AS Biology - Testing for Saccharides

Testing for Saccharides
All must be able to describe and carry out tests
for reducing and non reducing sugars – D/C grade
Most should be able to explain how a positive or
false result is produced – B grade
Some could suggest why certain foods contain
reducing sugars, non – reducing sugars, or both?
And how to compare the concentration of the
reducing sugar – A/A* grade
• On your sheet write down what you can
remember about food tests and try to give
examples (the bit labelled at the start of then
• This is what we will measure success on by the
end of the lesson
What is a reducing sugar?
Any sugar that forms an aldehyde or ketone in the presence of an alkaline solution is a
reducing sugar. Types of reducing sugars include glucose, fructose, glyceraldehyde,
lactose, arabinose and maltose.
Sucroses and trehaloses are not reducing sugars. Ultimately, a reducing sugar is a type
of sugar that reduces certain chemicals through an oxidation reaction.
Benedict's Test
To test for the presence of reducing sugars, a food sample is dissolved in boiling water.
Next, a small amount of Benedict's reagent is added and the solution begins to cool.
During the next four to 10 minutes, the solution should begin to change colours. If the
colour changes to blue, then no glucose is present. If a high amount of glucose is
present, then the colour change will progress to green, yellow, orange, red and then a
dark red or brown.
How Benedit's test works
Benedict's reagent is made from anhydrous sodium carbonate, sodium citrate and
copper(II) sulphate pentahydrate. Once added to the test solution, reducing sugars
reduce the blue copper sulphate from the Benedict's solution to a red brown copper
sulphide, which is seen as the precipitate and is responsible for the colour change.
Non-reducing sugars cannot do this. This particular test only provides a qualitative
understanding of the presence of reducing sugars.
Chemical test for saccharides(sugars)
• Reducing Sugars
1) If the food to be tested is liquid, go to 2.
If the food to be tested is solid, make an extract. Grind
crush or chop a small amount and put into a test tube
to a depth of about 2cm. Add a similar amount of
distilled water and stir with a glass rod. Allow to
stand for a few minutes.
2) Heat the sugar solution with an equal volume of blue
benedict's solution for 2-3 minutes at about 90°C
3) A positive result is a brick red precipitate
4) Benedicts solution contains blue Cu2+ ions, the sugar
reduces this to the insoluble brick red Cu+ compound
• Cu2+ Electron Cu+
From sugar
Non reducing sugar test
• Some sugars are non reducing.
• They do not reduce benedict's solution
• One example is sucrose, it must be
hydrolysed(broken-down by adding water) to
form glucose and fructose
• This can be done by heating with a few drops
of acid at 90°C for a few minutes. Then
neutralising the solution with an equal
amount of sodium hydroxide solution
• You will then get a positive result when
repeating the benedict's test
Describe/Explain and give examples of food tests – start of the lesson
Type of
Result of benedicts
test for reducing
Result of nonreducing sugar
Reducing or nonreducing sugar?
Describe/Explain and give examples of food tests – 3/4 through the lesson
1) Some species of Acacia tree produce gum Arabic. Gum Arabic is classed as a
heteropolysaccharide. This means that it is made up of a number of different sugars.
(a) Describe what happens during the hydrolysis of a polysaccharide molecule. [2]
(b) Describe how you could compare the reducing sugar content of the leaves with that of
the seed pods.
In your answer you should make clear how the steps in the process are sequenced.
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