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analyzed using GC. The FAMES are dissolved in a suitable organic solvent that is then
injected into a GC injection chamber. The sample is heated in the injection chamber to
volatilize the FAMES and then carried into the separating column by a heated carrier
gas. As the FAMES pass through the column they are separated into a number of peaks
based on differences in their molecular weights and polarities, which are quantified
using a suitable detector. Determination of the total fatty acid profile allows one to
calculate the type and concentration of fatty acids present in the original lipid sample.
5.5.4. Chemical Techniques
A number of chemical methods have been developed to provide information
about the type of lipids present in edible fats and oils. These techniques are much
cruder than chromatography techniques, because they only give information about the
average properties of the lipid components present, e.g. the average molecular weight,
degree of unsaturation or amount of acids present. Nevertheless, they are simple to
perform and do not require expensive apparatus, and so they are widely used in
industry and research.
Iodine Value
The iodine value (IV) gives a measure of the average degree of unsaturation of
a lipid: the higher the iodine value, the greater the number of C=C double bonds. By
definition the iodine value is expressed as the grams of iodine absorbed per 100g of
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lipid. One of the most commonly used methods for determining the iodine value of
lipids is "Wijs method". The lipid to be analyzed is weighed and dissolved in a suitable
organic solvent, to which a known excess of iodine chloride is added. Some of the ICl
reacts with the double bonds in the unsaturated lipids, while the rest remains:
R-CH=CH-R + ICl ’! R-CHI-CHCl-R + ICl
excess remaining
The amount of ICl that has reacted is determined by measuring the amount of
ICl remaining after the reaction has gone to completion (ICl =ICl - ICl ).
reacted excess remaining
The amount of ICl remaining is determined by adding excess potassium iodide to the
solution to liberate iodine, and then titrating with a sodium thiosulfate (Na S O )
2 2 3
solution in the presence of starch to determine the concentration of iodine released:
ICl + 2KI ’! KCl + KI + I
remaining 2
I + starch + 2Na S O (blue) ’! 2NaI + starch + Na S O (colorless)
2 2 2 3 2 4 6
Iodine itself has a reddish brown color, but this is often not intense enough to
be used as a good indication of the end-point of the reaction. For this reason, starch is
usually used as an indicator because it forms a molecular complex with the iodine that
has a deep blue color. Initially, starch is added to the solution that contains the iodine
and the solution goes a dark blue. Then, the solution is titrated with a sodium
thiosulfate solution of known molarity. While there is any I remaining in the solution
2
it stays blue, but once all of the I has been converted to I- it turns colorless. Thus, a
2
change in solution appearance from blue to colorless can be used as the end-point of
the titration.
The concentration of C=C in the original sample can therefore be calculated by
measuring the amount of sodium thiosulfate needed to complete the titration. The
higher the degree of unsaturation, the more iodine absorbed, and the higher the iodine
value. The iodine value is used to obtain a measure of the average degree of
unsaturation of oils, and to follow processes such as hydrogenation and oxidation that
involve changes in the degree of unsaturation.
Saponification Number
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The saponification number is a measure of the average molecular weight of
the triacylglycerols in a sample. Saponification is the process of breaking down a
neutral fat into glycerol and fatty acids by treatment with alkali:
Triacylglycerol + 3 KOH ’! Glycerol + 3 Fatty acid salts of potassium
The saponification number is defined as the mg of KOH required to saponify
one gram of fat. The lipid is first extracted and then dissolved in an ethanol solution
which contains a known excess of KOH. This solution is then heated so that the
reaction goes to completion. The unreacted KOH is then determined by adding an
indicator and titrating the sample with HCl. The saponification number is then
calculated from a knowledge of the weight of sample and the amount of KOH which
reacted. The smaller the saponification number the larger the average molecular weight
of the triacylglycerols present.
Acid value
The acid value is a measure of the amount of free acids present in a given
amount of fat. The lipids are extracted from the food sample and then dissolved in an
ethanol solution containing an indicator. This solution is then titrated with alkali
(KOH) until a pinkish color appears. The acid value is defined as the mg of KOH
necessary to neutralize the fatty acids present in 1g of lipid. The acid value may be
overestimated if other acid components are present in the system, e.g. amino acids or
acid phosphates. The acid value is often a good measure of the break down of the [ Pobierz całość w formacie PDF ]