Iodine number, Saponification number, Acid Number, Reichert-Meissl (RM) number & Rancidity of Oil/Fats

Acid number iodine number The Pharmapedia
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PROPERTIES OF TRIACYLGLYCEROLS/FAT/OILS/Fatty acids

The Following properties of TGs/Fat/Oil are mainly used to detect adulteration/deterioration of oil/fat.

S. No.PropertiesReactantSample amountTo be determinedUse/ Application
1Iodine valueI2 (No of gm of Iodine absorbed by)100 gm of Fat/oilIodine number  UnsaturationOil/Fat has specific Iodine no value, Use to check Adulteration
2Saponification numberNo of mg of KOH required to hydrolysis/saponify1 gm of fat/oilMeasures the average molecular size of Fatty acids/FA (Higher saponification no for fat/oil containing short chain FAto check Adulteration
3Acid numberNo of mg of KOH required to neutralise free Fatty acidPresent in 1 gm of fat/oilto determine free fatty acid (Rancidity liberate free acid)To check oil decomposition/rancidity/adulteration
4Reichert-Meissl (RM) numberNo of ml of 0.1 N KOH or 1/10 N KOH required to neutralise soluble volatile fatty acid distilledFrom 5 gm fatto determine volatile fatty acidUse to check purity of butter oil since butter oil contain volatile FA.
5Hydroxyl valueNo of mg of KOH required to neutralize acetic acid capable of combining by acetylationWith 1 gm fat/oil  
6Acetyl valueNo of mg of KOH required to neutralize acetic acidWith 1 gm acetylated fat/oil is saponified  
7Ester valueNo of mg of KOH require to combine with fatty acidsIn 1 g oil/fat Difference b/w saponification value & acid value is ester value.
8.Polenski valueNo of 0.1 N KOH require to neutralize water insoluble, steam-distilable acid liberated by hydrolysis of5 gm fat/oil  

1. Iodine Number:

It is defined as the grams (number) of iodine absorbed by 100 g of fat or oil. Iodine number is useful to know the relative unsaturation of fats, and is directly proportional to the content of unsaturated fatty acids. Thus lower is the iodine number, less is the degree of unsaturation.

2. Saponifiction:

The alkaline hydrolysis of triacylglycerols/fatty acid (esters)by alkali (KOH, NaOH) to produce glycerol (alcohol) and soaps is known as saponification. Lipids that contain fatty acid ester linkages can undergo hydrolysis. The mechanism of saponification is:

The reaction is used commercially to make soap, lubricants, and fire extinguishers.

3. Reichert-Meissl (RM) number:

It is defined as the number of ml 0.1 N KOH required to completely neutralize the soluble volatile fatty acids distilled from 5 g fat. RM number is useful in testing the purity of butter since it contains a good concentration of volatile fatty acids (butyric acid, caproic acid and caprylic acid). This is in contrast to other fats and oils which have a negligible amount of volatile fatty acids. Butter has a RM number in the range 25-30, while it is less than 1 for most other edible oils.

4. Acid number:

It is defined as the number of mg of KOH required to completely neutralize free fatty acids present in one gram fat or oil. Generally, refined oils should be free from any free fatty acids. Oils, on decomposition—due to chemical or bacterial contamination—yield free fatty acids. Therefore, oils with increased acid number are unsafe for human consumption.

5. Rancidity:

Unsaturated Fats and oils on exposure to air, moisture, bacteria etc. undergo rancidity (deterioration- unpleasant taste). This can be prevented by the addition of certain antioxidants (vitamin E, hydroquinone, gallic acid). To prevent rancidity of oil/fat, fat soluble anti-oxidant (vitamin E, hydroquinone, gallic acid) are added.

6. Hydroxyl value

No of mg of KOH required to neutralize acetic acid capable of combining by acetylation With 1 gm fat/oil.

7. Acetyl value

No of mg of KOH required to neutralize acetic acid With 1 gm acetylated fat/oil is saponified.

8. Ester value

No of mg of KOH require to combine with fatty acid in 1 gm oil/fat. Difference b/w saponification value & acid value is ester value.

9. Polenski value

No of 0.1 N KOH require to neutralize water insoluble, steam-distilable acid liberated by hydrolysis of 5 gm fat/oil sample.

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