Acrolein Test For Glycerol: A Brief Overview

Acrolein test works as a qualitative analysis which undergoes in detecting the presence or absence of glycerol, which can be present either free or esterified. This detection is based upon its oxidative dehydration reaction yielding acrolein aldehyde ,when heated with solid potassium hydrogen sulphate (KHSO4). Thus ,depending upon lipids’ reactive ability with the different chemical reagents, these tests also classify the different groups of lipids based. Thus, the qualitative analysis of lipids is an analytical method that actually detects lipids by the characteristic change or alteration in the sample’s colour and smell.

Chemically, the Lipids are Like heterogeneous mixtures. The lipids have only one common property that is their insolubility in water. Foods contain a large variety of lipids; the most necessary among them are fats, complex lipids, and steroids. Fats are actually triglycerides, which are the esters of fatty acids and glycerol. The Complex lipids also consist of fatty acids, but one difference is that their alcohol part may be either glycerol or sphingosine. These lipids also consist of other constituents such as phosphate, choline, ethanolamine, or mono– to oligosaccharides. A significant representative of this lipid group is lecithin, which is a glycerophospholipid, consisting of fatty acids, glycerol, phosphate, and choline. As we know , Different types of foods contain different proportions of these three groups of lipids.

In Structure , cholesterol has the steroid nucleus that is the very common core of all steroids: Steroid nucleus Cholesterol. The differentiation between cholesterol and lecithin is done by the test called the acrolein reaction.

Objective of Acrolein test:

• To study, identify ,and detect the presence of oils and fats or presence of glycerol or fat in the test sample.

Principle of Acrolein test:

The qualitative Acrolein test is utilised in detecting the presence of glycerol or fat in the given test sample. The Principle of Acrolein test is works as follows When the fat or lipids containing glycerol are treated strongly in the presence of a dehydrating agent like potassium bisulphate (KHSO4),hence ,it will remove the water molecules from the glycerol.Thus , the glycerol portion of the molecule undergoes dehydration to form an unsaturated aldehyde, acrolein (CH2=CH-CHO) which can be distinguished by its irritating acid smell or as burnt grease, that is it has a very pungent irritating odour.

On Further heating of the test solution it will results in to the polymerization reaction of the aldehyde group that yields acrolein as product, this product formed is demonstrated or shown by the conversion of the reaction mixture into a slight blackish colour.Both the unpleasant or pungent smell and the black color produced on further heating will indicate and detect the presence of glycerol,and thereby fat and/or lecithin. Cholesterol is a compound which gives a negative acrolein test.

Requirements:

1. Materials Required:

• Glassware: 10 mL ,7 boiling test tubes
• Pipettes (1-10 mL)
• Dropper
• Glass rod

Equipment and accessories:

• Bunsen burner
• Test tube holder
• Spatula and
• Blotting paper.

Test sample:

Use these six samples for Acrolein test:

1. (1) lecithin preparation
2. pure cholesterol
3. pure glycerol
4. corn oil
5. butter
6. egg yolk.

Reagents/Chemical Required:

• Potassium bisulphate (KHSO4)
• Or Concentrated H2SO4 (Sulphuric acid).

Procedure of Acrolein test:

The Acrolein Test for Glycerol:-

1. Take seven cleaned and dried test tubes to perform the test. And label these test tubes from 1,2,…..7.
2. Now, Place 1 g of potassium hydrogen sulfate, KHSO4, or Conc. Sulphuric acid in each of the labelled seven clean and dry test tubes.
3. In the very first step, To the labelled 1 test tube, add a few grains or some amount (0.1-0.5g) of pure lecithin; to test tube labelled 2, add a few grains or some amount of cholesterol.
4. And To the next test tubes which are labeled as 3 -6, add dropwise with the help of a dropper that is, put one separate drop which measures approximately 0.1g of glycerol (to the labeled test tube 3), corn oil (to test tube4), butter (to test tube 5), and egg yolk (to test tube 6).
5. In the last test tube which is the seventh test tube, put or add a few crystals of sucrose sample.
6. Set up a Bunsen burner near a hood.

Caution: Always Perform this test near the hood because of the pungent odor of the acrolein aldehyde formed.

7. When the fumes are released from the test tubes to be smelled, keep distance from the nose and do not inhale the fumes directly into the nostril. Move the test tubes sideways under your nose and then smell it or smell it by whirling the vapors toward your nose with a cupped hand.
8. While heating each test tube, be gentle and careful , heat them one at a time, over a Bunsen burner flame, and keep shaking ,and twirling it continuously from side to side. So that the heat is provided equally.
9. When the mixture melts and slightly blackens, you will notice the evolution of fumes. Stop the heating. Carefully smell the test tubes; here pay attention to the Caution provided above.
10. An unpleasant or pungent odor, resembling burnt grease or hamburgers, provides a positive test for glycerol.
11. The Sucrose in the seventh test tube will undergo dehydration by the potassium bisulphate and yields a black color.

Although , it doesn’t provide a positive test for acrolein because the smell in its case is different.

Interpretation of result:

Positive result: on heating ,if the mixture releases fumes having a pungent smell, it implies that glycerol is present in the sample. This is the positive result for the acrolein test.

Negative result: on heating if the mixture does not release or produce any fumes with a pungent smell ,then it implies that glycerol is absent in a sample. Cholesterol gives the negative result for this Acrolein qualitative test.

Precautions:

• Handle the chemicals with care. Do Not inhale chloroform since it has an anaesthetic effect.
• To take the reagents from the bottle always use droppers.
• While performing the experiment always use hand gloves and wear lab aprons.
• Due to the pungent odour of the acrolein Always perform this test near the hood .
• Be careful and never overheat the test tubes, because the residue will become hard,which makes it difficult to clean the test tubes.