Food Tests AS level Biology

Biuret test:

1. place sample in a test tube and place an equal amount of sodium hydroxide keeping the solution at room temperature 

2. add a couple of drop of dilute copper sulfate soltution and mix carefully 

3. if the mixture goes FROM blue TO purple then it is a protien as the colour change indicates peptide bonds.

(can refer to just adding biuret reagent which is the cupper sulfate solution)

Emulsion test:

1. Add your sample to a grease and streak free test tube (this is very important or you could get a false positive)

2. Add 5ml of alochol to a liquid sample (cruch if solid and filter) and mix throughly to dissolve the lipid

3. Add 5ml of water to the mixture and mix 

4. If a white emulsion is formed then a lipid is present 

all monosaccharides and some disaccharides are reducing sugars. This means that they can donate electrons to another chemical eg. Benedicts reagent

Benedicts test:

1. add 2ml of liquid sample (if solid grind in water) to a test tube

2. add an equal volume of benedicts solution and heat gently in a boiling water bath for 5 minutes

3. If the colour changes FROM blue then it has reducing sugars in it

If the coulour cahnge to green then there are very few reducing sugars, the more red it goes the more reducing sugars present.

If the misture goes brick red then there is high amounts of reducing sugars 

If you get a negative result for the reducong sugars test then try this test for non reducing sugars

1. Add 2ml of liquid sample to a test tube and add 2ml of dilute HCl and place in a gently boiling water bath for 5 minutes, the HCl should hydrolyse the glycosidic bonds in the disaccahride  

2. slowley add sodium hydrogencarbonate to the test tube neutralising the acid

3. re-test with the benedicts reagent, it should now give a positive result

Iodine test:
1. add your solid food sample into a test tube or add liquid sample to a spotting tile

2. add 2 drops of iodine ans shake/stir

3. if the sample turns a blue/black colour then starch is present