Definition and structure
Fatty acids are lipids consisting of a carboxy group and a hydrocarbon chain that is usually unbranched and may contain double bonds. The figure shows palmitic acid with 16 carbon atoms (C16): they typically exist in nature free or in the form of glycerides. Glycerides consist of a molecule of glycerol esterified with one, two or three fatty acids. Triglycerides contain three identical or different fatty acids. In waxes, long-chain fatty acids are esterified with long-chain and aliphatic alcohols. Fatty acids are also of central importance for phospholipids in cell membranes. Furthermore, fatty acids can also be esterified with cholesterol and can be found in sphingolipids.
Different chain lengths
Fatty acids differ in hydrocarbon chain length. The usual chain length is between 4 and 24, but longer fatty acids also occur, for example, with 30 carbon atoms. The short-chain carboxylic acids such as butyric acid (C4) are also counted as fatty acids. The lower fatty acids are liquid, the higher ones are semi-solid to solid. While the lower fatty acids are miscible with water, water solubility decreases with increasing length due to the lipophilic side chain.
Saturated and unsaturated fatty acids
Fatty acids without double bonds in the side chain are referred to as , those with double bonds as . In terms of configuration, a distinction is made between – and – fatty acids. In nature, mainly -fatty acids occur. The -fatty acids (TFA) are formed, for example, during fat hardening, industrial food processing and deep-frying. They are considered a health hazard and are therefore regulated by law in many countries. Oleic acid (C18) is a monounsaturated fatty acid with the -configuration: elaidic acid (C18) is the corresponding -fatty acid:
Essential fatty acids
Fatty acids are mostly of plant, animal or synthetic origin. They are excellent energy stores and are needed for the formation of adenosine triphosphate (ATP). The human body cannot produce the so-called essential fatty acids itself. These are in particular α-linolenic acid (ALA), which belongs to the omega-3 fatty acids, and linoleic acid, an omega-6 fatty acid. They must be ingested with food. Both are found in rapeseed oil, for example. The essential fatty acids were called vitamin F in the past, but they are not vitamins.
Alpha and omega
For nomenclature purposes, fatty acids are numbered. C-1 is the first carbon atom in the carboxy group. In lauric acid (C12), the last carbon atom is C-12. The position of the carbon atom next to the carboxy group (C-2) is named as α, the following as β, and that of the last carbon atom as ω. ω-3 means that the first double bond is located at the third carbon atom from the end of the chain.
Saponification
When glycerides are hydrolyzed with strong bases such as sodium hydroxide or potassium hydroxide, salts of fatty acids are formed, which are called soaps. In this context, they are referred to as saponification.
Salts
Salts (and esters, for that matter) of fatty acids are designated by the suffix -at, for example:
- Lauric acid: laurate
- Stearic acid: stearate
- Oleic acid: oleate
Fat hardening
Treatment with hydrogen and nickel as a catalyst removes the double bonds of fatty acids in glycerides. This is referred to as hydrogenation and fat hardening. In this process, the physicochemical properties change and liquid oils become fats. This is due to the fact that unsaturated fatty acids have a lower melting point than saturated ones with the same chain length. A typical example is hydrogenated peanut oil (Arachidis oleum hydrogenatum).
Representative
Some important representatives are listed below: Saturated fatty acids (SFA):
- Butyric acid (C4)
- Caprylic acid (C8)
- Lauric acid (C12)
- Myristic acid (C14)
- Palmitic acid (C16)
- Stearic acid (C18)
- Arachidic acid (C20)
Monounsaturated fatty acids (MUFA):
- Oleic acid (C18)
- Linolenic acid (C18)
- Erucic acid (C22)
Polyunsaturated fatty acids (PUFA):
- Linoleic acid (C18)
- Arachidonic acid (C20)
- Eicosapentaenoic acid (C20)
- Docosahexaenoic acid (C22)
Fields of application
- In pharmacy, fatty acids play a central role as excipients. They are present not only in semisolid preparations, but also in solid dosage forms such as tablets and in liquid drugs. For example, magnesium stearate serves as a lubricant in tableting.
- For the manufacture of cosmetics and personal care products such as soaps.
- Fatty acids are also used medicinally and therapeutically, for example, in the form of evening primrose oil for the treatment of eczematous skin diseases.
- As a dietary supplement for an adequate supply of essential fatty acids, even during pregnancy and lactation.
- For the prevention and treatment of disorders of blood lipid levels (hypertriglyceridemia) and cardiovascular disease (omega-3 fatty acids).
Adverse effects
Unsaturated fatty acids can oxidize and become rancid. Vitamin E, which is present in many oils, counteracts this process. Saturated fatty acids are less susceptible because they do not contain double bonds. They are more stable and have a longer shelf life. Fats have a very high energy density of over 800 to 900 kcal per 100 g (!) and can promote the development of overweight and obesity and associated diseases. By comparison, the calorific value of 100 g of chocolate is “only” > 500 kcal. Fatty foods should therefore not be consumed excessively. However, fats are not unhealthy per se, but essential components of the diet. Vegetable fats and oils and unsaturated fatty acids are recommended for the diet. Trans fatty acids, animal fats, and saturated fats should be avoided or taken in smaller amounts.
