A number of oils are available in the market for use by the general populace, including mustard oil, coconut oil, olive oil, rice bran oil, sunflower oil, sesame oil, linseed oil, and many more. Some are cheaper than others, and this is why many people try to mix low-cost oils with high-cost ones, so as to gain more profit. However, this is illegal and is a clear case of adulteration. The consumers are actually consuming low-quality products for which they have paid large amounts of money. Sometimes instead of cheaper oils, the greedy money-grubbers adulterate oils with synthetic chemicals that have a direct impact on the nervous system, jeopardizing the health of everyone who consumes it.
Here, we have listed some of the common methods to test edible oil for the presence of cheaper oils. These experiments can easily be conducted in a laboratory and can be used to check the same.
1. Detection of synthetically made artificial mustard oil by Sodium Azide Test
Mustard oil is available in the market at high costs as compared to other cheap substitutes. So, fraudulent often use cheap oils and color them using an oil-soluble yellow dye, and further add to it, synthetic Allyl isothiocyanate. Ultimately, the oil looks and smells similar to mustard oil, but is very cheap manufacturing, and is harmful when consumed.
1. Take 100ml of oil sample.
2. Add 100ml of 2% sodium azide solution.
3. Reflux on hot plate/by direct heating for 3 hours.
4. Transfer content to a 250ml flask and let cool.
5. Oily layer will separate.
6. Collect the lower aqueous layer and remove the oily layer.
7. Wash this aqueous layer twice with 50ml diethyl ether to remove any oil content.
8. Filter the solution and boil it till half of the volume remains.
9. Take 1ml of this solution and mix it with 1ml of Bismuth nitrate solution.
10. Immediate formation of Deep yellow precipitate means the sample contains synthetic mustard oil.
11. However, natural mustard oil gives a negative result.
2. Detection of Argemone oil
Argemone oil is a common adulterant in mustard oil as it is cheap. However, it is highly toxic when consumed, and causes necrosis, dropsy, high tension glaucoma, diarrhea, vomiting, anemia.
A modified form of nitric acid test is done to detect the presence of argemone oil in edible oil.
1. Take 5ml of sample oil, and add to it 0.5 ml of 2% salicylic acid in methanol, 2ml of concentrated nitric acid, 2-4 drops of sulfuric acid, and shake.
2. Within 20-30 seconds, if a deep orange-red color develops, then argemone adulteration is present.
Even 0.1% argemone adulteration can be detected by this method.
3. Detection of Rice bran oil by an azo dye test
Rice bran oil is similar to mustard oil in color and density but is cheaper as compared to the latter. Henceforth, it is commonly used as an adulterant.
1. Mix 1ml of sample oil with 2-4 ml of 10% sodium hydroxide solution, and shake for 5-10 minutes to form an emulsified solution. This is solution ‘a’.
2. Take 1-2 drops of aniline in another dry test tube and dissolve it in dilute HCl.
3. Cool to 0-5°C.
4. Add 2-3 ml of 5% sodium nitrite solution and shake. This is solution ‘b’
5. Mix solution a and b, and shake for a few seconds
6. If an orange-red color develops within 10-20 seconds, that indicates rice bran oil adulteration.
4. Detection of sesame oil by modified Baudouin test
1. Take 5ml of sesame oil or melted fat in a 25ml measuring cylinder provided with a glass stopper.
2. Add 5ml of HCl and 0.4 ml of furfural solution.
3. Insert the glass stopper, and shake vigorously.
4. Allow the mixture to separate. The development of pink color in the acid layer indicates the presence of sesamum oil.
5. To confirm, add 5ml of water and shake again. If the color in the layer persists, then sesame oil is definitely present. If the color disappears, then not present.
This test can detect up to 0.3% of sesame oil in other oils.
5. Detection of Linseed oil using Hexabromide test
1. Take 1ml of sample oil in a test tube, and add it to it, 5ml of chloroform.
2. Add 1ml of bromine dropwise. The solution should become deep red, which will indicate the presence of bromine in excess.
3. Cool the test tube in the ice water bath and add about 1.5 ml of rectified spirit dropwise.
4. Shake the mixture simultaneously, and add alcohol till the precipitate dissolves completely.
5. 10 ml of ether is added. Shake the test tube gently to mix the solution. The tube is again kept in the ice water bath for 30 minutes.
6. Appearance of a precipitate indicates the presence of linseed oil.
This test can detect the presence of up to 1.0% linseed oil in other oils.
6. Detection of Cottonseed oil, using Halphen’s test
The cylopropenic acids, a component of cottonseed oil, if present in high amounts cause toxicity.
1. Take 5ml of filtered and dried oil in a dry test tube.
2. Add 5ml of 1% (w/v) solution of sulfur in carbon disulfide and then add an equal amount of amyl alcohol.
3. Mix thoroughly by shaking and heating gently in water bath 70-80% for a few minutes, with occasional shaking till carbon disulfide is boiled off and foaming ceases.
4. Place tube in an oil bath for a brine bath maintained at 110-115C. Hold for 1-2 hours
5. If red color is present at the end, it indicates the presence of cottonseed oil.
7. Detection of Castor oil by Molybdate method
This method can detect castor oil to the extent of 1% or more in other oils.
1. Take 1ml of oil in a dry test tube and dissolve it in 10ml petroleum ether.
2. Shake vigorously for 2 minutes and add 1-2 drops of molybdate reagent (For making molybdate reagent, dissolve 1.25 g of ammonium molybdate in 100ml of concentrated sulfuric acid).
3. If white turbidity develops instantaneously, it indicates the presence of castor oil as an adulterant.
8. Detection of Palmolein by Solvent Partition method
Palmolein is the liquid fraction of palm oil, which is obtained from the fleshy mesocarp of the fruits of palm trees. Palmolein can be detected by phytosterol acetate test, gas-liquid chromatography, and thin-layer chromatography.
In the solvent partition method, the following steps are followed-
1. Take 5ml of sample oil in a beaker, and dissolve it in 5ml of hexane.
2. Transfer this solution to a separating funnel, after passing through anhydrous sodium sulfate.
3. Add 3ml of dimethyl formaldehyde (DMF) and shake the mixture gently for about a minute.
4. Settle till the two layers separate. Draw the lower DMF layer and reject it.
5. Go for a second washing if the DMF layer is deeply colored.
6. Collect the hexane solution in a porcelain dish, and evaporate the solution in a water bath.
7. Transfer this sample into a test tube.
8. Repeat the above steps using pure groundnut oil as the sample. (This is used for the reference purpose)
9. Observe both the test tubes containing samples under UV light.
10. In the case of pure groundnut oil, no greenish-yellow fluorescence will be detected. However, in the case of groundnut oil admixed with palm oil, Yellowish green fluorescence is observed under UV light.
These were the 8 Laboratory Tests that can be used for the identification of possible adulterants in Edible oil.
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