Determination of iodine value of fats and oils

The iodine value is a measure of the unsaturation present in fats and oils, particularly the degree of unsaturation of fatty acids within them.

The quantity of iodine that reacts with 100 grams of fat or oil determines the iodine value. Higher the iodine value, the higher the concentration of unsaturated fatty acids in the sample. Unsaturated fatty acids have double bonds between carbon atoms, which can react with iodine to produce a greater iodine value. Saturated fatty acids, on the other hand, lack double bonds and do not react with iodine.

It is an important parameter used to evaluate the quality and content of edible fats and oils, including vegetable oils, animal fats, and fat-containing processed foods.

Principle of Iodine value determination

It is defined as the amount of iodine in 100 grams of fats or oils that will react with the double bonds. Saturated and unsaturated fatty acids can be found in fat or oil. When halogens are introduced across the double bonds of unsaturated fatty acids, they generate new molecules. Allowing iodine monochloride or monobromide to react with the oil in the dark allows the iodine to be incorporated into the fatty acid chain where double bonds are present.

Iodine consumption is directly proportional to the amount of double bonds present. Titrating the sample with standard sodium thiosulphate and using starch as an indicator, the quantity of unused iodine monochloride or monobromide is calculated from iodine released on addition of excess potassium iodide. The degree of unsaturation is determined by the amount of iodine absorbed by the oil or fat throughout the process. Iodine value or number is a useful parameter in studying oxidative rancidity of oil since higher the unsaturation, the greater is the possibility of the oil to become oxidized by the atmospheric oxygen.

Apparatus required

• Balance machine
• Burette with stand
• Measuring cylinder
• IV flask
• Spatula
• Filler
• Pipette
• Filter paper
• Funnel
• Beaker
• Hot plate

Chemical and reagents

• Starch soluble
• Potassium iodide
• Sodium thiosulphate
• Carbon tetrachloride
• Wij’s solution

Chemical preparation

 Step-1 

1% starch solution

How is 1% starch solution prepared?

• Measure 500ml distilled water and take it into a beaker.
• Place the beaker on a hot plate and boil the water.
• Now, take a weight of 0.5g of starch solution.
• Transfer the weight starch into the boiling water.
• While boiling, stir the starch solution with a glass rod to dissolve it in water.
• A clean and transparent solution indicates that the starch has been completely dissolved.
• Prepare the filter paper by placing it in a funnel.
• Now, filter the hot starch solution.
• After 30 minutes, remove the filtrate.
• The 1% starch solution is ready for use as an indicator in the titration.

0.1N Sodium thiosulphate

How is 0.1N Sodium thiosulphate prepared?

• Calculate the weight of 2.5g of sodium thiosulphate crystal.
• Label a 100ml volumetric flask with 0.1N sodium thiosulfate.
• Fill the flask with approx.80ml of distilled water.
• Transfer the weight sodium thiosulfate into the flask.
• To dissolve the chemical in water, shake the flask.
• Heat the flask for 10 minutes to thoroughly dissolve the sodium thiosulfate
• After 10 minutes, turn off the heat and let the flask to cool to room temperature.
• After the flask has cooled, add water to make the final capacity of 100ml.
• Standardize the solution before used.

Step-2

Sample and blank preparation

• Take 2.5 to 3.1 g of oil sample into IV flask (expected IV of our sample is 10).
• Note the sample weight.
• Prepare a blank IV flask without taking sample into it.
• Pipette 25ml carbon tetrachloride and pour into the sample flask.
• Close the flask with its stopper immediately.
• Pipette 25ml carbon tetrachloride and pour into the blank flask.
• Close the flask with its stopper immediately.
• Pipette 25ml Wijs solution and pour into the sample flask.
• Close the flask with its stopper immediately.
• Pipette 25ml Wijs solution and pour into the blank flask.
• Close the flask with its stopper immediately.
• Shake and rotate both flasks together for proper mixing.
• Add potassium iodide crystal on all round surface of the stopper.
• Keep both flask in dark place for 30min.

Step-3

• After 30min titration will start.
• Take a standardized 0.1N sodium thiosulfate solution in a burette.
• Note the initial reading.
• Measure 100ml of distilled water.
• Pour the water into the sample flask washing the stopper into the flask.
• Shake and rotate the flask for proper mixing.
• Measure 1ml or 1% starch solution and keep ready to use later.
• Start titration using 0.1N sodium thiosulfate.
• Again add 1% starch solution when the solution color is changing to lighter.
• Shake the flask and resume the titration.
• Milky white color solution indicates the end -points of the titration.
• Shake the flask vigorously, if the blue color comes back and titrate again.
• Titration of sample is done.
• Note the final burette reading.
• Start titration of blank in the same manner as like as sample titration.
• Note the initial burette reading.
• Add 100ml distilled water washing the stopper and KI into the flask.
• Shake and rotate the flask for proper mixing.
• Measure 1ml or 1% starch solution and keep ready to use later.
• Start titration using 0.1N sodium thiosulfate.
• Again add 1% starch solution when the solution color is changing to lighter.
• Shake the flask and resume the titration.
• Milky white color solution indicates the end -points of the titration.
• Shake the flask vigorously, if the blue color comes back and titrate again.
• Titration of blank is also done.
• Note the final burette reading.

Step-4 

Calculation

Iodine value = 

Where, 

• WS= Sample weight
• VB= volume of sodium thiosulfate for blank
• VS= volume of sodium thiosulfate for sample
• N= normality of sodium thiosulfate

Approximate iodine values for some common fats and oils:

Application of iodine value in oils and fats

The iodine value is widely used in the food industry and has several applications in the analysis and characterization of fats and oils. Among its primary applications are:

Quality evaluation: 

The iodine value is used to evaluate the quality of fats and oils. It indicates the degree of unsaturation, which might affect the flavor, fragrance, and stability of the product. Oils with greater iodine concentrations are more prone to oxidation, which causes rancidity and off-flavors. The iodine value can be used to assess the quality and possible shelf life of fats and oils.

Detection of authenticity and adulteration: 

The iodine value may be used to determine the validity of fats and oils as well as detect potential adulteration. Iodine levels vary according on source and processing technique. Deviations from predicted values might indicate the presence of adulterants or the application of alternative oils.

Formulation of food products: 

The iodine value is applied in food product formulation to obtain desired qualities. It helps in the selection of suitable fats or oils depending on their degree of unsaturation. Oils with greater iodine levels, for example, may be favored for goods requiring a longer shelf life, whilst oils with lower values may be utilized for applications requiring stability at higher temperatures.

Oil blending: 

The iodine value may be used to improve the efficiency of oil blending processes.  Manufacturers can acquire the appropriate iodine value for a certain application or product by understanding the iodine values of different oils and blending them in specified ratios.

Research and development: 

The iodine value is an important parameter in fats and oils research and development. It facilitates in the investigation of the chemical and physical characteristics of various oils, as well as their behavior during processing, storage, and use.

Regulatory compliance: 

The iodine value is frequently mentioned in quality standards and regulations for fats and oils. Manufacturers must verify that their goods fulfill the stipulated iodine value criteria in order to comply with regulatory standards and labeling regulations.

References

1. Lívia C. Tomaszewski, et al. (2021). Determination of Iodine Value in Edible Oils and Fats: A Review. Molecules, 26(2), 346. doi: 10.3390/molecules26020346
2. AOCS Official Method Cd 1c-85. (2017). Iodine Value of Fats and Oils. Official Methods and Recommended Practices of the AOCS, 7th edition.
3. Talwalkar, A., & Dandekar, S. (2016). Analysis of Fats and Oils. In Food Analysis (pp. 433-434). Academic Press.
4. Pomeranz, Y. (2018). Fats and Oils. In Pomeranz’s Handbook of Food Analysis (pp. 318-319). CRC Press.
5. United States Pharmacopeia. (2019). Iodine Value Determination. In USP 42-NF 37, General Chapter <616>.

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