Test for Starch 🍚
Dilute iodine solution reacts with starch, forming a very dark blue colour. This test will work on a solid sample of food, such as potato, or a suspension of starch on water.
Test for Glucose 🍬
Glucose is called a reducing sugar. This is because the test for glucose involves chemically reducing an alkaline solution of copper sulphate to copper(I) oxide.
In I/GCSE Biology, the test for glucose is called the Benedict’s test. Simply add a few drops of Benedict’s solution, which contains alkaline copper sulphate, to some glucose. Add enough to turn the mixture blue. Then heat the mixture in a boiling water bath. The clear blue solution will gradually change colour, forming a cloudy orange or brick red precipitate of copper(I) oxide. All ‘single’ sugars such as fructose are reducing sugars, as well as some ‘double’ sugars, such as the milk sugar.
Test for Proteins
In I/GCSE Biology, the test for protein is called the Biuret test. It is done by firstly mixing some protein such as powered egg white to some water. Then, dilute potassium hydroxide solution is added to the mixture. Finally, a small amount of copper sulphate solution is added to the mixture. A mauve colour develops.
Test for Lipids
Fats and oils are insoluble in water, but will dissolve in ethanol. In I/GCSE Biology, the test is done by dissolving a lipid in ethanol. The mixture is then poured into cold water. A white cloudy layer forms on top of the water. This is caused by the ethanol dissolving in the water and leaving the lipid behind as a suspension of tiny droplets, called an emulsion.
Measuring Energy Content in Food – Such as a Peanut! 🥜
If a sample of food will burn well in air, you can measure its energy content using a simplified version of the food calorimeter.
- Find the mass of the peanut, and place 20ml of water into a boiling tube.
- Measure temperature of water.
- Spear peanut with a mounted needle and light it up with a Bunsen burner until it catches flame.
- Hold the lit peanut underneath boiling tube. If peanut stops burning, relight it and continue boiling the water with the lit peanut – not penis, until it can no longer burn.
- Measure the final temperature of the water and calculate the rise in temperature.
-4.2 joules raises the temperature of one gram of water by 10C
-1ml of water has a mass of 1g
To find the energy (joules per gram) = (rise in temperature x initial volume of water x 4.2)/mass of food (g)
That's the end of the topic!
Drafted by Joey (Biology)