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Food Test for Non-Reducing Sugars

Disaccharides are compound sugars formed when two monosaccharide molecules combine.

Food Test for Non-Reducing Sugars

Disaccharides are compound sugars formed when two monosaccharide molecules combine. Disaccharides are found in sugar cane (sucrose), malt (maltose), and milk (lactose). Some disaccharides are reducing sugars (lactose and mal-tose), while others are non-reducing sugars (sucrose).

Learning Objectives

To carry out food test for non-reducing sugar in a given food sample.

 

Materials

Benedict's solution*, cooking pot, kerosene stove or charcoal burner, plas-tic spoon, droppers*, empty plastic bottles, test tube*, test tube holders*, citric acid solution*, sodium hydroxide solution*, food sample containing non-reducing sugar like table sugar or fresh sugar cane.

Preparation Procedure

1. Make a solution of a food sample containing a non-reducing sugar.

Activity Procedure

1.           Put 2 mL of the sample solution in a test tube.

2.           Add 2 drops of citric acid solution to the food sample.

3.           Heat the mixture to boiling in a hot water bath.

4.           Remove the solution as soon as it boils and let the solution cool.

5.           Add 2 drops of sodium hydroxide solution to the food sample.

6.           Add 2 drops of Benedict's solution to the food sample.

7.            Heat the mixture in the water bath again and record your observations.

Clean Up Procedure

1.           Unused reagents should be stored in plastic bottles for further use. Do not store sodium hydroxide in glass bottles.

2.           Dispose of chemical waste in a pit latrine.

Hazards and Safety

Sodium hydroxide is corrosive - concentrated solutions can burn skin and wood and even dilute solutions can blind if they get into eyes.

Citric acid is irritating - keep out of eyes.

If sodium hydroxide solution spills, neutralize spills with citric acid solution or vinegar.

Close the container of sodium hydroxide solution after use to prevent reaction with atmospheric carbon dioxide.

Results and Conclusions

The colour of the food sample will change from green to yellow and nally to a brick red precipitate. This indicates the presence of a non-reducing sugar.

Notes

This experiment will also test positive for all reducing sugars. Therefore it is important to rst perform the test for reducing sugars before considering this test. If the test for reducing sugars is positive, there is no reason to perform the test for non-reducing sugars - the conclusion will be invalid.

 

Non-reducing sugars are a misnomer, that is, their name is incorrect. This test does not test for any sugar that is not reducing. Rather, this is a test for any molecule made of multiple reducing sugars bound together, such as sucrose or starch. When these polysaccharides are heated in the presence of acid, they hydrolyse and release monosaccharides. The presence of these monosaccharides is then identi ed with Benedict's solution.

 

The purpose of the sodium hydroxide is to neutralize the citric acid added for hydrolysis. If the citric acid is not hydrolysed, it will react with the sodium carbonate in Benedict's solution, possibly making the solution ine ective.

 

For information about Benedict's solution and reducing sugars, see the explanation in the previous experiment: Food Tests - Reducing Sugars.

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