Experiment 9 - The presence of starch and adulterant Metanil in food

Learning Objective:

To test a) the presence of starch in a given food sample and b) the presence of adulterant Metanil yellow in dal.

Materials Required:

Safety Precautions:

1. KEEP THE MOUTH OF THE TEST TUBE AWAY WHEN HANDLING HCL, USE A TESTTUBE HOLDER TO DO SO, IF USING SHARP TOOLS TO EXPOSE REGIONS CONTAINING STARCH, HANDLE WITH CARE

Chemical Reactions:

1. Iodine Test : In the presence of starch, Iodine solution changes from brown to blue-black Brown 🡪 Blue Black/Purple

2. HCL changes to pink in the presence of Metanil Yellow

Methodology:

Testing for Iodine:

1. Take a given food sample and expose the region suspected to store starch(e.g. slice a potato to reveal starchy regions)

2. Place the sample(in its exposed state) in a petri dish and add a couple of drops of iodine solution to its surface using a dropper

3. Observe any changes in color

Testing for Metanil Yellow:

1. Take a sample of dal(e.g.) peas and place it in a dry test tube

2. Add a few drops of HCl to the dal, and swirl the tube properly(using a test tube holder)

3. Observe any changes in colour

Observation:

Testing for Starch:

Regions of the given food sample that had starch turned dark blue/black when exposed to Iodine solution.

Testing for Metanil Yellow:

The reaction mixture should turn pink when exposed to HCl solution.

Experiment in Context:

There are various food tests that scientists employ to determine the presence of substances.
Some of these are:

Testing for reducing sugars:

1. Heat sample with an equal amount of Benedict's solution

2. Place test tube in a beaker of boiling water or over a gentle Bunsen Burner flame(if on top of the Bunsen burner, it must be moved constantly to ensure that the liquid doesn't boil away)

3. The solution should change from clear blue to cloudy green to yellow and to a red precipitate(deposit) of copper (I) oxide

Testing for Proteins:

1. Add dilute sodium hydroxide and an equal volume of 1% copper sulfate solution to the sample

2. A purple color should appear

3. If the copper sulfate is run into the food solution without mixing, a violet halo appears

Testing for Fats/Lipids:

Add dilute sodium hydroxide and an equal volume of 1% copper sulfate solution to the sample

A purple color should appear

If the copper sulfate is run into the food solution without mixing, a violet halo appears

Testing for Vitamin C:

Take lemon juice in a syringe, and add it drop by drop to a 0.1% solution of DCPIP(a blue dye) of equal volume to the total lemon juice

At one point, the DCPIP will turn colorless quite suddenly

Perform the above steps except, replace the lemon juice with the sample to be tested

If it takes less of the sample than lemon juice to decolorize DCPIP, the sample has more vitamin C

Solids are generally prepared for food tests by the following method:

1. The solid is crushed in a mortar and pestle and then dissolved in warm water(but if it’s a fat, then ethanol).

2. Separate samples of this mixture are then tested