Testing for Saccharides

1. Testing for Saccharides LO: 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

2. Starter • 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 lesson) • This is what we will measure success on by the end of the lesson

3. 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. • OILRIG

4. Chemical test for saccharides(sugars) • Reducing Sugars • 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 bluebenedict'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+ Cu+ Electron From sugar

5. Non reducing sugar test • Some sugars are non reducing. • They do not reduce benedict'ssolution • 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

6. Describe/Explain and give examples of food tests – start of the lesson Describe/Explain and give examples of food tests – 3/4 through the lesson

7. 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. 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. [6]