Experiment 3: Identification of food constituents in milk.
•To compare and analysis the composition of two different types of unknown milk, K1 and K2. •To determine the differences of the fat content in milk samples, K1 and K2. •To compare the reducing sugar in the both samples of milk, K1 and K2. •To compare the protein concentrations in the both samples of milk, K1 and K2.
Milk provides a useful dietary source of calcium, which is vital for the growth and maintenance of bone and tooth. Milk is the first source of nourishment as it played an important role in human nutrition. The constituents in milk that are vital in food preparation are enzymes, vitamins, pigments, salts, sugar, fat, and proteins. Different milk samples which contain different food constituents will have a different nutritional value respectively. Sudan (III) is fat-soluble dye used for staining of triglycerides (fat). It is used in the experiment to detect for the presence of fatty acids. It will appear as red globules if fatty acids are present in the milk sample. Benedict's reagent is used to test for the presence of reducing sugars. The solution will form a brick-red precipitate when the milk sample contains reducing sugar (glucose, maltose, lactose.) Sugar that can form an aldehyde and ketone in the presence of an alkaline solution is known as reducing sugars. The Biuret test is used to test for the presence of protein. The biuret test is based on the ability of Cu (II) ions to form a violet-coloured chelate complex with peptide bonds (-CONH-groups) in alkaline conditions. In Biuret’s test (consist of sodium hydroxide and copper (II) sulphate solution),the solution will turn to purple when the proteins are present in the milk sample. Proteins are organic compounds important for growth and repair. Protein molecules are composed primarily of amino acids linked together through peptide bonds.
Copper(II) sulphate solution, unknown milk sample (K1,K2), cooking oil, distilled water, salad oil, Sudan(III), full cream milk, Benedict reagent, 10% potassium hydroxide, 0.5% copper(II) sulphate
Test tubes, plastic pipettes, cork, water bath
(A) 1. A clean test tube is filled until 1cm of the top with copper (II) sulphate solution. 2. A small amount of K1 is pipette into a clean plastic pipette and a drop is gently released into the copper (II) sulphate solution as shown in Figure 3. Another drop of milk is further released. 3. Then the pipette is withdrawn slowly without disturbing the copper (II) sulphate solution. The movement of K1 is observed carefully and the observations are noted. 4. The procedure above is repeated using new, fresh copper (II) sulphate solution, new clean glass apparatus and the sample of K2. Observations on the movement of K2 are recorded and any differences in the behaviour of the drops of K1 and K2 are noted carefully.
(B) 1. 6 test tubes are obtained and numbered them from 1-6. 2. The materials listed in Table 1 are added.
3. The contents of each tube are mixed thoroughly.
4. The colour changed of the tubes contents are recorded. TUBETUBE SOLUTION
1.1ml cooking oil + 1ml distilled water
2.1ml salad oil + 5 drops of Sudan III
3. 1ml full cream milk + 5 drops of Sudan III
4. 1ml distilled water + 5 drops of Sudan III
5.1ml distilled water + 1ml cooking oil + 5 drops of Sudan III
6.1ml cooking oil + 5 drops of Sudan III
(C) (i) Reducing sugar test
1. 2cm3 of K1 obtained is poured into a test tube and added by 2cm3 of Benedict’s reagent. The reagent with the K1 solution is mixed thoroughly. 2. The test tube is shaken occasionally when placed in the water bath with the temperature of 94ᶿc and left for 5 minutes. 3. The colour changed is observed and the observations are recorded. 4. Procedures 1-3 are repeated with another sample of milk, K2. The observations are recorded.
(ii) Protein test