A "Sugar-Free" Squash, Benedict's Test and a Diabetic Classmate
A Class 11 student is given four tubes for a Benedict's test practical. Tube A contains 1% glucose, tube B 1% sucrose, tube C 1% soluble starch and tube D a commercial "sugar-free" orange squash. The label on D lists water, sorbitol, maltodextrin, citric acid and "fruit juice concentrate (6%)". After heating in a boiling water bath for three minutes, A gives a brick-red precipitate, B and C stay blue, and D gives a yellow-green colour. The student then boils 5 mL of each tube with 1 mL dilute hydrochloric acid for five minutes, cools, neutralises with sodium carbonate and repeats the test. Now B, C and D all give orange-red precipitates, while a fresh untreated sample of D again gives only yellow-green. The student concludes that the squash "contains no sugar" and is therefore safe for a diabetic classmate to drink freely. The teacher asks the group to justify or refute this conclusion using their own observations.
1. Account for the results in tubes B and C before and after acid hydrolysis, and explain why tube A behaved differently from the start.
Read the worked answer
Before hydrolysis
- A (glucose): a free aldehyde group on carbon 1 is available, so glucose is a reducing sugar. It reduces Cu(II) in Benedict's reagent to Cu(I) oxide, giving the brick-red precipitate.
- B (sucrose): the glycosidic bond joins the anomeric carbons of glucose and fructose, so neither carbonyl is free. Sucrose is non-reducing and gives no precipitate.
- C (starch): the chain has only one free reducing end per very large molecule, so the number of reducing groups per unit mass is negligible. The blue colour is unchanged.
After hydrolysis
- Dilute acid cleaves the glycosidic bonds. Sucrose yields glucose and fructose; starch yields many glucose units. The concentration of free reducing ends rises sharply, so both tubes now reduce Benedict's reagent.
Conclusion: the test detects free carbonyl groups, not "sugar" in general. A negative result before hydrolysis does not mean the carbohydrate is absent; it means it is present in a glycosidic, non-reducing form.
2. Tube D gives a weak positive before hydrolysis but a strong positive after. What does this tell you about its carbohydrate content, and how does it affect the label claim?
Read the worked answer
Before hydrolysis: the yellow-green colour is a weak positive. It indicates a small amount of free reducing sugar, consistent with the 6% fruit juice concentrate, which supplies glucose and fructose. Sorbitol is a sugar alcohol with no free aldehyde under these conditions, so it contributes little or nothing to the colour. The maltodextrin content also contributes very little before hydrolysis because it has few free reducing ends.
After hydrolysis: the strong orange-red colour shows that acid digestion released a large amount of reducing sugar. Maltodextrin is a glucose polymer that is rapidly hydrolysed, so it is the main hidden source.
Consequences for the claim:
- "Sugar-free" on a label normally refers to sucrose (and sometimes to added sugars), not to all digestible carbohydrate.
- Maltodextrin is a digestible glucose polymer with a high glycaemic effect; sorbitol is slowly absorbed but still supplies energy (about 2.6 kcal per gram) and can cause osmotic diarrhoea in quantity.
- The hydrolysis data directly contradict the inference that the drink is carbohydrate-free. The label claim and the diabetic-safety claim are two different statements.
3. Would you advise the diabetic classmate to drink D freely? Give a reasoned recommendation based on the data and the label.
Read the worked answer
Recommendation: no, not freely.
The reasoning must rest on the observations, not on the phrase "sugar-free":
- The post-hydrolysis result is the decisive evidence. A large reducing-sugar load appears only after acid treatment. That is exactly what a rapidly digested glucose polymer such as maltodextrin would do, and rapidly released glucose raises blood glucose.
- The pre-hydrolysis result shows there is also some free reducing sugar from the fruit concentrate, so even the untreated drink is not inert.
- Sorbitol is not "free": it is absorbed slowly and metabolised, supplies calories, and in larger amounts draws water into the gut.
- Practical advice: water or an unsweetened drink is the safer default. If the drink is used at all, it should be a measured portion counted as carbohydrate, and the classmate's blood glucose response should be monitored rather than assumed.
The key judgement is that a label claim about sucrose cannot be used as evidence about total carbohydrate or glycaemic effect.