Fatty acids and lipids

Overview

Lipids are a very heterogeneous group of compounds belonging to the constitution of living beings and have the common property of being insoluble in water (Lipos) and soluble in apolar organic solvents such as hexane, benzene, chloroform and ether.

In the body, lipids have 4 main functions:

Reserve of energy stored as triglycerides in adipose tissue, lipids are an energy reserve mobilization (1 gram of fat provides approximately 9.3 kcal).

A structural role: the fatty acids used for synthesis of other lipids, including phospholipids that form the membranes around cells and organelles. The fatty acid composition of the phospholipid membrane provides special physical properties (elasticity, viscosity).

A messenger role of the fatty acids are precursors of several messengers intra and extra-cellular. For example, arachidonic acid is the precursor of eicosanoids, hormones involved in inflammation, blood clotting, etc..

A transport role of vitamins: fat food convey four fat-soluble vitamins A, D, E and K.

In camels and dromedaries, lipids constitute a reserve of water for these animals: the degradation of lipids stored as triglycerides in the bosses of these animals leads to the formation of water.

Classification of lipids

Lipids are classified into two broad categories: lipids based on the fatty acid and lipid basis of isoprene (polyisoprene lipids).

In the category of lipid-based fatty acids are the fatty acids themselves, and that two families named simple lipids and complex lipids. Simple lipids include glycerides, and the CERID sterides. Lipids complex lipids designate phosphorus, nitrogen and lipids lipids sulfur. Each of these lipid families is itself divided into several classes of lipid compounds grouped by their structural homology .

The lipid-based fatty acids are also called "saponifiable lipids (lipids treated with NaOH or KOH, give soap). In contrast, lipids do not lead to the formation of soap by alkaline treatment are called "unsaponifiable lipids." This last group contains lipids polyisoprene which are divided into three categories: terpenoids, carotenoids, quinones and isoprene chain steroids.

Fatty acids

Fatty acids are carboxylic acids, aliphatic carbon chain more or less derived from or contained in animal fats and vegetable. By extension, the term is sometimes used to designate all the carboxylic acids in non-cyclic carbon chain.

The fatty acids generally have a sour taste and a pronounced odor. They are insoluble in water but soluble in these organic solvents such as ether.

Fatty acids differ between them by the length of the carbon chain (4 to 18 carbon atoms for the fatty acids known, usually a pair) and the type of bonds that meet their carbon atoms: one says that they are saturated when containing only single carbon-carbon, unsaturated, and when at least one double bond carbon = carbon.

Despite their quantitative importance as constituents of lipids, they are in very small quantities in the free state. In the human body, the essential fatty acids derived from enzymatic degradation (enzymes with lipase) of lipids in the digestive system.

There is a quarantine of natural fatty acids, the most important are the butyric acid (or butano), found in butter, the palmitic acid (palm oil), the stearic acid (tallow) , the linoleic acid (peanut oil) which is derived the arachidonic acid and the linolenic acid (borage oil). The linoleic and linolenic acids are called essential fatty acids: the animals are unable to synthesize and therefore must necessarily be found in their diet.

In industry, the fatty acids are produced by the hydrolysis of ester bonds of triglycerides (lipids made of glycerol and three fatty acids).

Saturated fatty acids

A saturated fatty acid is a fatty acid fully saturated hydrogen bonds between the carbons are simple (no double bonds). Saturated fats are usually solid at room temperature (as fat) with the exception of the butyric acid (C₄H₈O₂) and caproic (C₆H₁₂O₂). They are found in foods of animal origin such as butter, milk and cheese.

Saturated fatty acids have the general chemical formula: H₃C - [CH₂]_n- COOH where n is an integer equal to or greater than 2.

Each saturated fatty acid in general has two names: a common name that recalls its origin (eg, caproic acid which is found in goat's milk, its name is derived from the Latin word "capra" means goat) and a systematic name that describes its structure (number of carbons, number of unsaturation, etc.). and from the chemical nomenclature (Table 1). In addition, a classification often used in physiology and biochemistry, which is based on the number of carbon atoms and the number of unsaturation (eg fatty acid Cx: 0 where x indicates the number of atoms carbon and 0 indicates that there is a double zero carbon = carbon and therefore, the saturated fatty acid).

Unsaturated fatty acids

A fatty acid is an unsaturated fatty acid containing one or more unsaturation (presence of double carbon = carbon). It is monounsaturated if it contains one double bond and carbon = carbon polyunsaturated if it contains two or more carbon double bonds = carbon. The presence of a double bond in a fatty acid causes a cis-trans isomerism.

 H       H                   
  \     /
   C = C      
  /     \
 CH3     COOH

The 2 hydrogens are on the same side

 H       COOH 
  \     /
   C = C      
  /     \                    
 CH3     H
The 2 hydrogens are opposed

The fatty acids are generally natural cis configuration. However, there are natural trans fats in some foods such as dairy products, fats and meat from ruminants (beef fat and mutton: approx. 4.5%, dairy cow and goat milk: approx . 3.3%, the beef and mutton: approx. 2%). These trans fatty acids from the bacterial conversion of unsaturated fatty acids in the rumen. Another source of trans fatty acid is the catalytic partial hydrogenation of polyunsaturated fatty acids.

At room temperature, unsaturated fatty acids are liquid (oils) they are usually found in foods of plant origin. It is possible to turn oils into fats by hydrogenation of their double bonds (addition of hydrogen atoms), which corresponds to a saturation of double bonds. This operation is used eg for margarine from vegetable oils.

In the same way as saturated fatty acids, unsaturated fatty acids also have a name related to their common origin and a systematic name describes their structure (Table 2). In addition, a classification often used in physiology and biochemistry that is based on the number of carbon atoms, the number of unsaturation and position of the first double bond C = C. For example, the fatty acid Cx: y w-z consists of carbon x, y double C = C bonds and the first of which is the z^thposition from the side opposite the acid group (see figure below).

Definition of omega (w)

The significance of the letter omega (w) comes from the physiological and biochemical nomenclature of unsaturated fatty acids. This nomenclature, as we stated earlier, just shows the number of carbons, the number of unsaturation and position of the first double bond (from the opposite side of the acid group). The latter is expressed by the letter omega (w). Sometimes the letter omega is replaced by the letter or the letter d.

This nomenclature 'w-z "comes from the biology, the unsaturation appear every 3 carbons in the majority of cases. This allows to classify them into families. For example, arachidonic acid is called acid C20: 4 w -6. The unsaturation is first on the C14 carbon (6^th position from the side opposite the acid group) and three other carbons on C11, C8 and C5 (see figure above). The arachidonic acid is a fatty acid of the omega 6 family.

Essential fatty acids and essential fatty acids

Nutritionists call essential fatty acids, fatty acids that the body can not synthesize itself. These fatty acids should be made mandatory for food. From them the body is then able to synthesize the other fatty acids, which the body needs to function. These fatty acids can be synthesized take the name of essential fatty acids.

For chemists, however, fatty acids are called essential if the body needs to live and if it can not synthesize itself. It is in fact what nutritionists call essential fatty acids. The other acids are simply called fatty acids by chemists while nutritionists call essential fatty acids. Thus it will always be careful in the names of fatty acids, ie, we place a view chemist or nutritionist.

The one most common and most used is that of chemists and it is this definition that we consider in our case: A fatty acid is called "essential" when the body can not make that small amount if at all, they must therefore be made by the daily food supplements or appropriate.

The existence of these fatty acids known as "essential" and their functions were discovered by scientists who had observed linolenic acid deficiency in patients suffering from stunting, skin diseases, loss of reproductive rate, heart problems and kidney ...

Essential fatty acids are two: the linoleic acid (C18: 2, w -6) and the alpha linolenic acid (C18: 3, w -3). Each belongs to a different family, the first is the family of omega 6 and the second is the family of omega 3. The linoleic acid content is mainly in vegetable oils and so called virgin first cold pressing (peanut oil, evening primrose oil, sunflower, safflower, ...), in eggs, dairy products, in wild game meat (especially liver). The alpha-linolenic acid comes from green plants, some aquatic plants (eg, Spirulina), seafood (eg fish oils such cold seas salmon, halibut, mackerel, ...) certain vegetable oils (borage, walnut oil, soybeans, flax ...). The two acids together constitute what is known as vitamin F.

The different families of fatty acids electively contribute to the formation of specific kinds of tissue: nerve cells feed mainly acids of the omega 3 family, whereas the development of muscle uses fatty acids of the omega 3 family and the family of omega 6.

Simple lipids

Lipids or simple homolipides are lipids that contain only carbon, hydrogen and oxygen. They are esters of an alcohol and fatty acids. The simple lipids are classified into three groups: glycerides, and the CERID sterides.

Glycerides

The glycerides are also called simple lipid fats. These are esters of glycerol and of fatty acids (one, two or three fatty acids). Depending on the number of fatty acids combined with glycerol, there are monoglycerides, diglycerides and triglycerides. Triglycerides are the main constituents of animal fats and vegetable oils (95%). Monoglycerides and diglycerides are much less abundant than triglycerides.

In the body, lipids are stored mainly in the form of triglycerides. Lipids from the diet are lipolysis in the intestines into fatty acids and glycerol. These are absorbed and are again reconstituted in the body to form fat triglycerides.

CERID

The CERID waxes are also known. They are esters of an aliphatic primary alcohol of long chain fatty alcohol known, and of higher fatty acids with palmitic acid (16 carbon atoms). They are both in plants than in animals. In plants, they are represented by a cuticle more or less impervious to the surface of leaves and fruits, and play a protective role.

Sterides

The sterides are esters of fatty acids and sterols. Sterols are tetracyclic alcohols attached to the group of steroids.

According to the origin of sterides, there are 3 groups: fungistérols (which are specific to fungi), phytosterols (constituents of the unsaponifiable plant) and zoostérols (found in animal tissues). The main representative is zoostérols cholesterol.

Cholesterol, despite its bad reputation, is essential to our health: it joins phospholipids to form membranes of animal cells (there is no cholesterol in plants) and also to form different molecules such as essential steroid hormones, vitamin D and bile salts (the latter are contained in the bile, which helps digest fats in the intestine).

Most of our cells produce cholesterol. Nearly 80% of cholesterol in the body is synthesized. The rest comes from food. Consumption of food rich in cholesterol and saturated fat tends to raise blood cholesterol.

All studies show that there is a correlation between high blood cholesterol levels high and the risk of heart disease, particularly the atherosclerosis.

Cholesterol is present in two forms in the body: the good cholesterol and bad cholesterol. The good cholesterol (HDL: High Density Lipoproteins) is that which, united with the essential fatty acids of virgin olive oil, is absorbed by the body. Bad cholesterol (LDL: Low Density Lipoproteins) is one that does no more than fat and is deposited anywhere.

Complex lipids

The complex lipids are lipids which contain more carbon, hydrogen, oxygen and one or more heteroatoms (nitrogen, phosphorus, sulfur). Depending on the nature of the heteroatom can be distinguished: the lipid phosphorus, nitrogen and lipids lipids sulfur.

Lipid phosphorus

Called phospholipids (phosphorus or lipid) lipid compounds containing phosphorus. These are the main constituents of biological membranes. Designated by the term "phospholipid" all glycerophospholipids and sphingophospholipides.

The glycerophospholipids are esters of glycerol, 2 fatty acids, a phosphate and an alcohol. Depending on the type of alcohol, there are the phosphatidylcholine (alcohol = choline), the phosphatidylethanolamine (alcohol = ethanolamine), the phosphatidylserine (alcohol = serine) and phosphatidylinositol (alcohol = inositol ).

The phosphatidylcholine is known as "lecithin". This compound is present in all living tissue, particularly in nerve tissue and red blood cells. There are also lecithins in plants and egg yolk. Lecithin is used as an emulsifier in margarine and other processed food. The commercial lecithins are produced mainly from soybeans.

The sphingophospholipides are membrane lipids not containing glycerol. It is composed of a fatty acid to long chain fatty amino alcohol such as sphingosine or a derivative and a phosphate. The sphingosine is composed of 18 carbon atoms, a trans double bond, an amino group and 2 hydroxides .

Lipid nitrogen

Called lipids lipid nitrogen compounds containing the nitrogen. They are constituents of biological membranes. Distinguished this group: the ACYLATED ceramides or sphingosine on the one hand and sphingosidolipidiques or glycolipids (cerebrosides) on the other.

Fat sulfur

Lipids are also known as sulfur sulfolipides or sulfatides. It 's sulfuric esters of cerebrosides (glycolipids).

Fat polyisoprene

Polyisoprene lipids are lipid-based isoprene. This group of lipids is also called unsaponifiable lipids and play a fundamental biological role (hormones and vitamins). They are divided into four categories: terpenoids, carotenoids, quinones isoprene chain and steroids. Carotenes (orange-red pigment), the xanthophylls (yellow pigment) and vitamin A are among the carotenoids. Vitamin E, vitamin K, ubiquinones and plastoquinone are quinones isoprene chain. Steroids include sterols, bile acids, steroid hormones and vitamin D. These last three are derivatives of sterols.

The plant essential oils (geraniol, limonene, menthol, pinene, camphor) that contribute to odor and flavor of certain species of lipids are also polyisoprene.

Behavior of lipids in water

Lipids are amphiphilic molecules: they have a polar head (hydrophilic) who loves the water and a tail apolar (hydrophobic), which pushes the water. The amphiphile is very widespread in the phospholipids, the fatty acids in soaps and sphingolipids. It is less widespread in the glycerides and sterides. This amphiphilic character that determines the organization of lipids in water: single-layer, bi-layer (liposomes) or micelles.

By depositing a small amount of oil on the surface of water, lipid molecules form a mono-molecular layer (mono-layer) at the air-water interface: the hydrophilic parts of lipid are heading towards the water and hydrophobic parts are directed to the air.

Some lipids such as soaps or detergents are apparently soluble in water. In fact, it is a combination of lipids to form micelles that remain suspended in water. In a micelle the parties polar (hydrophilic) head to the outside (in contact with water) and parties apolar (hydrophobic) is moving inwards (in contact with other hydrophobic parts). The diameter of a micelle is about 20 nm.

The action of washing detergent is due to the property micellage. When the agitated water containing a detergent, the droplets of oil or grease (greasy dirt) associated with the central part of micelles because of their hydrophobic character. Thus, grasses are dirt suspended in the micelles. When rinsing, these micelles are driven by water and dirt are separated from their support.

Strongly amphiphilic lipids such as phospholipids or sphingolipids do not form micelles or mono-layers, but bi-layer lipid (liposomes). In a bilayer polar parts are on the outside and apolar parts are inside.

Chemical reactions of lipids

Saponification reaction

Fatty acids are weak acids. They react with bases to obtain mineral soap. Soaps are the most famous soap proper sodium (hard soap) or potassium (soaps pouts).

(RCOO^-K⁺) is the soap. Soap is a compound bipolar head hydrophilic COO^- and a hydrophobic side chain R. The soaps are endowed with the power due to their detergent property of lowering adhesion forces between the dirt and the machine which frees and moves in the aqueous phase (See above: Behavior of lipids in the water).

The fatty acids can be regenerated from the soap and in the presence of an acid such as hydrochloric acid (HCl) according to the following reaction:

(RCOO- , K+)    + HCl <=======> RCOOH + (Cl- , K+)

The acid can be simply carbon dioxide from the air (CO₂) dissolved in water. Therefore, a soap solution in contact with the air loses its power detergent. In this case the reaction is as follows:

2(RCOO- , K+)    + H2CO3 <=======> 2(RCOOH) + CO3K2

In food industry, the saponification reaction is used to remove excess acidity of crude oils (Operation neutralization) and the production of soap. The amount of KOH (in mg) required to neutralize 1 gram of fat is called "saponification (IS):

The saponification also shows the average length of chains of fatty acids constituting the lipid in question. More SI, the higher the chain is short.

Esterification reaction

The esterification reaction is a reaction of an acid and an alcohol. The product of the reaction is an ester. The reverse reaction is the hydrolysis.

Acide + Alcool  <======>    Ester + Eau

The esterification reaction is important for the following reasons:

• Naturally, the lipids are esters of an alcohol and a fatty acid.
• For the analytical separation of fatty acids by gas chromatography or by fractional distillation (distillation in several stages), it transforms to the state of methyl or ethyl esters. The latter have a boiling point of 30 to 40 ° C below that of their non-esterified fatty acids.

In the food industry, the esterification reaction is used to determine the fatty acid composition of vegetable oils and animal fats in order to identify fraudulent mixtures (mixed with other oils of different nature). The reverse reaction of esterification is used for the production of alcohols from fats.

Hydrogenation reaction

The hydrogenation of unsaturated fatty acids is by using hydrogen (H₂) under a pressure of 100 to 200 bar, a temperature of 200 to 400 ° C and in the presence of catalysts, especially transition metals such as platinum (Pt), nickel (Ni), zinc (Zn), etc.. Under these reaction conditions, the unsaturated fatty acids determine the hydrogen to give saturated fatty acids.

— CH2 — CH = CH — CH2 —  + H2  ———> — CH2 — CH2 — CH2 — CH2 —   

The hydrogenation of the lipid is a process to make oils solid or semi-solid (margarine) and less susceptible to oxidation (rancidity). The partially hydrogenated fatty acids are used in the food industry as a texture to make food more firm and less leaking, as a preservative to prevent rancidity, or deodorant in certain fish oils. They are thus in many processed foods, including margarine.

The hydrogenation of a mixture of fatty acids may be selective or non-reactive under the conditions. The selectivity of the reaction is related to the degree of unsaturation of the fatty acid or lipid. The fatty acid is the most unsaturated hydrogenated first. For example, in a mixture of oleic acid (1 double bond), linoleic acid (2 double bonds) and linolenic acid (3 double bonds), linolenic acid is saturated first, and linoleic acid and the oleic acid.

Selective hydrogenation is obtained by increasing the temperature of the reaction, while the non-selective hydrogenation is obtained from increased pressure or agitation.

The hydrogenation may be partial or total. In the first case, part of the unsaturated fatty acids is converted to saturated fatty acids. The finished products meet specifications, especially solids, depending on the intended purpose. In the second case, all the unsaturated fatty acids are transformed into saturated fats.

The hydrogenation reaction is not without drawbacks, especially if the hydrogenation is partial. This is the formation of geometric isomers Cis and Trans. These are less digestible than the Cis and are involved in diseases of atherosclerosis (reduction of the cavity of the arteries by the formation of atherosclerotic plaques made up of fatty deposits). Therefore, this reaction must be conducted carefully, especially if hydrogenated fat is produced for food use.

Interesterification reaction

The interesterification is the change in the structure of fatty glyceridic by molecular rearrangement of fatty acids on the glycerol. This leads to significant changes in physical properties of fat. It is a method of transforming an oil grease without changing the nature of its fatty acids, only their distribution on the glycerol being changed. When this is done, not a single body fat, but on a mixture of two different fats or oils, one speaks of transesterification. This process leads to the formation of fatty acids no trans. " Interesterification allows a better control of the quality of both functional and nutritional fats and is an alternative to the method of hydrogenation.

The interesterification reaction may be chemical or enzymatic:

The chemical interesterification occurs spontaneously by heating above the melting temperature of triglyceride. It is easier and faster in the presence of metal catalysts such as metal oxides (zinc oxide, iron oxide, etc.) Or alkali (NaOH, KOH, LiOH). Chemical interesterification is relatively cheap and is used on an industrial scale to produce plastic saturated fat content in low or no trans fat.

The enzymatic interesterification offers better control of products generated during the reaction. Enzymes are very specific and can be selected to cleave specific bonds esters, ie occupying a particular position on the molecule. As enzymatic interesterification can be done at lower temperatures than chemical interesterification, there is less degradation.

Unlike chemically interesterified oils, the enzymatically interesterified do not need to be washed and bleached.

Reaction of lipid oxydation

The main factors determining the life of the lipid oxidation reactions can be classified according to their mechanism, in: autooxydation, photooxidation and enzymatic oxidation. Substrate oxidation reactions are mainly unsaturated fatty acids. They oxidize faster in general when they are free and more unsaturated. Saturated fatty acids do not oxidize at temperatures above 60 ° C, whereas the same oxidize polyunsaturated acids during storage of foods in a frozen state. The main problem in these reactions is the formation of volatile odor (rancidity) and the formation of peroxides which are carcinogenic molecules.

The reaction of oxidation of lipids is developed in part related to chemical weathering reactions of food.

Reaction of fatty acids with halogens

Unsaturated fatty acids determine the halogen by a reaction of addition.

CH2 — CH = CH — CH2 — + I2 ————> — CH2 —CHI — CHI — CH2 — 

This reaction is mainly used with iodine and bromine to assess the degree of unsaturation of fatty acids. It is actually an assessment of the suitability of the rancid fatty acids: there are more of unsaturation on fatty acid, it would be more sensitive to O2.

The amount of iodine (in grams) needed to set 100 grams of a fatty acid is called "iodine index".

Use of lipid

In addition to their metabolic roles, lipids are used in many food preparations:

Hydrogenated vegetable oils are used as raw material for the manufacture of margarine and "shortening" and as an ingredient in other preparations such as biscuits.

Some lipids such as lecithin are used as emulsifiers in margarine, mayonnaise and other preparations.

Lipids are also used for the manufacture of soap. The oil soap is a by-product that comes from the neutralization of crude edible oils.