To test for Lipids we will use the emulsion this test will indicate the presence of lipids. Lipids are insoluble in water, but soluble in ethanol. To test for lipids, we will add ethanol to a food sample in a test tube then shake vigorously. If lipids are present, the molecules will clump together into tiny droplets spread throughout the liquid; this is called an emulsion. To test for proteins, we add some Biuret solution to a food sample or solution, a purple colour will indicate the presence of protein.
Background Information
Starch
Plants manufacture starch during photosynthesis. Starch molecules are of two kinds. In the first kind, amylose, which constitutes about 20 per cent of ordinary starch, the C6H10O5 groups are arranged in a continuous but curled chain somewhat like a coil of rope; in the second kind, amylopectin, considerable side-branching of the molecule occurs. Starch is almost insoluble in cold water and in alcohol, but with boiling water it gives a colloidal suspension that may form a jelly on cooling.
Hot water changes starch slowly into smaller molecules called dextrins. This reaction, an example of hydrolysis, is catalysed by acids and by some enzymes. Dextrin’s, like starch, react with water, giving still simpler molecules, the ultimate products being maltose, C12H22O11, a disaccharide, and glucose, C6H 12O6, and a monosaccharide. The biochemical test for starch is by the use of iodine. In the presence of starch the iodine will turn blue-black.
Lipids
Lipid is one of the largest groups of fatty organic compounds in living organisms. They include animal fats, vegetable oils and natural waxes. Lipids form an important food store and energy store in plant and animals. Lipids are a mixture of hydrophobic compounds made of carbon hydrogen and oxygen. They contain fats and oils and do not dissolve in water but are soluble in alcohol, ether, or other organic solvents.
Each molecule of fat or oil is composed of building blocks called fatty acids linked by ester bonds to an alcohol, glycerol. Fats and oils contain twice as much stored energy, per unit of weight, as carbohydrates or proteins. The biochemical test for lipids is the emulsion test, knowing that lipids are insoluble in water but soluble in ethanol means that ethanol is used for this test. If lipids are present when adding ethanol to the food sample the molecules will clump together into tiny droplets and spread through the liquid.
Sugars
Chemical compounds in the carbohydrate group that are readily soluble in water; are colourless, odourless, and usually crystallisable; and are more or less sweet in taste. The biochemical test for sugars is the use of Benedict’s reagent. To test reducing sugars, add some benedict’s solution and heat, and the solution will turn from blue to brick red or yellow if a reducing sugar is present. For non-reducing sugars, first hydrolyse the sample by heating it with hydrochloric acid, then neutralise with sodium hydrogen carbonate, this breaks the bonds and releases monomers. A benedict’s test is then carried out which will be positive because the monomers are now free.
Proteins
Proteins are molecules, made up of amino acids. These amino acids contain carbon, oxygen, hydrogen and nitrogen. Sulphur is often present as well as iron and phosphorus. Each amino acid is made to the same design, with an amino group, a carboxyl group, a hydrogen atom, and a distinct R group (or side chain) all of which are bonded to an a-carbon atom.
Amino acids are linked together in proteins by a peptide bond, made by the reaction of the carboxyl group of one amino acid with the amino group of the next. Polypeptides are just a string of amino acids, but they fold up and form the different structures of the proteins. The primary function of protein is to build body tissue and to synthesize enzymes. The biochemical test for proteins is the Biuret test. By adding some Biuret solution to a food sample; the presence of proteins would turn the food sample purple.
Prediction
I can predict that according to background knowledge that there will be some starch in the bread, sweet biscuit, the crisp and the cream crackers. The olive oil, nuts, the cream crackers and the crisp will also contain some lipids. It can also be predicted that the milk, eggs, nuts and cream crackers would contain some proteins. The cream crackers, sweet biscuits and crisps would be likely to contain some sugars.
Materials and Method
Health and Safety
- Handle all chemicals with care
- Handle glass ware with great care
- Only go near the water bath if necessary
Precaution
- Make sure the apparatus used is clean to prevent contamination
Materials
- Test tubes Sweet biscuits
- Biuret solution Some olive oil
- Iodine Some nuts
- Ethanol Some crisps
- Benedict’s reagent Some egg mixture
- Some bread Some milk
- One potato some cream crackers
- A test tube holder a water bath
Method
- Take four test tubes and put a small sample of olive oil in each of them
- Only use a small amount of the food
- Add a few drops of iodine solution in one of them and shake
- Add a few drops of benedict’s solution in another one and shake. If there is no change in colour, put the test tube in a the water bath to heat
- Add a few drops of ethanol in another test tube and shake
- Add some Biuret solution in the last one and shake
- Record your results in a table
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Repeat the experiment with all the sample of foods (Bread, milk, cream crackers, potato, crisps, nuts, egg, sweet biscuits)
Raw Results
Analysis of Results
The results obtained show more or less what was predicted. The cream crackers had a great amount of each biological molecule. According to the results the potato contains some sugars and starch. The eggs and the milk only contained protein I would have expected them to contain a little lipid as well. The nuts were predicted to have both proteins and some lipids as it is shown in the results.
Most of the foods tested contain some starch or some saturated fats, they might not contain these nutrients naturally but the manufacture of them may well imply using some ingredients containing saturated fats. E.g. crisps are potatoes containing starch and sugars, which are deeply fried in oil containing lipids with added salt and flavouring, therefore in the crisps there is a content of lipids, which do not occur naturally, they are added in the manufacture.
Evaluation of Materials and Method
The equipment used was quite precise; however, the focus is more on the investigation itself. The methods used were quite simple and easy to follow. In order to get accurate results, a constant amount of the each food sample and solution needed to be used. A variety of food types were used to test for the four different biological molecules, by doing this we obtained different results. Having a lot of data gives biologists a lot to compare and discuss.
Another factor affecting the experiment was time. We didn’t have enough time so we did not repeat any of the experiments. However if any test had to be repeated it would have to have been the emulsion test for all the foods. The results obtained might be unreliable due to the emulsion test. After the ethanol was added there was not an addition of water, the procedure for the emulsion test was not conducted the way it was meant to be. If this investigation were to be carried out again we would use a variety of foods such as meat, fish, different types of animal foods, this could help to compare the human diets and the animal diet. The results that would be obtained might be more or less the same, as all living organisms need the same nutrients, however might have different purposes in one’s body.
Time would also have to be an important factor. With enough time the experiments could be repeated and more accurate results would be taken. The method used could be used again however. The experiment would be repeated at least twice to obtained so reliable results; this will enable us to be aware of the mistakes in the procedures or recording of the results.
Another way to improve this investigation could be to use raw materials not manufactured foods. If we used the ingredients contained in each food the results would be the same but it would help to explain and understand how certain foods contain certain nutrients.
Conclusions
It can be concluded that different kinds of food have different types of biological molecules and not all of them are the same. Eating different types of food which does not contain the same the biological molecules will provide the body with necessary nutrients.
References
Books:
Phillips Concise Encyclopaedia
CXC Biology
Oxford Dictionary
Online references:
MSN Encarta
Software:
Euro press AS/A2 Biology 2005/2006
Letts GSCE Biology 2005