To establish which food substances contain lipids.
1. All diary products will contain lipids.
2. All animal tissue will contain lipids.
3. All plant storage organ products, i.e. seeds, will contain lipids.
Lipids are insoluble in water but soluble in organic solvents. Ethanol is an example of an organic polar solvent which will dissolve lipids to a limited extent. However ethanol is miscible with water so when a solution of lipid and ethanol is added to water the ethanol will combine preferentially to the water, leaving the ethanol to form tiny globules in the water. These globules then form a white emulsion.
Sudan III is a dye which binds preferentially to lipids and will stain them red. When lipids are in an aqueous suspension, the dye will colour them red whilst the water remains a light pink.
The presence of lipids was tested for in various food substances using the ethanol emulsion test, the grease spot test and the Sudan III test.
1. The liquid substances were all placed in 250cm3 beakers; vegetable oil, full fat milk, egg yolk and egg white. The watsits were ground using a mortar and pestle. All of the other substances were placed into petri dishes; the butter, margarine and lard were placed in directly; the bacon and cheese were thinly sliced using a scalpel and white tile and the chocolate was grated.
2. A boiling water bath should then be set up for use in the Sudan III test; A beaker containing 250cm3 of water should be set up on a tripod and gauze over a Bunsen burner. Once lit the air hole on the Bunsen burner should be opened so that it burns with a blue flame.
The Grease Spot Test
3. Two pieces of filter paper were folded into eight segments. Each segments was numbered to represent a particular food sample.
4. A small quantity of food material was then placed onto its segmented. If the material was a solid it was rubbed on to the filter paper. If the material was a liquid one drop was added using a 3cm3 plastic pipette. The filter paper was then left to dry before the results were recorded. A permanent, greasy translucent residue left on the paper indicted that lipids were present.
Sudan III Test
5. A spatula of the prepared food samples or a few drops of the liquid substances were added to labelled clean dry test tubes. The liquid substances were added using 3cm3 plastic pipettes.
6. 5cm3 of distilled water was then added to each test tube. This was then placed into the boiling water bath to allow for the food to break down. This caused the lipids to melt and enter the solution. The lipids then floated to the top surface of the water.
7. Tongs were then used to remove the test tubes from the water bath and to place them into a test tube rack.
8. 3 drops of Sudan III were then added to each test tube using a glass Pasteur pipette and they were carefully shaken.
9. The Sudan III stained any lipid present red. Final observations of the test tubes were then written into the results table.
Ethanol Emulsion Test
10. A small sample of the solid food materials and a few drops of the liquid material were placed into clean dry test tubes using either a spatula or 3cm3 plastic pipette.
11. 3cm3 of 100% ethanol was then added to each test tube using a pipette. The solution was then shaken vigorously.
12. The test tubes were then placed back into the test tube rack and left to allow for any solid material to settle.
13. A quantity of the supernatant liquid (ethanol containing dissolved lipids) was removed using a glass Pasteur pipette and added drop wise to half a test tube full of tap water.
14. The observations were then recorded in the results table. The presence of lipids was indicated by a white turbid suspension.
Goggles were worn through out the experiment as Sudan III can be extremely harmful (it has known...