CELL AND MOLECULES
LABORATORY PRACTICAL 1
COMPARISON OF CHEMICAL COMPOSITION OF
VARIOUS FOOD STUFF
1. LOO MING JING
2. TEE ANN JO
3. SEN YUN JAN
4. WONG KIM JIN
5. RINI AMIRA BINTI ZUL BAHRI
Title : Comparison of Chemical Composition of Various Food Stuff
Carbohydrate is a macromolecule that consists of atoms of carbon, hydrogen & oxygen. The carbohydrates or saccharides exists in 4 groups, monosaccharides, disaccharides, oligosaccharides and polysaccharides. Starch is among one of the polysaccharides and it consists of a large number of units of glucose molecules joined together by glycosidic bonds. Starch consists of two types of molecules , the linear and helical amylose and the branched amylopectin. Potatoes have large amount of starch as it is a staple food. The starch in potato can be tested by the iodine test. When starch is present, the iodine will turn purplish black in colour.
Benedict’s solution is used to test the presence of reducing sugar. All monosaccharides are reducing sugars since all of them have an active carboxyl group. The example of reducing sugars are glucose, fructose, lactose and maltose. Some disaccharides that are exposed to a carboxyl group are also reducing sugar, but less reactive than monosaccharides. By mixing the sugar solution solution with Benedict’s solution and heating them under water bath of 80℃, a redox reaction will occur. The copper(II) sulphate present in Benedict’s solution which exists as light blue in colour reacts with electron from aldehyde or ketose group of reducing sugars to form copper oxide, which is a brick-red precipitate.
The purpose of doing Benedict’s test is to detect the presence of reducing sugar in a substance. Benedict’s solution, which is the reagent for the test will detect the presence of aldehydes and alpha-hydroxy-ketones. If positive result is shown when Benedict’s test was conducted, the substance will change in colour from clear blue to brick-red, and also will form precipitate which is suspended at the bottom of the test tube. Benedict’s test is to detect the presence of reducing sugar. But this test also can be conducted on non-reducing sugar substance. One of the examples of non-reducing sugar is sucrose. It contains two sugars, namely fructose and glucose, which are joined together by glycosidic bond in such a way to prevent the glucose isomerize to aldehyde, or the fructose isomerize to alpha-hydroxy-ketone form. But non-reducing sugar can also give a positive result if it is heated with dilute hydrochloric acid. The acidic conditions and heat will break the glycosidic bond in sucrose through hydrolysis. When the glycosidic bond is broken, the isomerisation of glucose to aldehyde, and fructose to alpha-hydroxy-ketone will occur. The products of sucrose decomposition (fructose and glucose) will then can be detected by Benedict’s solution. This then leads to the positive result of the test. The change of the colour of the solution...