1. Reducing and non-reducing sugars test
Lab activity in preparation for practical exam
AS level biology
Reducing and non-reducing sugars test

2. A Food Test  is a chemical test which determines the presence of nutrients/biological molecules commonly found in food.
These biological molecules include sugars - reducing and non-reducing,  starch, protein and lipids. 
Food tests use reagents which react predictably with the biological molecules  to give observable color changes.
Food Tests

3. Food Tests
Name at least 3 foods for which you would predict a positive result
Name the reagents used
Outline the steps in sequential order
Explain the principle or basis of the test
Interpret the results obtained
For each of the food tests you should be able to:

4. What is Benedict's Test for Reducing Sugars?
Benedict's Test for non-reducing Sugars  is a test which determines the presence of non-reducing sugars in a test solution.
The principal reagent in Benedict's Test for Reducing Sugars is Benedict's Solution which contains
copper(II) sulphate
sodium carbonate
sodium citrate

5. What are reducing sugars?
Sugars are classified as reducing or non-reducing based on their ability to act as a reducing agent during the Benedict's Test.
A reducing agent donates electrons during a redox reaction and is itself oxidized.
The aldehyde functional group is the reducing agent in reducing sugars.
Reducing sugars have either an aldehyde functional group or have a ketone group - in an open chain form - which can be converted into an aldehyde.
What are reducing sugars?

7. Reducing sugars are simple sugars and include all monosaccharides and most disaccarides.
Some examples of monosaccharides are glucose, fructose and galactose.
Examples of reducing disaccharides are lactose and maltose.
Note that the disaccharide sucrose is not a reducing sugar. In fact, sucrose is the most common non-reducing sugar.
Reducing sugars

8. What is the procedure for the Benedict's Test for reducing sugars?

9. What is the procedure for the Benedict's Test for reducing sugars?
Add 2 cm3 of the sample solution to a test tube. 
Add an equal volume of Benedict's solution to the test tube and swirl or vortex the mixture.
Leave the test tube in a boiling water bath for about 5 minutes, or until the colour of the mixture does not change.
Observe the colour changes during that time as well as the final colour.
To prepare a control, repeat the steps above using 2 cm3 of distilled water instead of sample solution

10. What are the expected observations for the Benedict's Test for reducing sugars?
Observations No Colour Change (Blue) Green Yellow Orange Brick-red
Interpretations No reducing sugars present Trace amounts of reducing sugars present Low amounts of reducing sugars present Moderate amounts of reducing sugars present Large amounts of non-reducing sugars present

11. What is the principle of the Benedict's Test for reducing sugars?
Reducing Sugars  have an aldehyde functional group which can reduce soluble copper (II) ions  - in copper (II) sulphate -  to insoluble copper (I) ions - in copper (i)oxide.
The copper (I) oxide is seen as a precipitate.
What is the principle of the Benedict's Test for reducing sugars?

12. State the role of copper sulphate in Benedict's Solution.
Reduced Species. The blue copper(II) ions from copper(II) sulphate are reduced to red copper(I) ions by the aldehyde groups in the reducing sugars.
This accounts for the colour changes observed. The red copper(I) oxide formed is insoluble in water and is precipitated out of solution. This accounts for the precipitate formed.
State the role of copper sulphate in Benedict's Solution.

13. Benedict Test Reducing Sugars

14. What is Benedict's Test for non-reducing sugars?
Benedict's Test for non-reducing Sugars  is a test which determines the presence of non-reducing sugars in a test solution.
Non-reducing sugars are sugars which do not have an aldehyde functional group  - the reducing species. As non-reducing sugars do not have the aldehyde group, they cannot reduce copper (I) (blue) to the copper(II) (red).
What is Benedict's Test for non-reducing sugars?

15. Sucrose is the most common disaccharide non-reducing sugar.
Sucrose is a non-reducing sugar

16. Careful
Many students confuse the Benedict's Test for Non-Reducing Sugars with the Benedict's Test for Reducing Sugars, perhaps because some of the steps in the procedure are similar.

17. What is the procedure for the Benedict's Test for Reducing Sugars?
Add 2 cm3 of the sample solution to a test tube.
Add 1 cm3 of dilute hydrochloric acid and boil for one minute.
Allow the tube to cool and then neutralize the acid with sodium hydrogen carbonate. Exercise caution due to 'fizzing' or effervescence.
Check with blue litmus paper or pH paper. Do not worry if the resulting solution is slightly alkaline; Benedict's solution is also alkaline since an alkaline medium in needed for oxidation of the copper (II) ions by the sugars
What is the procedure for the Benedict's Test for Reducing Sugars?

18. What is the procedure for the Benedict's Test for Reducing Sugars?
Leave the test tube in a boiling water bath for about 5 minutes, or until the colour of the mixture does not change.
Observe the colour changes during that time as well as the final colour.
To prepare a control, repeat the steps using 2 cm3 of distilled water instead of sample solution.
What is the procedure for the Benedict's Test for Reducing Sugars?

19. Observations No Colour Change (Blue) Green Yellow Orange Brick-red
Interpretations No reducing sugars present Trace amounts of reducing sugars present Low amounts of reducing sugars present Moderate amounts of reducing sugars present Large amounts of non-reducing sugars present

20. The test for non-reducing sugars is often conducted on a food sample which tested negative for reducing sugar.
If reducing sugars have been shown to be present, a heavier precipitate is often observed when the test for non-reducing sugar is conducted.
What results are expected if a food sample has already been tested for reducing sugars?

21. What is the principle of the Benedict's Test for non-reducing Sugars?
Disaccharides are hydrolyzed to their constituent monosaccharides when boiled in dilute hydrochloric acid.  The monosaccharide products of hydrolysis are reducing sugars i.e. have the aldehyde functional group and can reduce copper in the presence of alkali  producing the colour changes.

22. Why was sodium hydrogen carbonate added to the mixture?
Neutralization. Sodium hydrogen carbonate is necessary for neutrallization as the reduction of the copper(II) ions will not take place in acidic conditions - of excess acid is present.

23. Benedict Test Non-Reducing Sugars

24. What is the Iodine Test for Starch?
The Iodine Test for Starch is used to determine the presence of starch in biological materials. The test can be qualitative or quantitative. As a Biology Student, you will be testing for the presence of this complex carbohydrate in foods or in leaves as part of a photosynthesis experiment.
The sole reagent required for the test is bench iodine solution  (0.1 M potassium triiodide solution).
What is the Iodine Test for Starch?

25. What is the procedure for the iodine test for starch?
The procedure  for the iodine test for starch depends on whether the test sample is a solid or liquid.

26. What is the procedure for the iodine test for starch?
SOLID SAMPLE
Peel off the skin of any vegetables e.g. potato and fruits as these are often impermeable. Use a clean spatula to remove samples of powdered food. Avoid cross contamination with other foods.
Add a few ( 2-3) drops of bench iodine solution potassium to a piece of solid food on a white tile.
Make observations.

27. What is the procedure for the iodine test for starch?
LIQUID SAMPLE
Add 10 cm3 of the liquid food sample to a clean, dry test tube.
Add about 5 drops of iodine solution to the test tube.
Note any colour changes.
To prepare a control, perform steps 1 -3 for de-ionized water.

28. What are the expected observations and interpretation of the iodine test for starch?
No change (Iodine remains brown) A blue-black colour develops
Interpretation
Starch is not present Starch

29. Describe the structure of starch and state which structural feature is key to the colour change in the iodine test for starch.
Starch is a polysaccharide consisting of glucose units joined together by glycosidic bonds. The chains formed during the condensation reaction are either linear or highly branched molecule

30. Where branched molecules of starch are called Amylopectin.
Linear - both straight and helical - molecules of starch are referred to as Amylose.
Where branched molecules of starch are called Amylopectin.

31. Natural starches - from plants - consist of a mixture of amylose ( %) and amylopectin (75-90%).The structure of the helical amylose is key to the Iodine-starch reaction. A helix is a coil or a spring.
The tri-iodide and penta-iodide ions formed are linear and slip inside the helix of the amylose (form of starch).
Potassium Iodide

32. Describe the composition of the iodine/potassium tri-iodide reagent in the iodine test for starch.
Iodine on its own (small non-polar molecule) is insoluble in water. Therefore Potassium triiodide solution - Iodine dissolved in potassium iodide solution - is used as a reagent in the test. To be more specific, potassium iodide dissociates, and then the Iodide ion reacts reversibly with the Iodine to yield the triiodide ion. A further reaction between a triiodide ion and an iodine molecule yields the pentaiodide ion. Since molecular iodine is always present in solution, the bench iodine solution appears brown; the iodide and triiodide pentaiodide ions are colourless.

33. Explain the principle or the basis of the colour change in the  Iodine Test for Starch.
The starch-iodide complex is formed as charge - recall electrons are charged particles - is transferred between the starch and iodide ions  - tri-iodide or pentaiodide. The transfer of charge between the starch and the iodide ion changes the spacing between the energy levels/ orbitals. This change results in the starch-iodide complex absorbing light at a different wavelength - than any other species aforementioned - resulting in an intense purple colour; Biologists call this colour blue- black.

34. Explain the principle or the basis of the colour change in the  Iodine Test for Starch.

35. Potassium Iodide test

36. CREDIT m