Practical: food testsSpec 2.9
You can use chemical reagents to identify the main biological molecules in a food. If the food is a solid, crush it with a little water and use the liquid. For the fat test, shake the food with ethanol instead. Always wear eye protection.
Practical:
A negative result means the reagent keeps its original colour. Learn the start colour as well as the positive result.
| Food | Reagent | Method | Positive result |
|---|---|---|---|
| Glucose (a reducing sugar) | Benedict's solution | Add Benedict's solution and heat in a hot water bath for a few minutes | Blue to green, yellow, orange or brick-red, depending on the amount of sugar |
| Starch | Iodine solution | Add a few drops of iodine solution to the sample | Orange-brown to blue-black |
| Protein | Biuret reagent | Add biuret reagent to the sample and mix | Blue to purple (lilac) |
| Fat | Ethanol | Shake the sample with ethanol, then pour the mixture into water | A cloudy white emulsion forms |
In a negative result, iodine stays orange-brown, Benedict's solution and biuret reagent stay blue, and the ethanol test stays clear. Ethanol is flammable, so keep it away from naked flames. Benedict's solution is heated in a water bath, not directly over a flame.