food tests

preparing food sample

Get piece of food and crush it up using the mortar and pestle. Get crushed food and add it to a beaker with distilled water. Stir with glass rod until some of it dissolves. Filter out the solution with a funnel that’s been lined with filter paper to get rid of all the solid bits of food that we don’t need. Solution we end up with will be our sample

test for (reducing) sugars

benedict’s test: transfer 5cm³ of sample to a test tube. prepare water bath at 75°C. add 10 drops of benedict’s solution to the test tube then place it in the water bath for 5 minutes. If the sample changes from blue to brick red, reducing sugars are present.

For safety reasons, use a test tube holder in the water bath and point the test tube away from you and anybody else.

benedict’s test colour change

blue to (green, yellow, or) brick red.

[green - low concentration, yellow - medium concentration, red - high concentration]

test for starch

iodine test: add 5cm³ of the food sample to a test tube then add a few drops of iodine solution. This will turn the solution a brown-orange colour. Give the test tube a gentle shake. If the sample did contain starch the solution will change from the browny-orange colour to a blue-black.

iodine test colour change

orange-brown to blue-black

test for proteins

biuret test: add 2cm³ of your sample to a test tube then add another 2cm³ of the biuret solution to the sample, turning it blue. Give it a gentle shake and if the colour changes from blue to pink or purple, protein is present.

biuret test colour change

blue to pink/purple

test for lipids

ethanol test: take your food sample and add 2cm³ of ethanol and shake it vigorously. Then add 2cm³ of distilled water and if a cloudy emulsion forms, lipids are present.

ethanol test colour change

clear to cloudy emulsion