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Chapter: Biology laboratory Practicals: Biology Activities

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Food Test for Lipids

Lipids are an organic food substance made of carbon, oxygen, and hydrogen.

Food Test for Lipids

Lipids are an organic food substance made of carbon, oxygen, and hydrogen. Lipids occur in two forms: fats and oils. Oils are liquid at room temperature whereas fats are solid. Lipids provide the body with energy and create a layer of insulation to help keep the body warm. The main sources of lipids are milk, animal fats, groundnuts, coconuts, and avocado.

Learning Objectives

To carry out a test for lipids in a given food sample.

Materials

Iodine solution*, water, empty plastic bottles, test tubes*, droppers*, and a cooking oil that is liquid at room temperature, e.g. sun ower oil.

Hazards and Safety

Iodine solution is harmful to swallow.

Iodine solution can stain clothing. Remove stains promptly with a solution of crushed vitamin C (ascorbic acid). Iodine will also migrate into wood stain, permanently discolouring tables - prevent spills.

Preparation Procedure

1.           Mix about 10 mL (one cap full) of cooking oil and about 100 ml of water in a plastic bottle.

2.           Close the bottle and shake vigorously.

Activity Procedure

1.           Pour 2 mL of the food sample solution into a test tube. You should shake the bottle of sample solution each time before pouring it to pre-vent the oil from separating.

2.           Add 3 drops of iodine solution to the test tube.

3.           Shake the test tube and let the mixture settle.

4. Record results.

Results and Conclusions

You should see the formation of a red ring at the top of the sample solution. This indicates the presence of lipids.

Clean Up Procedure

1.           Unused iodine solution should be stored in a labeled bottles for future use.

2.           None of the waste from this experiment requires special disposal.

Notes

Many biology books call for a chemical called Sudan III to test for lipids. Sudan III is a bright red pigment that is much more soluble in oil than in water. For this reason, Sudan III solution is usually prepared using ethanol to bring the Sudan III pigment into the solution. In mixtures of oil and water, the oil separates and moves to the top. When shaken with Sudan III, this oil absorbs the Sudan III, turns red, and produces a "red ring" at the top of the test tube. However, the ethanol used to make Sudan III causes the water and oil to form an emulsion. In an emulsion, the oil is broken into very small particles and it takes a long time for this emulsion to break down and form an oil layer on the top. Hence testing with Sudan III takes a long time to show a clear result.

 

Iodine is another coloured molecule that is more soluble in oil than in water. When a mixture of oil and water is shaken with iodine solution, the iodine moves to the oil layer, colouring it orange or red. This also gives the result of a "red ring" at the top of the test tube. To prevent an emulsion forming - as happens with Sudan III - it is very important to make iodine solution from pharmacy tincture that is without ethanol. Another bene t of using iodine is that while Sudan III is always red, iodine is uniquely yellow in water and red in oil, making the difference between positive and negative results easier to see. Because there is no ethanol in iodine solution, the result also comes much faster, usually within 10-20 seconds.

 

Note that if the oil and water mixture settles before you transfer it to the test tube, there may be too little or too much oil in the test tube. Shake the food sample solution before taking each sample.

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