What are the common chemical tests of oils and fats and their principles
Fats are molecules made up of a fatty acid chain with a glycerol head. There are several different kinds of fats determined by the number of carbon atoms in the fatty acid chain and a number of double bonds in the chain. It is a subgroup of lipids. The principal difference between fats and oils is that fats are composed of high amounts of saturated fatty acids which take a solid form at room temperature whereas oils are composed of mainly unsaturated fatty acids which take a liquid form at room temperature. In this article, we will discuss the various tests of oils and fats.
Analysis of Fats and Oils
Fats and oils can be tested by using the following methods:
1. Acrolein Test
In the acrolein test, we take the given food sample, prepare the extract of the given food sample. Then add potassium bisulphite to the above extract. A Pungent irritating odour evolves to form the solution confirms the presence of Fats in the food.
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2. Baudouin Test
This test is used to identify the adulterant (vanaspati ghee) in the desi ghee. The food sample is treated with the mixture, containing 5 ml hydrochloric acid, 2% fufral solution in the alcohol. A rose-red colour appears after some time (around 5-10 minutes) in the solution confirms the presence of vanaspati in the desi ghee.
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3. Translucent Spot Test
Take the food sample and place it between the folds of filter paper and rub it lightly. Presence of translucent spots on the filter paper confirms the presence of fats in foodstuff.
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4. Solubility Test
Take the given food sample, prepare the extract of the given food sample. Then add the above extract to the chloroform solution or alcohol solution. If the extract gets dissolved into the solution, it confirms the presence of Fats in the food.
By using the above methods, the analysis of fats and oils can be done.
Did You Know?
Our nervous system is covered with a sheet of fat.
Vitamin A, D, E, K are fat containing vitamins.
Fats play an important role in the brain development of children.
Unsaturated fats are healthier than saturated fats.
FAQs on Tests of Oils and Fats in Chemistry
1. What are the tests of oils and fats in chemistry?
The tests of oils and fats are chemical and physical methods used to determine their quality, purity, degree of unsaturation, and extent of rancidity. These tests are commonly used in organic chemistry and food analysis.
- Acid value – measures free fatty acids (rancidity).
- Saponification value – indicates average molecular weight of fatty acids.
- Iodine value – measures degree of unsaturation.
- Reichert–Meissl value – detects volatile fatty acids (butter analysis).
- Peroxide value – measures oxidative rancidity.
2. What is the acid value of oils and fats?
The acid value is the number of milligrams of KOH required to neutralize the free fatty acids present in 1 g of oil or fat. It indicates the extent of hydrolytic rancidity.
- Formula: Acid value = (mg of KOH used) / (g of sample).
- High acid value → more free fatty acids → spoilage.
- Determined by titration with standard KOH using phenolphthalein indicator.
3. What is the saponification value of oils and fats?
The saponification value is the number of milligrams of KOH required to saponify 1 g of fat or oil. It provides information about the average molecular weight of the fatty acids present.
- High saponification value → shorter chain fatty acids.
- Low saponification value → longer chain fatty acids.
- Based on the hydrolysis reaction:
Fat + 3KOH(aq) → Glycerol + 3RCOOK(aq)
4. What is the iodine value of oils and fats?
The iodine value is the number of grams of iodine absorbed by 100 g of fat or oil and measures the degree of unsaturation. It indicates the number of carbon–carbon double bonds (C=C) present.
- High iodine value → more unsaturated (e.g., vegetable oils).
- Low iodine value → more saturated (e.g., butter, ghee).
- Unsaturation reaction (addition):
RCH=CHR + I2 → RCHI–CHRI
5. What is the peroxide value test for oils?
The peroxide value measures the amount of peroxide (–O–O–) oxygen per kilogram of oil and indicates oxidative rancidity. It shows the extent of lipid oxidation during storage.
- Expressed in milliequivalents of active oxygen per kg of oil.
- High peroxide value → oil is becoming rancid.
- Based on liberation of iodine from potassium iodide by peroxides, followed by titration with sodium thiosulfate.
6. What is the Reichert–Meissl value in fat analysis?
The Reichert–Meissl (RM) value measures the amount of water-soluble volatile fatty acids in fats, especially butter. It helps detect adulteration of butter with vegetable oils.
- Expressed as mL of 0.1 N alkali required to neutralize volatile fatty acids from 5 g of fat.
- High RM value → presence of short-chain fatty acids (e.g., butyric acid in butter).
- Low RM value → possible adulteration.
7. How do you test for unsaturation in oils and fats?
Unsaturation in oils and fats is tested using the bromine test or iodine value test, which detect carbon–carbon double bonds (C=C).
- Bromine test: Decolorization of bromine solution indicates unsaturation.
Reaction: RCH=CHR + Br2 → RCHBr–CHBrR - Iodine test: Iodine adds across double bonds; amount absorbed gives iodine value.
8. What is the difference between acid value and saponification value?
The acid value measures free fatty acids present, while the saponification value measures the total fatty acids (free and combined) in the form of triglycerides.
- Acid value: Indicates rancidity and spoilage.
- Saponification value: Indicates average molecular weight of fatty acids.
- Acid value involves neutralization only.
- Saponification value involves complete hydrolysis of ester bonds.
9. How is rancidity detected in oils and fats?
Rancidity in oils and fats is detected by measuring the acid value (hydrolytic rancidity) and peroxide value (oxidative rancidity).
- Increase in acid value → formation of free fatty acids.
- Increase in peroxide value → formation of peroxides due to oxidation.
- Rancid oils often develop unpleasant odor and taste.
10. Why is the iodine value important in classifying oils?
The iodine value is important because it classifies oils based on their degree of unsaturation into drying, semi-drying, and non-drying oils.
- Drying oils: High iodine value (>130), e.g., linseed oil.
- Semi-drying oils: Moderate iodine value (100–130).
- Non-drying oils: Low iodine value (<100), e.g., coconut oil.
