Lecithins: what they are, and what they are not

This is the translation of an article that you will find in the Encyclopedia of the Académie d'agriculture de France : 

 

 

 

Lecithins

Many food products contain "lecithins", but what are they?

Food products from the industry sometimes display "lecithins" on the packaging, but what is this ingredient, listed by the European classification of additives under the number E322?

Here a review of the chemical history of food compounds will show us that the regulations would benefit from being more in line with the definitions given internationally by chemists.

About lecithin, there is the same question as with the term "chlorophyll", which was initially introduced by chemists to designate the green material that can be extracted from green plants... before it was discovered that it was actually a variable mixture of many compounds.

Having understood that this green material was made of many green, blue, yellow, orange and red pigments, chemists decided internationally to reserve the name "chlorophylls" (in the plural) for particular pigments, and more particularly for green pigments with a very particular molecular structure.

The same historical sequence can be found with many animal and plant materials. For example, still following the work of chemists (in this case from the 18th century), the material isolated from egg white by evaporation of water was called "albumin" for a long time, before the progress of chemistry made it possible to understand that it was a variable mixture, which led to the use of the word "albumins" (in the plural), to designate particular proteins

Lecithin ? No, lecithinS

For lecithins, they were discovered in 1845 by the French chemist Theodore Nicolas Gobley (1811-1876), who succeeded in extracting it from egg yolk. He created the name from the Greek lekythos, which means "egg yolk", and this definition persisted until 1850. The chemical nature of lecithin remained unknown until 1874. Then the progress of chemistry clarified the composition of the material isolated by Gobley, which was in fact a mixture of several compounds.

Where the cacophony sets in - and this is the breeding ground for fraud, dishonesty, misunderstandings, etc. - is that technical or technological publications have not kept up with the progress of chemistry, and that one finds various definitions in these circles.

For example, some have defined the product marketed under the name of lecithin as "a mixture composed of polar lipids (glycolipids, phospholipids) and triglycerides, obtained from animal or plant tissues" (we will see later that these are the compounds). Others have designated under this name "lipids containing phosphorus, extracted from eggs or brain tissue". And a third definition refers to phosphatidylcholine. According to the International Lecithin & Phospholipids Society (ILPS, 2020), lecithin is "a complex mixture of glycerophospholipids of plant, animal, or microbial origin, containing varying amounts of triglycerides, fatty acids, glycolipids, sterols, and sphingophospholipids. "Recently, one researcher (Leonard, 2017) even provided his own definition, naming lecithin "a group of lipid substances found in animal or plant tissues that are essential for cell function."

In less technological texts, we find other definitions. For example, in the Encyclopedia Britannica (2020), we find the third of the previous definitions, but also as a "natural" mixture containing notable proportions of phosphatidylcholine (PC), cephalin (phosphatidylethanolamine, PE) and phosphatidylinositol (PI).

Towards the same clear definition for all

 

 

All this should be swept away, because the 1905 law on the food trade imposes healthy, marketable and... fair products: horse is not beef! However, fairness imposes a single, common definition... which has moreover been given very clearly by the International Union of Pure and Applied Chemistry (IUPAC): for chemists around the world, lecithins are "cholic esters of phosphatidic acids" (IUPAC, 2019). Since chemists introduced the word "lecithin," they are the ones with the definition, right?

To understand what it means, let's start by analyzing the following representation, of a phosphatidic acid molecule:

On such a representation, the vertices carry carbon atoms, the letters O, H, P indicate oxygen, hydrogen and phosphorus atoms respectively, the segments represent bonds between atoms, and many hydrogen atoms are omitted for clarity (and we know that there are enough of them for each carbon atom to have a total of four bonds). That leaves R1 and R2, which we will consider later, but, before that, let's observe that, in the center of this structure, we find the following:

Here, we have replaced the eliminated parts with hydrogen atoms (H)... and we find the glycerol molecule, a "sugar" with three carbon atoms.

In the phosphatidic acid molecule, we also find the phosphorus atom with its neighbors, which corresponds to a phosphoric acid residue. Finally, the "R1" and "R2" designate chains of carbon atoms linked to hydrogen atoms; together with the doubly linked oxygen atoms, this makes "fatty acids"... but as there are missing atoms, lost during the assembly of the complete molecule, we should rather speak of "fatty acid residues". In lecithins, fatty acid residues have between 6 and 26 carbon atoms, depending on the source: lecithins of animal origin have longer fatty acid residues, while the number of carbon atoms is limited to about 20 for lecithins of vegetable origin.

Finally, we mentioned "cholic" esters of phosphatidic acids, which means that lecithins contain a residue of a compound called "choline":

Here, the letter N represents a nitrogen atom.

And for the complete lecithins, the molecules are :

Commercial names to be revised

We have seen that there is no ambiguity in the chemistry of lecithins... and this must quickly change the vocabulary of the industrial world. Can we really admit that this world designates under the name of lecithin mixtures of phospholipids, but also of glycolipids, triglycerides, water and sugars?

Glycolipids have nothing to do with "cholic esters of phosphatidic acid": they are compounds whose molecules include a lipidic part and a small sugar. In the lipid part, two fatty acid residues are attached to a glycerol residue, while the sugar residue is often a D-glucose, D-galactose or inositol residue. Like phospholipids, these compounds are present in cell membranes. The sugars leave the phospholipid bilayer, in the aqueous solutions that bound the cell membranes. Glycolipids are found in plant and animal tissues, but are most abundant in photosynthesizing algae and plants.

Triglycerides, on the other hand, have nothing to do with "cholic esters of phosphatidic acid" either: they are the compounds that make up oils, with a glycerol residue linked to three fatty acid residues.

By the way, why do food products contain lecithins?

Now that we know what lecithins really are, let's examine their usefulness by first considering the case of chocolate making. Let's skip the roasting of the cocoa seeds, the pressing of the roasted seeds, to produce cocoa butter, and focus on the "conching" stage, where sugar is added to this cocoa butter with vegetable matter added. This is traditionally done in a heated millstone, which rotates until the beans are reduced to very small particles, smaller than the 15 thousandths of a millimeter that remain perceptible between the teeth. When the millstone turns like this, in the mixture, it struggles, and consumes a considerable amount of energy, especially because the grains of sugar are surrounded by a thin layer of water... which does not mix well with the melted fat. As soon as lecithins are added to the mixture, the effect is seen: the grinding wheel starts to turn more easily, the rotation being facilitated, because the lecithins favour the contact of the fat and the water.

In various food products, too, lecithins have this "surfactant" role, allowing to reduce the energy necessary for the dispersion of fat, in the form of droplets, in an aqueous solution or conversely. Think of the famous mayonnaise sauce, where the egg yolk provides both water and lecithins, and where oil is dispersed and added drop by drop (although, in this case, the proteins also provided by the egg yolk are much more active than lecithins).

Are these lecithins used by the food industry "dangerous", as some of those who criticize "additives" claim? The first answer is that lecithins are present in the cells of our body: these cells are limited by a "membrane", which is a double layer of phospholipids or glycolipids. Eating meat, fish, vegetables or fruit means consuming lecithins en masse, even without the slightest intervention of an industry that some criticize for reasons that we will not analyze here.

References

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