Q. Detect the presence of synthetic food colours in the given samples?
Objectives
After conducting this activity, you will be able to:
• describe the kind of natural and synthetic colours used in the food industry,
• recognize the permissible limits of the synthetic food colours,
• isolate the synthetic colours in various food products, and
• identify these synthetic colours.
Principle
Synthetic colours form a major group of food colours which are classified into synthetic colour dyes and mineral pigments. These synthetic colours are futher divided into acidic and basic dyes. The isolation is carried out with the wool dyeing technique.
To identify them, you must study the colour reactions of these water soluble, acidic dyes which are characteristic to each colour and thus can be used as conformatory tests. The reactions of various colours with different chemical medium are given in Table earlier. Write down the reactions once again here in the format given:
Procedure
Carry out the activity following the procedure enumerated herewith:
1) Preparation of the Sample
Before carrying out the isolation of acidic colours, it is necessary to remove the interfering substances such as starches and fats, which hinder with the extraction procedures. The pre preparation of the sample for the extraction process would involve:
1) For Alcoholic drinks
Boil the alcoholic drinks to remove alcohol.
2) For the starchy products carry out the following steps:
Grind the starchy products like custard powders, cakes, pulses, candied fruits.
Treat 10 g of the ground sample with 50 ml of 2% NH3 in 70% ethanol.
Keep aside for one hour.
Centrifuge and take the liquid portion in a beaker.
Evaporate on the water bath.
Dilute the residue with 30 ml of dilute acetic acid.
3) For fatty food products like sausages, meats, fish pastes, processed nuts etc.
Defat the product with light petroleum in soxhlet apparatus.
Treat the defatted sample with 50 ml of 2% NH3 in 70% ethanol.
Keep aside for one hour.
Centrifuge and take the liquid portion in a beaker.
Evaporate on the water bath.
Dilute the residue with 30 ml of dilute acetic acid.
4) For the acidic products like fruit preserves and sauces
Solid samples are mixed with water and acidified with glacial acetic acid.
2) Dyeing of Wool
1) Immerse a 1m piece of pure white wool in the acidified solution of the sample.
2) Warm the solution till the wool gets dyed.
3) Take out the wool wash in water.
4) Put the dyed wool in 25 ml of 2% NH4OH and boil it.
5) Add NH4OH in excess till the colour gets extracted and comes in the solution.
6) Take 24 ml of this aqueous solution add 6 ml of 10% NaOH and 30 ml of solvent ether.
7) Take them in separating funnel, shake gently to mix all the layers.
8) Remove the aqueous fraction in another beaker.
9) To the ether layer add 5 ml of glacial acetic acid, if pink colour appears in the ether layer, it indicates the presence of rhodamine.
10) Take the aqueous layer from the 8th step, add 10 ml of glacial acetic acid and 30 ml of ether. Shake well.
11) Remove the aqueous layer in separate beaker and to the ether layer add excess of 10 ml of conc. NH4OH. If pink colour appears in the ether layer, it indicates the presence of Erythrosine.
12) Take 25 ml of the aqueous layer, add 10 ml of 20% NaCl shake and add 30 ml amyl alcohol. Add excess of conc. HCl so that the total volume becomes 60 ml.
13) Remove the aqueous layer and to this add 30 ml ether. Shake. Pink colour will appear in the water layer.