Analysis of dietary fats
THE DEGREE OF UNSATURATION: CHARACTERISTIC OF THE COMPOSITION
The iodine value indicates the degree of unsaturation of a fat. It is used to determine an average number of double bonds present in the constituent fatty acids.
Even if iodine, which binds to the double bonds too slowly, is replaced in practice with a halide (iodine chloride), the usual procedure is to express the unsaturation by the amount of iodine (I2) that can bind the fat analyzed.
The iodine value (IV) corresponds to the number of g of bonded iodine per 100 g of fat. It is expressed in g/100 g.
DEGREE OF UNSATURATION: CHARACTERISTIC OF THE COMPOSITION
PrincipleThe determination of the iodine value is based on the dissolution of the fat in a solvent (cyclohexane/glacial acetic acid) and the addition of Wijs reagent (iodine chloride). After a given time, potassium iodine and water are added. Then, the released iodine is titrated with a solution of sodium thiosulfate.
R1-CH=CH-R2 + ICi → R1-CHI-CHCi-R2 + ICiexcess
ICiexcess + KI → I2 + KCi
I2 + 2 S2032- → 2 I- + S4062-
with R1-CH=CH-R2: unsaturated fatty acid (free or esterified)
Method adapted from the NF EN ISO 3961 standard ("iodine value")
DEGREE OF UNSATURATION: CHARACTERISTIC OF THE COMPOSITION
Equipment & Reagents
- Standard laboratory equipment, in particular: analytical scale, magnetic stirrer, burette, glassware.
- Cyclohexane/Glacial acetic acid (50/50, v/v)
- Wijs reagents (iodine chloride)
- Aqueous solution of potassium iodine at 100g/L (to be prepared extemporaneously)
- Starch in solution (mix 5 g of soluble starch with 30 mL of water and add this mixture to 1 L of boiling water; boil for 3 mins, then leave to cool down).
- Sodium thiosulfate 0.1 mol/L (aqueous solution)
Method adapted from the NF EN ISO 3961 standard ("iodine value")
DEGREE OF UNSATURATION: CHARACTERISTIC OF THE COMPOSITION
Experimental procedure- Accurately weigh 0.2 g of fat in a 250 mL Erlenmeyer flask with ground-glass stopper (weight adapted to most vegetable oils) and solubilize in a mix of cyclohexane/glacial acetic acid (50/50, v/v).
- Add exactly 25 mL of Wijs reagent. Do not pipette this extremely corrosive reagent, pour it using a burette (under the fume hood).
- Cork, shake and place the Erlenmeyer flask in the dark for 1 hour.
- Add 20 mL of a freshly prepared potassium iodine solution at 100 g/L, then 150 mL of water. The formation of two immiscible phases can be observed.
- Titrate the released iodine with the sodium thiosulfate 0.1 mol/L by vigorously shaking the corked Erlenmeyer flask so as to mix the two phases. Once the yellow color due to the iodine has nearly disappeared, add a few drops of the starch solution and continue titrating until the blue tone disappears.
- Perform a blank (free of fats) under the same conditions.
≈ 2 hours / sampleMethod adapted from the NF EN ISO 3961 standard ("iodine value")