose, maltose) and it needs alkaline conditions plus heat. The blue result in tube B does not prove absence:
- The drink is acidic (pH 2.8). The acid neutralises the sodium carbonate in Benedict's, so the reagent cannot oxidise the sugar and stays blue. This is a false negative, not evidence of no sugar.
- The strong red colouring could mask a small brick-red precipitate. The tube should be left to settle and viewed against a white tile.
- The ingredients list glucose-fructose syrup, which is a mixture of reducing sugars, so a negative result contradicts the label.
To check properly, neutralise a measured volume of the drink with dilute sodium hydroxide until about pH 7, then boil with Benedict's. A brick-red precipitate confirms reducing sugar. Even then the test shows presence, not quantity, so it cannot by itself prove the label claim is false - a quantitative method would be needed.
2. The iodine test was also negative for all three samples. Explain what this result does and does not show about the drink.
Read the worked answer
Iodine solution turns blue-black only in the presence of starch, forming a starch-iodine complex. A negative result therefore shows only that there is no detectable starch in the drink.
It says nothing about sugars. The sweetener listed is glucose-fructose syrup, which contains monosaccharides - these give no colour with iodine. Fruit juice concentrates also contain mainly glucose, fructose and sucrose rather than starch. So the iodine result is irrelevant to the claim being tested and cannot be used as supporting evidence.
A good answer notes the logic error: the student has tested for the wrong molecule and then treated a negative result as confirmation. Only a sugar-specific test (Benedict's, on a neutralised sample, or a quantitative glucose test strip) can address the label claim.