1. Structure, properties, classification and analysisof lipids. 

Lipids are a group of compounds exhibiting intermediate to low polarity and low water solubility, which constitute one of the three main components of living organisms. One of their peculiarities is that when metabolized, they are capable of generating more than twice the amount of energy obtained from equivalent weight of carbohydrates (contributing ca. 39 kJ/kg). Though a large majority of lipids are fatty acid triesters of glycerol, the other lipids' importance is out of proportion with their relative abundance, because of their structural roles, namely at the interfaces separating hydrophylic an hydrophobic parts of the tissues and organs.

Fats and oils are triacylglycerols separated by the slightly arbitrary distinction of solid or liquid state at room temperature. In triacylglycerols of vegetable origin, fatty acids esterified onto position 2 significantly differ from those esterified onto positions 1 and 3 - which exhibit little overall difference in substitution pattern, whereas in products of animal origin random substitution seems to predominate.

Lipids exhibit peculiar melting properties which form the basis for their selection for a number of uses and underlie apparent physiological selectivities. These will be dealt with in section 3.5 .


Lipid ClassComposition
AcylglycerolsMono-, di- and triesters of Glycerol
WaxesFatty acid esters of fatty alcohols
Phosphoacyl glycerols Diacylglycerol and an amine or alcohol esterified to phosphoric acid
Sphyngomielins Sphingosine plus a fatty acid esterified to phosphatidylcholine
Cerebrosides Sphingosine , fatty acid and a hexose
Gangliosides Sphgingosine, a fatty acid and a sialic acid carbohydrate
Steroids C30 Tetracyclic triterpenes
Carotenoids C40 polyunsaturated tetraterpenas
Tocols Tocopherols and tocotrienols, phenolic antioxidants including vitamin E
Other Phytomenadione,...

Analysis of lipids is usually performed by gas chromatography after selective extraction normally performed with chloroform/methanol (2:1 by volume), for instance by thin layer chromatography.

Oil and fat analysis includes routinely the determination of unsaturation, free acidity, unsaponifiable materials, and fatty acid profile, and may include stereospecific analysis. Chromatographic determination of actual triacylglycerols, either by HPLC or GLC methods, is also usual when dealing with solid fats or when analysing oils in search of their genuine origin. In this respect it might also be useful to determine the stereochemical configuration of carbon 2 of glycerol - characterizing the populations of positions 1 and 3 separately in order to find out whether significant differences exist. This may be achieved using a combination of chemical and enzymatic analytical methodologies, which yields the absolute configuration around the carbon atom 2 of glycerol. For this purpose, pancreatic lipase hydrolysis is carried out under controled mild conditions yielding a mixture of 1,2-diacylglycerols, 2,3-diacylglycerols and fatty acids characteristic of positions 1 and 3. Under stronger conditions the 2-acylglycerol is obtained. Fatty acid separation and analysis yields the composition at these two positions, 1+3. The mixture of diacylglycerides from the first reaction is then subject to diacylglycerol kinase phosphorilation, which occurrs given ATP, but specifically at position 3 of the 1,2-diacylglycerol. The phosphorylated diacylglycerol is separated from the 2,3-diacylglycerol. Fatty acid profiles are obtained after chemical ester hydrolysis, derivatisation and chromatography, for position 2 (from the 2-acylglycerol), for 1+2, from the phosphorylated compound, yielding the composition for position 1 by difference, and for position 3 from the 2,3-diacylglycerol, and also from the composition obtained for the fatty acids resulting from lysis of 1+3 ester bonds when preparing the 2-acylglycerol. Selective hydrolysis using some of the phospholipases may also be used to characterize fatty acid composition at position 2 or at position 3.

Unsaturation is usually given as the iodine value, a numerical quantity proportional to the weight of iodine which adds to a given quantity of lipid (generating viccinal diiodo compounds from pre-existing double bonds). For this purpose iodine in solution is allowed to add to the lipid, and the excess is back titrated with thiosulphate.

Free acidity measures the amount of free acids present in the sample, and may further distiguish between free fatty acids and inorganic acidity.

Unsaponifiable content is measured to determine the total content of non acylglycerol material, and is a quantity usually specified for oils and fats.

Fatty acid profile determination is performed by GLC after either saponification and neutralization or, more recently, acid hydrolysis of the oil. In the latter, BF3 or acetyl chloride may be used to liberate the fatty acids.

Physical properties of fats are extremely important. They are the consequence of the triacylglycerol composition, which influences the nature, stability and structure of ordered phases. Influences are felt which relate to the number of carbon atoms, unsaturation and conformation of the fatty acids, others mainly to the triacylglycerol structure itself.

Even numbered fatty acid moieties, the most abundant ones throughout, tend to show higher solid/liquid transition temperatures due to higher Van der Waals interactions per unit weight. Saturated fatty acid residues tend to adopt mainly a zigzag type of conformation when in the crystal lattice of triacylglycerols, and the inclusion of a trans double bond does not significatively affect this situation, despite the slight shortening of the carbon chain which it entails. The existence of a cis double bond introduces a bending of the chain, and lowers transition temperatures. Much the same situation arises in the somewhat simpler situation of fatty acid crystals, themselves, and is illustrated by the relative values of stearic acid (69.6 °C), which is the saturated 18 carbon atom fatty acid often abbreviated as (18:0), which compares with 62.9 ºC for palmitic acid (16:0) and only 61.3 for margaric acid (17:0), and also with 13.4 °C for oleic acid (18:1, (9-cis)), and 46 °C for elaidic acid(18:1, (9-trans)). This feature is even stronger when various isolenic cis double bonds are present,as in a-linoleic acid (18:2, (9-cis, 12-cis) which melts at -5 °C and a-linolenic acid (18:3, (9-cis, 12-cis, 15-cis)), which melts at -11 °C.

A special positional identification system for the cis double bonds is commonly used for lipids.

This system is based upon the distance to a double bond when starting from the methyl group end of the fatty acid. Thus w3 C18 acids are those which have a double bond linking carbons 15 and 16, such as a -linolenic acid

Despite the fact that this nomenclature differs from the IUPAC nomenclature, it is useful especially because the number of carbon atoms which separate the methyl group from the double bond is related to the physiological role of the fatty acid, apparently, and also indicates common biosynthesis. The role of w6 acids such as linoleic (9-cis,12-cis-octadienoic), and g-linolenic 6-cis,9-cis,12-cis-octatrienoic) is different from that of eicosapentaenoic acid (6-cis,9-cis,12-cis,15-cis, 18-cis -eicosapentaenoic acid) or of its twenty two carbon atom homologue docosahexaenoic acid, both of which are important fish oil components and which belong to the w3 family together with a-linolenic acid. Oleic acid (9-cis-octenoic acid) belongs to the group of w9 acids.

The patterns of distribuition of the various fatty acid residues among the glycerol positions also critically affect the crystallisation behaviour, polymorfism, compatibility and rheology of each particular fat. This topic bears importance, for instance in determining the plausibility of using a given triacylglycerol moiety as a replacement for the expensive cocoa butter, but outside the scope of this work.


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