Unsaturated Fatty Acids - Important Components of Plant Oils

Demonstration Experiment on Video

Unsaturated Fatty Acids - Important Components of Plant Oils

Objetive: Test for the Unsaturated Character

Peter Keusch

German version

Supermarket products:
safflower oil (Distel oil)
palmin
vinegar essence
starch
Betaisodona® solution

Apparatus and glass wares:
2 test tubes 200 mm × 20 mm
test tube rack
4 beakers 100 mL
watching glass
2 rubber stoppers for test tubes

Experimental procedure:

One test tube is filled with 20 mL of Distel oil. Palmin is placed in a second test tube. 75 mL of acetic acid (10 mL of vinegar essence mixed with 65 mL of water) and some drops of Betaisodona® solution are added to each of the two test tubes. The stoppered test tubes are shaken. After some minutes a spatula full of starch powder is added to each of the two test tubes. The test tubes are again shaken.

Results:

The sample containing palmin turns intensely blue (iodine-starch complex). In Distel oil all the iodine will be consumed. Consequently no color change can be observed after the addition of starch.

Video clip (Download RealPlayer .rm file)

Discussion and background:

Distel oil contains unsaturated fatty acids. Iodine adds across the double bonds in an unsaturated bond. Palmin consists mainly of glycerides from saturated fatty acids (e.g. palmitic and lauric acid).

Fats and oils are more correctly called triglycerides. Triglycerides contain three fatty acids esterified with glycerol.

As a general rule animal fats are saturated and vegetable fats are unsaturated with some exceptions. The most common fatty acids found in animal fats are butyric and stearic acid. Most of the plant fats are rich in the unsaturated fatty acids, linoleic and oleic acid, which the body cannot manufacture. For this reason, monosaturated fatty acids are called essentially fatty acids. Plant oils, such as safflower, linseed, sun flower and corn oil contain polysunsaturated (two and more double bonds) fatty acids.

The more unsaturated and the longer are the fatty acids, containing in a fat, the more fluid it will be in its consistence. Distel oil has a high fraction of monounsaturated and polyunsaturated fatty acids.

The iodine number indicates level of unsaturation. The iodine number of a fat is the number of grams of iodine that will just still be consumed (decolored) by 100 g of fat or oil. The more double bonds a fat contains, the more iodine is required for the addition reaction; thus, a high iodine number means a high degree of unsaturation. In order to determine the iodine number, a certain amount of iodine bromide is added in excess to the fat. IBr reacts with the double bonds through an addition reaction. When reaction is complete an appropriate amount of potassium iodide is added. Iodide reacts with unreacted iodine bromide to form bromide and iodine. The released iodine is determined by titration with standardized sodium thiosulfate using a starch indicator.

Palmin is a 100 % plant source (coconut fat). But the fat is "hardened" by hydrogenation in the presence of a nickel catalyst. For this reason the added iodine is not completely consumed in the above described experiment. It reacts with starch to form the blue iodine-starch complex.

Reference:
Oil Quality

Index of Lecture Experiments