Iodine number

The iodine number (IZ) is a fat number used to characterize fats and oils . It is a measure of the amount of unsaturated compounds in a fat - more precisely: unsaturated fatty acid residues in the glycerides . It is the amount in grams of iodine that can formally be added to 100 g of fat. The more olefinic double bonds (unsaturated compounds) there are in a fat, the more iodine can formally be added and thus the higher the iodine number. Other olefinic accompanying substances such as sterols are also recorded.

The method can be used for identification and quality control. Lipids are classified according to their degree of saturation, since this is decisive for aging ("drying") during storage; Fats and oils with a higher iodine number age faster.

When unsaturated fats are heated (e.g. when deep-frying ), the iodine number decreases and their viscosity increases due to polymerization.

Classification

Classification

IZ	Type	Examples
<10	not drying	saturated oils, beeswax , coconut fat , coconut oil , neutral oil
010- 020		Palm kernel oil , babassu oil , murumuru fat
020- 050		Beef suet , butter , cocoa butter
050-100		Olive oil , avocado oil , jojoba oil , almond oil , palm oil , shea butter
100-130	half drying	Corn oil , sunflower oil , rapeseed oil , sesame oil , wheat germ oil
130-170		Soybean oil , peanut oil , safflower oil , hemp oil , poppy seed oil , grape seed oil , walnut oil , wild rose oil , blackcurrant seed oil , kukui nut oil
000> 170	drying	Linseed oil (169-192), perilla oil , Isanoöl , tung oil , stillingia oil , Lallemantiaöl

Significance for the properties of fats

Fatty and fatty oils always consist of a mixture of fatty acid glycerides. Their properties are therefore determined proportionally from the sum of the fatty acid components occurring in them.

Saturated fatty acids do not contain any carbon-carbon double bonds and therefore have a low iodine number. These fats are good for making soap .

The higher the proportion of unsaturated fatty acid residues among the fatty acid residues of the triglycerides, the higher the iodine number and the more the oils tend to crosslink ( polymerize ). Easily polymerizing oils such as linseed oil have an iodine number of more than 140 and are called drying or hardening oils. Semi-drying oils have an iodine number between 100 and 140; for non-drying oils it is below 100.

Methods

Hübl

The iodine number was introduced by Arthur von Hübl , who titrated fats with iodine in the presence of mercury chloride, but the actual reagent (probably iodine chloride) is formed in situ from mercury chloride and iodine. Pure iodine does not attach itself to alkenes, which is why the definition of the iodine number that is still valid today is only a formal one.

Above, a triglyceride in an oil with a saturated fatty acid residue marked blue , a monounsaturated fatty acid residue marked green and a triple unsaturated fatty acid residue marked red . The triple acylated glycerine ( marked in black ) can be seen in the center of the structural formula above . Such a triglyceride has a high iodine value . - Below is the reaction product after adding four equivalents of iodine chloride or bromine to the four C = C double bonds of the unsaturated fatty acid residues.

Wijs

The Wijs iodine number is determined in accordance with DIN 53241-1: 1995-05 (withdrawn) or EN 14111: 2003. It is based on the addition of iodine monochloride to the olefinic double bond .

The sample is dissolved in chloroform (or another organic solvent , e.g. cyclohexane ) and then an excess of iodine solution according to Wijs is added. The iodine solution according to Wijs contains an organic solvent, pure acetic acid (glacial acetic acid), elemental iodine and iodine trichloride . Iodine and iodine trichloride are completely converted into iodine monochloride in the solution.

${\displaystyle \mathrm {ICl_{3}+I_{2}\rightarrow 3\ ICl} }$

It is important that the iodine solution is anhydrous, as iodine monochloride breaks down in water into hydrogen chloride , iodine and iodic acid . In order to rule out the formation of iodine radicals and thus a falsification of the result, the sample is kept in the dark after the iodine solution has been added. There is an electrophilic addition reaction of iodine monochloride to the double bond.

${\displaystyle \mathrm {R_{1}{-}CH{=}CH{-}R_{2}+ICl\rightarrow R_{1}{-}CHI{-}CHCl{-}R_{2}} }$

The remaining iodine monochloride is then reacted with potassium iodide .

${\displaystyle \mathrm {ICl+KI\rightarrow I_{2}+KCl} }$

The resulting iodine is titrated with sodium thiosulphate and starch as an indicator.

${\displaystyle \mathrm {I_{2}+2\ Na_{2}S_{2}O_{3}\rightarrow 2\ NaI+Na_{2}S_{4}O_{6}} }$

Merchant

( Bromination of the double bonds with exclusion of light, reduction of the excess bromine with iodide , back titration of the iodine with thiosulphate )

When determining the iodine number according to HP Kaufmann , the fat is mixed with an excess of bromine. Here, bromine is added to the double bonds in the unsaturated fats. This reaction has to be carried out in the dark, as this prevents the formation of bromine radicals through light. This would lead to undesirable side reactions and thus to a result which would adulterate the excess consumption of bromine.

${\displaystyle \mathrm {Br_{2}+\ R_{1}{-}CH{=}CH{-}R_{2}\longrightarrow R_{1}{-}CHBr{-}CHBr{-}R_{2}} }$

Then the unused bromine is reduced to the bromide with iodide.

${\displaystyle \mathrm {Br_{2}+2\ I^{-}\longrightarrow I_{2}+2\ Br^{-}} }$

The amount of iodine formed is then determined by titration with sodium thiosulphate solution.

${\displaystyle \mathrm {I_{2}+2\ S_{2}O_{3}^{2-}\longrightarrow 2\ I^{-}+\ S_{4}O_{6}^{2-}} }$

Individual evidence

1. ^ Siegfried Hauptmann : Organic Chemistry , 2nd revised edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1985, p. 654, ISBN 3-342-00280-8 .
2. ↑ Reinhard Mattisek, Gabriele Steiner, Markus Fischer: Food Analysis . 4th edition. Springer, Berlin 2010, ISBN 978-3-540-92205-6 .
3. ↑ Information on the iodine number in the food dictionary from Lebensmittelwissen.de .
4. ^ Alfred Thomas: Fats and Fatty Oils . In: Wiley-VCH (Ed.): Ullmann's Encyclopedia of Industrial Chemistry . Weinheim 2002, doi : 10.1002 / 14356007.a10_173 .
5. ↑ Iodine number, HLB value, type, riding behavior of vegetable oils. In: olionatura.de. Heike Käser, archived from the original on March 21, 2017 ; accessed on March 28, 2017 . 
6. ↑ fats and oils. (PDF; 85 kB) In: ichemlab.at. Retrieved March 28, 2017 . 
7. ↑ Grape seed oil (Vitis Vinifera (Grape) Seed Oil). In: olionatura.de. Heike Käser, accessed on March 28, 2017 . 
8. ^ Fatty Acids and Properties of Oils Chart | SAP Values ​​| Iodine Value. In: thesoapdish.com. Retrieved March 28, 2017 (English). 
9. ↑ Walnut Kernel Oil (Juglans Regia (Walnut) Oil). (No longer available online.) In: olionatura.de. Heike Käser, archived from the original on March 21, 2017 ; accessed on March 28, 2017 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  

Web links

Wikibooks: Internship in food chemistry / iodine value  - learning and teaching materials