Fats and fatty oils (also called lipids) are water-insoluble substances with a liquid or solid consistency. Fats which are still liquid at temperatures below 20 °C are generally referred to as oils.
(* For the purposes of simplification, the term "fat" will hereafter be used as a generic term. )
1 Triglycerides
All fats, whether animal, plant, liquid or solid, have the same structure.
The fat molecule always consists of a glycerol molecule (alcohol). This forms the backbone of the fat molecule. The three fatty acids (hydrocarbon chains) are attached to the glycerol molecule. The chemical term for fats is therefore triglyceride. The “tri” represents the three attached fatty acids, the “glyceride” the glycerol molecule to which they are attached. (1)
All natural fats usually have different fatty acids attached to the glycerol. They are also referred to as mixed triglycerides (see Fig. 1).
2 Fatty acids
Fatty acids consist of a chain of carbon atoms (C) strung together, to which the hydrogen atoms (H) are attached. Natural fatty acids usually have an even number of carbon atoms (C), as the chains are compiled from C-C units. The fatty acids are classified according to their chain length (short-, medium- or long-chain), their degree of saturation (saturated or unsaturated) and the position of the double bonds (e.g. between the 9th and 10th carbon atom).
Saturated fatty acids (2)
If the maximum number of hydrogen atoms which the carbon chains can carry are bonded to the chain, the chains are described as "saturated" (Fig. 2).In these chains, all four valences (the "arms" of the carbon atoms) are "neutralised".Saturated fatty acids are "saturated and inert" and therefore stable. In terms of their use, this means that they can withstand high temperatures and can be stored for a long time (3). An extremely common saturated fatty acid is stearic acid with 18 carbon atoms (see Fig. 2).
The single bonds between two carbon atoms (C-C) can rotate freely. The fatty acid molecule is therefore extremely mobile, the carbon chains of the fatty acids can arrange themselves in straight lines and take up less space. For this reason, fats with a large number of saturated fatty acids are solid at room temperature. Due to their inert reactivity, fats with a high share of saturated fatty acids are preferred for deep fat frying.
Unsaturated fatty acids (3)
Unsaturated fatty acids are divided into monounsaturated and polyunsaturated fatty acids.Monounsaturated fatty acids are missing two hydrogen atoms, which means that the two free arms bond and form a second bond, referred to as a "double bond", between two carbon atoms. The most common monounsaturated fatty acid is oleic acid. It is derived from stearic acid and also has 18 car- bon atoms (see Fig. 3).
Polyunsaturated fatty acids are missing several pairs of hydrogen atoms. An example of a polyunsaturated fatty acid is linoleic acid with 18 carbon atoms and two double bonds.The more double bonds there are, the more unsaturated and reactive the fatty acids are.
Unsaturated fatty acids have a special role in nutritional physiology. Polyunsaturated fatty acids (e.g. linoleic and linolenic acid) cannot be produced by the body itself, but the body needs them for building cells, for example. For the same reason, animal fats have relatively few of these "essential" fatty acids. Plant oils such as sunflower oil, on the other hand, contain a large number of unsaturated fatty acids (see Fig. 4).
Fats consisting largely of monounsaturated and polyunsaturated fatty acids have a lower melting range than fats with a large number of saturated fatty acids, i.e. they are liquid at room temperature.
As a general rule, the longer the chain and the more double bonds there are, the lower the temperature at which the fats become liquid. (4), (5), (6).
Fats with a higher proportion of monounsaturated and polyunsaturated fatty acids are more prone to fat ageing than saturated fatty acids and are therefore not suitable for deep fat frying. From a health point of view, however, it is advisable to use cooking fat with the maximum possible proportion of unsaturated fatty acids.
Modern cooking fats have a high proportion of the beneficial fatty acids and have been modified so that they remain stable at high temperatures.
Trans fatty acids
Another form of unsaturated fatty acids are the trans fatty acids. Their double bonds have a special spatial structure described in the chemistry field as the trans form (Fig. 6), as op- posed to the cis form (Fig. 5).
In the cis fatty acid the two hydrogens (shown in green in the illustration) are on the same side, in this case the top side.
In the trans fatty acid on the other hand, the two hydrogen atoms (shown in pink in the illustration) are opposite each other. Trans fatty acids are mainly found in nutritional fats from animal sources. They are produced, for example, as a result of the conversion of natural cis fatty acids by microorganisms in the digestive tract of ruminant animals and are passed from there into their milk or meat.
In plant fats, trans fatty acids are primarily produced in the intermediate stage during hardening. In the so-called partially hardened fats, the proportion of trans fatty acids is considerably higher than in fully hardened fats.
In terms of nutritional physiology, the trans fatty acids are on a par with saturated fatty acids. The feature common to both types of fatty acids is that they increase the cholesterol level in the blood and are suspected of increasing the risk of cardiovascular diseases.
Cis fatty acids, on the other hand, reduce the cholesterol level and therefore have a positive impact on health.
During deep fat frying, the aforementioned fatty acids are separated from the glycerol radical as a result of various reactions, and in addition to the free fatty acids monoglycerides and diglycerides, polymeric trigylicerides or oxidative degradation products such as aldehydes and ketones are some of the substances produced. They are grouped under the term total polar materials, TPM for short, and used as a benchmark for measuring the rate of decomposition of the fat (see Fig. 7).
(1) Structure of fats, p. 10; from: Natür- lich mit Pflanzenöl, 2nd edition, Margarine-Institut; Hamburg.
(2) Structure of fats, p. 10; from: Natür- lich mit Pflanzenöl, 2nd edition, Margarine-Institut; Hamburg.
(3) Structure of fats, p. 11; from: Natür- lich mit Pflanzenöl, 2nd edition, Margarine-Institut; Hamburg.
(4) http://www.biorama.ch/biblio/b20g- fach/b35bchem/b12lipid/lip010.htm. Last updated: 10 Aug. 2005.
(5) http://www.margarine-institut.de/ presse2/index.php3?id=88.Last updated 08 April 2014
(6) http://www.margarine-institut.de/ presse2/index.php3?rubrik=1&id=88. Last updated: 10 Aug. 2005.
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