Experiment 2: Food Tests
* To study the presence of reducing sugars.
* To study the presence of protein.
In this experiment, glucose, maltose, lactose and sucrose are used for testing reducing and non-reducing sugars. Glucose is monosaccharide while maltose, lactose and sucrose are disaccharides of carbohydrates. Monosaccharaides are the monomers which make up all other carbohydrates and cannot be broken into smaller molecules by hydrolysis. Disaccharides are formed from the condensation reaction between two monosaccharaides and are linked by glycoside bond. All monosaccharaides and some disaccharides such as maltose and lactose are reducing sugars due to the presence of aldehyde or keto groups in the sugars. Sucrose is a non-reducing sugar because the aldehyde group of glucose is linked to the keto group of fructose and loses its reducing ability. The Benedict’s test is used as it allows the detection of the presence of reducing sugars. All monosaccharaides and some disaccharides which have exposed carbonyl groups will react with Benedict’s solution, whereas other disaccharides will not react with Benedict’s solution. The electrons from the aldehyde or ketone group of reducing sugar will react with the copper sulfate from the Benedict’s solution to form cuprous oxide, a red-brown precipitate. A non-reducing sugar will be verified by a negative test of Benedict’s solution before hydrolysis by acids, followed by a positive test of its hydrolysis into monosaccharide components. The iodine test is done to identify glycogen and starch. The polysaccharides combine with the iodine which will give out a black-blue coloration. There are two tests that are commonly used to test for protein which are the Millon’s and Biuret’s test. A Millon’s test detects the amino acid tyrosine and if the proteins possess a certain substance, only then it will give out a positive result. However, the Biuret test is done to identify the presence of proteins in a substance by detecting the peptide links. The peptide bond is the bond between the carboxyl acid group and the amino group on adjacent amino acids in a protein. Proteins, containing peptides bonds, when reacted with copper sulphate solution of Biuret reagent gives a violet complex. The Biuret test is mostly used to check for enzymes which are made up of proteins.
* Distilled water
* Potassium Iodide solution
* Benedict’s Reagent
* Sodium Hydrogen Carbonate solution
* Dilute Hydrochloric Acid solution
* Starch solution
* Albumin solution
* Potassium Hydroxide solution
* Copper Sulfate solution
* Test tubes
* White tile
* Water bath
A) Tests for Reducing and Non-reducing Sugars
i) Test for Reducing Sugar
1. The water in the water bath is brought up to boiling point and the source of heat is turned down maintaining the temperature. 2. 2cm2 of glucose solution is poured into the test tube and 2cm2 of Benedict’s reagent is added into the test tube. 3. The solution is then shaken till the reagent is mixed with the glucose solution thoroughly. 4. The test tube is then placed in the water bath and is left for 5 minutes. 5. The colour change is observed and recorded.
6. Steps 2-5 is repeated with sucrose, lactose and maltose in separate test tubes.
ii) Test for Non-reducing Sugar.
1. A sample of non-reducing sugar is identified from the test done in Section A(i). 2. A new sample of 2cm3 of the sucrose solution is poured into a clean test tube and 1cm3 of hydrochloric acid is added into the new sample of sucrose. 3. The solutions are mixed thoroughly and left in the water bath for approximately 2-3 minutes. 4. After boiling the solution, litmus paper is dropped into the solution and sodium hydrogen carbonate is added drop by drop into the test tube until the solution turned slightly...
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