Food Tests Carbohydrates (Sugars and Starches) Reducing Sugars. The reducing sugars include all inconsistencies, Such as glucose and fructose, and some disaccharide, such as maltose. Use 0. 1 – 1% sugar solutions. Test for reducing sugar: Benedicts test: Add 2 cam of a solution of the reducing sugar to a test tube. Add an equal volume of benedicts solution. Shake and bring gently to the boil, shaking continuously to minimize spitting. Observation: The initial blue coloration of the mixture turns green, then yellowish and may finally form a brick red precipitate. Basis of test: Benedicts solution contains copper sulfate.
Reducing sugars reduce soluble blue copper sulfate containing copper (II) ions (CUE+) to insoluble red – brown copper oxide containing copper (I). The latter is seen as a precipitate. Non- Reducing sugars. Non reducing sugars, the most common non reducing sugars is sucrose, a disaccharide. If reducing sugars have been shown to be absent (negative result in a above test) a brick red precipitate in the test below indicates the presence of a non reducing sugar. If reducing sugars have been shown to be present, a heavier precipitate will be observed in the following test than with the reducing test if non reducing sugar is also present.
Tests for non reducing sugar: Add 2 cam of sucrose solution to a test tube. Add 1 cam dilute Hydrochloric acid. Boil for 1 minute. Carefully neutralist with sodium hydrogenation’s (Check with pH paper) Care is required because effervescence occurs. Carry out benedicts test. Observation: As benedicts test. Basis of test: Disaccharide can be hydrolysis to it’s moneychangers constituents by boiling with dilute hydrochloric acid. Sucrose is hydrolysis to glucose and fructose, both of which are reducing sugars and give the reducing sugar result with the benedicts test. Test for starch: Iodine/potassium iodide test.
Add CACM of l%starch solution to a test tube. Add a few drops of iodine potassium solution. Alternatively add the latter to the solid for of starch. Observation: A blue-black coloration. A polyploidy complex is formed with starch. Test for a lipid: Emulsion test: Add CACM fat or oil to a test tube containing CACM of absolute ethanol. Dissolve the lipid by shaking vigorously. Add an equal amount of cold water. Observation: A cloudy white suspension. Lipids are immiscible with water. Adding water to a solution of the lipid in alcohol results in emulsion of tiny droplets in the water which reflect light and vive a white , opalescent appearance.
Test for a protein: Beirut test: Add CACM protein solution to a test tube. Add an equal volume of 5% potassium hydroxide solution and mix. Add 2 drops of 1% copper sulfate solution and mix. No heating is required. Observation: A mauve or purple color develops slowly. A test for peptide bonds. In the presence of dilute copper sulfate in alkaline solution, nitrogen atoms in the peptide chain for a purple complex with copper(al) ions (CUE+). Burier is a compound derived from urea which also contains the COHN- group and gives positive results.