Starch test using iodine in potassium iodide solution:
What is starch?
Plants store their food that is glucose in the form of polysaccharide starch. Starch is a mixture of two polymers: amylose and amylopectin.
Natural starches consist of 10% – 30% amylose and amylopectin is present about 70% -90%. Amylose consists of linear chains, and amylopectin consists of huge branched glucose. Amylose become soluble in hot water, but amylopectin is completely insoluble.
Why do we use iodine in potassium iodide solution for the starch test?
In iodine solution, Amylose present in starch is responsible for deep blue colour. Iodine is not soluble in water too much so iodine is made by dissolving iodine reagent in potassium iodide solution which is soluble and give a very dark blue-black colour when starch is present.
How do you carry out this starch-iodine test?
- Take a sample and add 2-3 drops of iodine reagent on it.
- After a few minutes, we can observe the Result.
Blue- Black colour will be observed.
Brown colour will be observed.
Reducing sugar (glucose) Test/ Benedict’s test:
- Before starting this test, prepare the sample in liquid form. Food sample will be crushed/ground/cut/dissolved (depends upon the type of food sample that will be given) in water to extract all glucose present in the sample.
- In this test, Benedict’s reagent(a solution of sodium citrate and sodium carbonate mix with the solution of copper sulphate) will be used.
- In this test, a range of colours will appear in the test tubes according to the amount of reducing sugar present in each sample.
How do you carry out Benedict’s Test?
- Prepare the food sample(crushing/grinding/cutting/dissolving depend upon the type of food provided.
- Take prepared food sample(inform of liquid) and Benedict’s solution(blue) in equal amount.(2cm3 each )
- Place the test tubes in water-bath and wait till boiling the test tube content.
- Then them for two minutes.
Strong positive Result:
Medium positive Result:
will remain blue
Benedict’s solution is harmful to skin and eye so wear gloves, goggles and lab coat for your safety.
Ethanol Emulsion Test(test for the presence of fat)
- An Emulsion is a mixture of two liquids, and they usually are immiscible(are those liquids that cannot mix in one phase and remain in two phases)
- Before starting this test, prepare the sample in liquid form. Food sample will be crushed/ground/cut/dissolved (depends upon the type of food sample that will be given) in water to extract all fats present in the sample.
- In this test, Ethanol is used.
- In this test, fats will be emulsified in the form of white precipitates.
How do you carry out Ethanol Emulsion Test(oil/fats/lipids)?
- Take the same quantity of food sample and Ethanol in a test tube.
- Shake them so they can mix well.
- Pour in an equal amount of water in it and shake well to mix this properly.
- Place the tube in a test tube rack and observe the Result.
a cloudy emulsion is formed.
liquid remains clear.
Ethanol is alcohol so keep it away from any fire source.
Biuret test(test to check the presence of Protein)
- Biuret test is also called Piotrowski’s test a chemical test to identify the presence of Protein.
- Biuret reagent, blue in colour, is used in this test.
How do you carry out a Biuret test?
- Take food sample (in liquid ) and add a few drops of biuret solution in the test tube.
- Wait for a few minutes.
- Observe the Result.
purple colour appears
remains blue because Biuret reagent is blue in colour.
Try yourself for Food Tests:
- Which test required heating?
- Name the test when the test solution added contents of the test tube turned blue.
- Name the test completed by the addition of water.
- Name the test that was positive when the content turned black.
- Name the test in which the material is a carbohydrate.
- In which test, the Result was green in colour and why?
- Why is it essential to use water-bath to carry out Benedict’s test?
- Which type of sugar is present in a germinating seed?
- Name the test used to identify the presence of Protein.
- Name the test if you get the final Result brick red in colour.
- Name the test to identify lipids.
- What colour will appear if you drop iodine solution on a piece of bread?
- Name the building block of fats.
- Name the building block of carbohydrates.
- Name the building block of Protein.
- Name the building block of starch.
- In which colour does iodine turn in the presence of complex carbohydrates(polysaccharides)?
- Name the biomolecule does biuret solution test for.
- Name the biomolecule does Bandict’s solution tests for.
- What do you do to prepare the sample solution for any food test?