Food Test. Aim: To conduct tests for reducing and non-reducing sugars, starch, lipids and proteins
Title: Food Test
Aim: To conduct tests for reducing and non-reducing sugars, starch, lipids and proteins
Reducing sugars are disaccharides formed by the reaction of the aldehyde group of one monosaccharide with a hydroxyl group of another free aldehyde group. Reducing sugars such as glucose are simple sugars while non-reducing sugars such as sucrose are complex sugars. They are both soluble in water, sweet in taste and turns blue Benedict's solution to brick-red with a precipitate upon boiling. Moreover, starch is soluble in water and turns iodine solution from yellow-orange to blue-black. Proteins such as meat, fish and egg are large chemical compounds. They are insoluble in water and turns blue Biuret solution to purple. On the other hand, fats such as palm oil and margarine are large chemical compounds. They are insoluble in water and forms a white cloudy suspension when tested with ethanol and cold water.
In this experiment, a number of foods would be tested using a series of chemical tests including the Benedict's test, Iodine test, Emulsion test and the Biuret test, to determine whether these foods contain the different macromolecules of reducing and non-reducing sugars, starch, proteins and fats. Also, in this experiment, the observations during each food test would be recorded and the colour changes would be observed to identify whether each food contain any of the macromolecules which are being tested.
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In the Benedict's test for reducing sugars, 2ml of the glucose solution was added to a clean test tube. An equal amount of Benedict's solution was added to the test tube. The test tube was then shaken to mix the solution and placed in a boiling water bath.
In the Benedict's test for non-reducing sugars, 2ml of the sucrose solution was added to a clean test tube. 1ml of dilute hydrochloric acid was added. The test tubes was then shaken to mix the solution and placed in a boiling water bath for 1 minute. Drop by drop, sodium hydrogen carbonate was added until the fizzing stopped. When the fizzing stopped, 2ml of Benedict's solution was added. The test tube was then shaken to mix the solution then placed in a boiling water bath for 1 minute.
In the Starch test with iodine/potassium iodide solution, one spatula full of bread crumbs was added to a clean test tube. 3-4 drops of I2/ KI solution was then added to the test tube. Procedure 1 and 2 was repeated for crushed potato.
In the Emulsion test for fat, 2 ml of ethanol was added to a clean test tube and 2ml of vegetable oil was added to the same test tube. The test tube was then shaken to dissolve the molecules formed. To another clean test tube, 2ml of cold water was added and the contents of the test tube containing the mixture of vegetable oil and ethanol was added to the test tube.
In the Biuret test for protein, 2ml of egg albumen mixture was added to a clean test tube. In this test tube, 2ml of KOH (potassium hydroxide) was added followed by 2-3 drops of CuSO4 (Copper Sulphate solution).
The following tests mentioned previously were completed and all observations were recorded in a table.
Table showing the results obtained from the tests for reducing and non-reducing sugars, starch, lipids and proteins
In the reducing sugars test, the solution turned from a blue colour to brick-red colour. This is because of the Benedict's solution, which when added turned the solution blue. Benedict's solution is an alkaline solution of copper (II) sulphate (CuSO4). The aldehyde or ketone group of the glucose solution was able to reduce Cu2+ ions to Cu+, while being oxidized to a carboxyl (-COOH) group. This gave rise to the formation of a brick-red precipitate of copper (I) oxide.
In the non-reducing sugar test, the solution also turned from a blue colour to a brick-red colour. In this test, sucrose was hydrolysed to its constituent monosaccharides (glucose and fructose) when it was boiled with dilute hydrochloric acid (a catalyst). The products of hydrolysis, glucose and fructose, which are reducing sugars, then reduced the Benedict's solution. In the Benedict's solution, the aldehyde and ketone group of glucose and fructose were able to reduce Cu2+ ions to Cu+, while being oxidized to a carboxyl (-COOH) group and this gave rise to the formation of a brick-red precipitate of copper (I) oxide.
In the starch test, a blue-black precipitate resulted. The colour was the result of a complex formed between the amylose helix and the iodine molecules. Starch has a structure of a helical amylose which is the essential in the iodine-starch reaction. When starch was added to the water, a coloured starch-iodine complex was formed. The starch formed a linear arrangement in the middle of the amylose coils and this created the exchange of charges between the starch and the iodine, which changed the arrangement of the electrons spacing. This change resulted in the starch-iodine complex absorbing visible light differently and this also resulted in the deep blue colour.
In the protein test, the solution turned from blue to purple. This resulted from where there were peptide bonds and a coloured complex was formed. The sodium hydroxide (NaOH) solution raised the pH to alkaline levels. As a result of the peptide bonds being present in the alkaline solution, the copper ions (Cu2+) present in the copper sulphate solution formed a complex with the nitrogen atoms that was present in the peptide bonds that held the amino acids together. The complex of the copper ions (Cu2+) and nitrogen atoms, caused the colour of CuSO4 solution from blue to purple.
In the fats test, a cloudy white suspension resulted. Lipids are soluble in ethanol and insoluble in water. When the lipid was added to the ethanol, it dissolved the lipid and when it was added to the cold water, it formed a cloudy suspension which is caused by tiny droplets of lipids that are dispersed in the water.
- The quantity of sugar present is unknown.
- The potato could have been oxidized due to exposure to the air.
- The chemicals may have been left opened for too long which could affect the results.
Conclusion: From the tests conducted in the experiment, reducing and non-reducing sugars were present in the food samples as the blue Benedict's solution turned to brick-red with a precipitate. Moreover, starch was also present in the food sample as the Iodine solution turned from yellow-orange to a blue-black colour with a precipitate. Protein was present in the food sample as the blue Biuret solution turned to purple. Conclusively, fat was present in the food sample as a white cloudy suspension resulted when ethanol and cold water was added.