Chapter 09.08 Von Liebig and the Theory of Proteins of Gerard Mulder.

Introduction to Bacon & the Art of Living

The story of bacon is set in the late 1800s and early 1900s when most of the important developments in bacon took place. The plotline takes place in the 2000s with each character referring to a real person and actual events. The theme is a kind of “steampunk” where modern mannerisms, speech, clothes and practices are superimposed on a historical setting.  Modern people interact with old historical figures with all the historical and cultural bias that goes with this.

Von Liebig and the Theory of Proteins of Gerard Mulder

Copenhagen, September 1891

Dear kids,

It is the first day of autumn. Denmark is not home, but there is a beauty to this world. Copenhagen is an amazingly beautiful city. It is much smaller than I thought it would be, but it is very organised. The buildings are old and beautiful!

Andreas became a brother. He is an amazing soccer player. I can’t keep up with him, either when I play with or against him. I try to teach them to play cricket and rugby, but it is difficult. I have given up, to the great amusement of his dad (and the relief of Andreas).

Even autumn is colder than the coldest winters we have in Cape Town. As the cold sets in I miss you guys more every day. My only consolation is that Minette is here! Every week we make time to go on a long hike. I miss Table Mountain! I miss my mom and sitting by the kitchen table as she cooks one of her legendary lunches!  I miss my dad. I miss Oscar and our crazy late-night dreaming. The fact that I learn on the one hand and do when I work in the bacon factory makes the learning more effective.

Copenhagen Harbour. The new Frihavns must impressive building was Silo warehouse on Midtermolen, built in 1892-94 by William Dahlerup
The Copenhagen harbour. The new Frihavns’ most impressive building was the Silo warehouse on Midtermolen, built in 1892-94 by William Dahlerup.

Copenhagen is not Africa!  It seems to me that all great dreams begin on horseback, on a farm, looking for stray cattle. The vlaktes of the Wes Transvaal seem so far. Like a dream.  I remember the day after we tasted the pork that we tried to cure on Oscar’s farm with the saltpetre that we bought from the Danish spice trader in Johannesburg and discovered that the pork was off. We tried to do it according to the Danish curing system as it was explained to us.  We were so disappointed!  Trudie told us that we must have done something wrong. We were sure that we did everything that the Danish guy told us.

The next day Ou Jantjie came to the house and told us that he saw some of Oscar’s cattle on Atties farm, close to the dam, nearest to Atties house. The off-tasting pork was out of our minds and we were on Poon and Lady, riding to look for the cattle.

Oscar said that Trudie is right, that we must have done something wrong and that we must learn much more. I think he knew from the beginning how difficult it would be to take David de Villiers Graaff on when it comes to curing bacon. Oscar’s mind is fast.

I reminded him that the spice trader said that if we really want to learn how it’s done, we must get on the next steamer leaving the Cape for Copenhagen. We decided to get everybody who will support the dream together for a meeting at his house one evening. Then we will decide.

The wind was in our faces and we had great dreams. I am learning how important those initial dreams are. It is like building up steam pressure before the engine starts to turn the big pistons on a steamship. If the pressure is not build up first, it will never be enough for the first “turn”. As soon as it’s turning, momentum takes over and the engine takes on a life of its own. The initial dreams are the building up of pressure.

This art of curing meat has been developing over thousands of years. On the one hand, people wanted to prevent meat from spoiling and on the other hand, cured meat developed into a culinary delicacy. The key ingredient is saltpetre (1).

Jeppe and I have the best of times during lunchtime. Since Minette arrived, it gave our lunchtime lessons a dynamic character. He would go through the relevant scientific discoveries of the previous few years, pointing out the direct application of science to the art of curing bacon. His lessons give both Minette and me enjoyment that is hard to communicate. The Wednesday, following our visit to the University, one of the Chemistry Professors decided to visit us at the bacon factory. It set the stage for another volcanic afternoon!

Justus von Liebig

He heard about our visit to the University and was eager to assist in our education. We were ready with notebooks, pens, and inquiring minds! He overviewed what we learned at the University and promptly changed the focus from the French to the German schools, in particular the formidable scientist in the person of Justus von Liebig. 

His father was a chemical manufacturer and had a small laboratory attached to his shop. Here Justus loved performing experiments and an exceptional life was inspired. After studying pharmacy, he received a doctorate from the University of Erlangen in Bavaria in 1822. The Grand Duke of Hesse-Darmstadt and his ministers noticed him and funded his further studies in chemistry under Joseph-Louis Gay-Lussac in Paris between 1822 and 1824. Gay-Lussac himself found all plant seeds “contain a principle abounding in azote.” It was, in Paris when a meeting with Alexander von Humboldt, according to Von Liebig, set his career on the path it took. Up to that point, it was the French chemists who were responsible for the progression of a field of study we touched on at the University namely protein metabolism, but with Von Liebig, this was about to change.

Humboldt is one of my heroes and as a child, I practically committed his books to memory. Humboldt arranged an appointment for Von Liebig at the small University of Giessen in May 1824. Liebig wrote about this meeting  that “at a larger university, or in a larger place, my energies would have been divided and dissipated, and it would have been much more difficult, perhaps impossible, to reach the goal at which I aimed.” Applying the techniques that he learned under Gay-Lussac he changed the face of organic chemistry and became the father of agricultural chemistry. The study of protein metabolism was now firmly in the hands of the Germans.

At the University of Giessen, Liebig created the most productive school of organic chemistry in existence at the time.  He perceived that his work could be logically extended to the chemistry of the living body. In 1840 his book, “Thierchemie in Ihrer Aufwendung auf Physio logie” appeared, and an English translation of the work entitled “Animal Chemistry, or Organic Chemistry in its Applications to Physiology and Pathology” appeared 1842.  Liebig believed that the basis of protein metabolism was chemical. Some believe this is his most important contribution to the subject.

Von Liebig was well prepared to make such a contribution on account of his training in France and his own studies in organic chemistry. The Danish Chemistry professor brought Von Liebig’s work, Animal Chemistry (1842) along and quoted liberally from us to show the various aspects of Liebigs views on protein metabolism. He quotes (p. 40): “… If we hold that increase of mass in the animal body, that development of its organs, and the supply of waste,—that all this is dependent on the blood, that is, on the ingredients of the blood, then only those substances can properly be called nutritious and considered as food which is capable of conversion into blood. To determine, therefore, what substances are capable of affording nourishment, it is only necessary to ascertain the composition of the food, and to compare it with that of the ingredients of the blood. Two substances require special consideration as the chief ingredients of the blood: . . . fibrine, which is identical in all its properties with muscular fibre, when the latter is purified from all foreign matters. The second principal ingredient of the blood is contained in the serum and gives to this liquid all the properties of the white of eggs, with which it is identical. When heated, it coagulates into a white elastic mass, and the coagulating substance is called albumen. Fibrine and albumen, the chief ingredients of blood, contain, in all, seven chemical constituents, among which nitrogen, phosphorus, and sulphur are found. . . . Chemical analysis has led to the remarkable result that fibrine and albumen contain the same organic elements united in the same proportion…. In these two ingredients of blood the particles are arranged in a different order, as shown by the difference of their external properties; but in chemical composition in the ultimate proportion of the organic elements, they are identical. . . . Both albumen and fibrine, in the process of nutrition, are capable of being converted into muscular fibre, and muscular fibre is capable of being reconverted into blood. . . . All part of the animal body which have a decided shape, which forms parts of organs, contain nitrogen; all of them likewise contain carbon and the elements of water. . . The chief ingredients of the blood contain nearly 17% of nitrogen and no part of an organ contains less than 17% nitrogen.”

The importance of nitrogen in all muscles and in blood in this way became known to humanity. It is therefore nothing strange to find nitrogen and its conversion into ammonia, ammonium and nitrate and nitrite as fundamental to the building blocks of animal life. Just as the same compounds play an essential role in plant nutrition and in all processes of the soil and water in our world.

The chemistry professor paused for a second to look up and see if we are following. He saw us hanging on his every word and he continued reading. “The most convincing experiments and observations have proved that the animal body is absolutely incapable of producing an elementary body, such as carbon or nitrogen, out of substances which do not contain it; it obviously follows, that all kinds of food fit for the production either of blood, or of cellular tissue, membranes, skin, hair, muscular fibre, etc. must contain a certain amount of nitrogen, because that element is essential to the composition of the above-named organs; because the organs cannot create it from the other elements presented to them; and, finally, because no nitrogen is absorbed from the atmosphere in the vital process.”

“The nutritive process in the carnivora is seen in its simplest form. This class of animals lives on the blood and flesh of the graminivora; but this blood and flesh are, in all its properties, identical with their own. . . . In a chemical sense, therefore, it may be said that a carnivorous animal, in supporting the vital process, consumes itself. That which serves for its nutrition is identical with those parts of its organisation which are to be renewed. The process of nutrition in graminivorous animals appears at first sight altogether different. Their digestive organs are less simple, and their food constituents consist of vegetables, the great mass of which contains but little nitrogen. … Chemical researches have shown, that all such parts of vegetables as can afford nutriment to animals contain certain constituents which are rich in nitrogen; and the most ordinary experience proves that animals require for their support and nutrition less of these parts of plants in proportion as they abound in the nitrogenised constituents. Animals cannot be fed on matters destitute of these nitrogenised constituents. . . . These nitrogenised forms of nutriment in the vegetable kingdom may be reduced to three substances, which are easily distinguished by their external characters. Two of them are soluble in water. The third is insoluble.”

He then states that he recognises “a vegetable fibrin, vegetable albumin and vegetable casein” which is similar in characteristics to these animal products. He continues (p. 48):  “How beautifully and admirably simple, with the aid of these discoveries, appears the process of nutrition in animals, the formation of their organs, in which vitality chiefly resides! Those vegetable principles, which in animals are used to form blood, contain the chief constituents of blood, fibrine and albumen, ready formed, as far as regards their composition. . . . From what has been said, it follows that the development of the animal organism and its growth is dependent on the reception of certain principles identical with the chief constituents of blood.”

Liebig’s view on nitrogen in nutrition is summarized by himself as follows (p. 95): . . . “According to what has been laid down in the preceding pages, the substances of which the food of man is composed may be divided into two classes; into nitrogenised and non-nitrogenised. The former is capable of conversion into blood; the latter incapable of this transformation. Out of those substances which are adapted to the formation of blood are formed all the organised tissues. The other class of substances, in the normal state of health, serve to support the process of respiration. The former may be called the plastic elements of nutrition; the latter, elements of respiration. Among the former, we reckon—vegetable fibrine, vegetable albumen, vegetable caseine, animal flesh, animal blood. Among the elements of respiration in our food are—fat, starch, gum, cane sugar, grape sugar, sugar of milk, pectine, bassorine, wine, beer, spirits.”

You will see that none of Von Liebig’s views was new.  These were concepts that originated with Magendie, 25 years earlier. Note in particular that Von Liebig did not have an inkling of the possibility of digestion and reconstruction of proteins taken in the diet.  (Munro and Allison, 1964)

One of the many productive directions of the work of Von Liebig and his students was the application of oxidizing agents (example, manganese dioxide, and chromic acid) during acid hydrolysis of proteins and in the process identifying a series of acids and aldehydes. The concept of studying the degradation products of protein originated with Von Liebig and was to play a crucial role in the next generation  (Sahyun, M. (Editor). 1948) There is an interesting point of application of this work to the modern bacon curer.  It has emerged that there is a link between foaming and the length of the amino acids that remain after proteins have been “digested” with the aid of an acid.  This became clear when we tried to dissolve the result of our “digestion experiments” in water.  If for some reason, such “digested” proteins must be used in a bacon brine, if the foaming is excessive and interferes with the curing operation, it will be of great help to “digest” the proteins for longer in acid before it is recovered and hydrated.

The atomic theory

At this point the Chemistry professor paused. He asked how much we know about Dalton’s atomic theory. Of course, I knew it well from high school in Cape Town.

I was very surprised when the professor said that Dalton’s work had an important application in the field of nutritional studies. John Dalton was by all accounts not the brightest of students. Some said that his main characteristic was not being bright but rather, determination. He was poor and largely self-taught. He worked as a  schoolmaster in the north of England and developed a very important notion. The notion was that all of the elements are made up of indivisible particles, or “atoms,” and, importantly, that for each element, every atom is identical. He came to the conclusion that in chemical combinations, two or more different atoms come together to form a firm union and this union, was, as far as the new substance is concerned, always in the same simple ratios by weight. So, for example, the gas, carbon dioxide has exactly twice the weight of oxygen (by unit weight of carbon) compared to what is present in another gas called carbon monoxide. So, the different elements in any compound are fixed. When comparing two different compounds, the same two elements will always be in a simple ratio by weight.

He further concluded that when gasses combine, they always do so in the same simple relation by volume. Let’s take the formation of ammonia as an example. When it is formed, 3 volumes of hydrogen combine with 1 volume of nitrogen and they form exactly 2 volumes of ammonia gas. A conclusion from these is that equal volumes of different gases contain the same numbers of molecules if one sees that many elements, such as hydrogen, oxygen, and nitrogen, have two atoms combined together to form a single molecule.

Early on there was uncertainty if carbon and oxygen each have one-half of the atomic weights that we now assign to them. Prout in England used improved methods of analysis and arrived at the formula C2H4N2O2.  Double the atomic weights for C and O and you arrive at the modern formula of CH4N2O. In the early 1800s, Friedrich Wöhler achieved what may believe to the start of organic chemistry when he obtained urea by heating silver cyanate with ammonium chloride. He wrote to his professor: “I can make urea without the use of kidneys.”  By doing this, he demonstrated that an organic compound produced in living systems could also be produced in the laboratory without the aid of any “vital force.”

Wöhler and Von Liebig’s Free Radical

Wöhler worked with Liebig and developed the idea of a common radical that would combine with other reagents, but still retain its own nature and be recoverable by further reactions. In chemistry, a free radical is a species that contains one occupied orbital.  A characteristic of a free radical is that they are neutral and they tend to be highly reactive.  The first such ree radical was “benzoyl”.

Starting with benzaldehyde (C6H5CHO), it can be oxidized to benzoic acid (C6H5CO2H).  Note the addition of an oxygen atom. Alternatively, a chlorinated derivative can be formed. The original benzaldehyde can be created by reducing or removing oxygen.”  (Carpenter, 2003)  It is easy to see the similarity in what we are doing with nitrate, nitrate, and ammonia and this, in turn, is build upon the logic of the atomic theory.

Gerard Mulder and the nature of animal substance

The Dutch chemist Gerard Mulder (1802–1880) published a paper in a Dutch journal in 1838 which was reprinted in 1839 in the Journal für praktische Chemie. Mulder examined a series of nitrogen-rich organic compounds, including fibrin, egg albumin, gluten, etc., and had concluded that they all contained a basic nitrogenous component (~16%)  to which he gave the name of “protein” (Munro and Allison, 1964) from a Greek term implying that it was the primary material of the animal kingdom.

The chemistry professor was adamant that we should know the building blocks of living organisms is proteins. Bacon curing is in the first place related to the manipulation of proteins and if we want any chance to understand the chemical reactions associated with bacon production, we should know the chemistry of proteins. Apart from the manipulation of proteins, it is key in our work to understand the concept of nutrition.

The term protein was coined by Jöns Jacob Berzelius, and suggested it to Mulder who was the first one to use it in a published article. (Bulletin des Sciences Physiques et Naturelles en Néerlande (1838); Hartley, Harold (1951) “Ueber die Zusammensetzung einiger thierischen Substanzen” 1839)). Berzelius suggested the word to Mulder in a letter from Stockholm on 10 July 1838. (Vickery, H, B, 1950) Mulder suggested using the symbol “Pr” for the radical, that egg albumin could be expressed as “Pr10 · SP” and serum albumin as “Pr10 · S2P,” and that the radical itself had the molecular formula “C40H62N10O12.  (Carpenter, 2003)

This common nucleus was linked with phosphorus and sulfur to give the various compounds referred to above. “Die organische Substanz, welche in allen Bestandtheilen des thier ischen Körpers, so wie auch, wie wir bald sehen, im Pflanzenreiche Vorkommt, könnte Protein von Tporetos primarius, genannt werden. Der Faserstoff und Eiweissstoff der Eierhaben also die Formel Pr + SP, der Eiweissstoff des Serums Pr + SP.” (The organic substance which is found in all the constituents of the animal body, as well as, as we shall soon see, in the vegetable kingdom, might be called protein of Tporetos primarius. The fiber and protein of the eggs thus have the formula Pr + SP, the protein of the serum Pr + SP)  (Munro and Allison, 1964)

Liebig initially liked the concept. He wrote, “… When animal albumen, fibrine, and caseine are dissolved in a moderately strong solution of caustic potash, and the solution is exposed for some time to a high temperature, these substances are decomposed. The addition of acetic acid to the solution causes, in all three, the separation of a gelatinous translucent precipitate, which has exactly the same characters and composition, from whichever of the three substances above mentioned it has been obtained. Mulder, to whom we owe the discovery of this compound, found, by exact and careful analysis, that it contains the same organic elements, and exactly the same proportion, as the animal matters from which it is prepared; insomuch, that if we deduct from the analysis of albumen, fibrine, and casein, the ashes they yield, when incinerated, as well as the sulphur and phosphorus they contain, and then calculate the remainder for 100 parts, we obtain the same result as in the analysis of the precipitate above described, prepared by potash, which is free from inorganic matter.”  (Munro and Allison, 1964)

If we look at it in this way, the main ingredients of blood and casein in milk can be regarded as a mixture of phosphates and other salts, and of sulphur and phosphorus, with a compound of carbon, nitrogen, and oxygen, in which the relative proportion of these elements is fixed.  This compound can then be regarded as the commencement and starting-point of all other animal tissues because these are all produced from the blood. Mulder had an insight that since the insoluble nitrogenised part of wheat flour (vegetable fibrine) when treated with potash, the exact same product is yielded namely protein.  He found that the true starting-point for all the tissues is albumen and that all nitrogenised articles of food, whether derived from the animal or from the vegetable kingdom, are converted into albumen before they can take part in the process of nutrition.  Liebig, like Mulder, ascribes the formula C4s H36N6O14 to protein, and albumen becomes C18H38N6014 + P + S, fibrine is C48E36. N6014 + P + 2 S, and so on.

– Liebig’s Opposition

Liebig eventually rejected Mulder’s concept of a nucleus protein based on work he continued to do on protein chemistry. He sets forth his arguments against Mulder at some length in his book “Researches on the Chemistry of Food,” published in an English edition in 1847. Here Liebig indicates that several chemists were unable to repeat some of Mulder’s basic experiments and that his formulas for fibrin, albumin, etc., as compounds of protein with sulfur and phosphorus in specific relations, do not agree with the results of more recent analyses of these substances. In this, he conveniently forgets his own earlier enthusiasm for Mulder’s view, and says (p. 18): “… A theoretical view in natural science is never absolutely true, it is only true for the period during which it prevails; it is the nearest and most exact expression of the knowledge and the observations of that period. It ceases to be true for a later period, inasmuch as a number of newly acquired facts can no longer be included in it. . . . But the case is very different with the so-called protein theory, which cannot be regarded as one of the theoretical views just mentioned, since, being supported by observations both erroneous in themselves and misinterpreted as to their significance, it had no foundation in itself, and was never regarded, by those intimately acquainted with its chemical groundwork, as an expression of the knowledge of a given period.” (Munro and Allison, 1964). “Mulder was enraged by the tone of the criticism from Liebig, who was now denying what he himself had previously asserted.” (Carpenter, 2003)

Despite his criticism, Von Liebig suggests the direction which eventually led researchers to ultimately resolved the structure of the protein molecule. He says (p. 27) : … The study of the products, which caseine yields when acted on by concentrated hydrochloric acid, of which, as Bopp had found, Tyrosine and Leucine constitute the chief part, and the accurate determination of the products which the blood constituents, caseine, and gelatine, yield when oxidised, among which the most remarkable are oil of bitter almonds, butyric acid, aldehyde, butyric aldehyde, valerianic acid, valeronitrile, and valeracetonitrile, have opened up a new and fertile field of research into numberless relations of the food to the digestive process, and into the action of remedies in morbid conditions.” (Munro and Allison, 1964)

It is an interesting thought that in the word “protein” we refer to the important class of body constituents, and we, at the same time, commemorate an erroneous oversimplification of protein structure.  We must also remember that we use the word in a meaning different from that originally intended. The German word for protein is “Eiweiss.” The reason may very well be because of Von Liebig’s eventual rejection of Mulder’s hypothesis.” (Munro and Allison, 1964). Dennis M Bier states that despite these nuances, Berzelius and Mulder were, in the most basic analysis, right: “Protein is the essential general principle of the constituents of the animal body. Thus, one might briefly summarize the physiological roles of protein in metabolism as “responsible for just about everything.” (Bier, D. M., 1999). “The notion of the protein radical disappeared from the literature and the term “protein” gradually began to be applied to all the materials previously described as “animal substance.”   (Carpenter, 2003)

Is Protein the only true nutrient?

Our good professor was on a roll.  Nitrogen, as the key nutrient was firmly established, but is this the only one?  While he is on the subject, he gave us a history lesson on the further development of thoughts around this matter.  As a food producer, this remains one of the overall biggest subjects.  Nutrition!  It is the original and main reason why we eat!

Von Liebig wrote the following in his book, Animal Chemistry or Organic Chemistry in its Application to Physiology and Pathology, that “because his analyses of muscles failed to show the presence of any fat or carbohydrate, the energy needed for their contraction must come from an explosive breakdown of the protein molecules themselves, resulting in the production and excretion of urea. Protein was, therefore, the only true nutrient, providing both the machinery of the body and the fuel for its work.

What is the reason then that we would need the other parts of the food that we consume?   Why is carbonic acid produced in much higher volumes during exercise? The explanation of Von Liebig was that increased respiration was needed to keep the heart and other tissues from overheating. This led to more oxygen finding a way into the tissues, which unfortunately potentially cause oxidative damage and a loss of protein tissue. Fats and carbohydrates then acted as mopping agents of this excess by being themselves preferentially oxidized.

Von Liebig’s book quickly gained a reputation as an important intellectual synthesis.  His ideas gained wide acceptance, the influence which was felt for many years. The Professor of Medicine at Edinburgh University was, for example, asked to investigate an unexpected and very serious outbreak of scurvy in a Scottish prison, he immediately concluded that it must be the result of an inadequate intake of protein. He calculated the average daily protein intake of a prisoner to be an ample 135g. Only 15g of this was from animal sources and 102g from gluten.

His conclusion was to raise the average daily intake of milk to increase the intake of animal protein because, he argued, the power of the body to convert gluten to animal protein was limited.  There were, however, a problem with this logic, as was spotted by another Scottish physician who replied that the value of lemon juice in the prevention of scurvy was well established and could not possibly be attributed to its protein content, given that a curative dose contained only a negligible amount of nitrogen.

The theory that muscular work is required to break protein down was problematic.  The traditional diet of labourers was of lower protein content that of the less active rich. Now, remember the book, Foods, we are reading every night with Andreas and his family.  Edward Smith, the author, and a British physician, and physiologist is another scientist of the time who was interested in the welfare of prisoners.  He was worried about the stressfulness of them having to work on a treadmill.  He measured their urea excretion in the 24h during and after their 8 hours of work, and again on their subsequent rest days, and found no difference. His findings were in complete opposition to the position of Von Liebig who would have said that on the basis that the energy expended all came from the breakdown of protein that resulted in the production of urea.  (Carpenter, 2003)

Liebig and Urine

Von Liebig drew attention to urea as an end-product of protein breakdown in the body. He did not get it right completely. In his work, “Animal Chemistry” (1842), (p. 62) he wrote, “… We know that the urine of dogs, fed for three weeks exclusively on pure sugar, contains as much of the most highly nitrogenised constituent, urea, as in the normal condition. Differences in the quantity of urea secreted in these and similar experiments are explained by the condition of the animal in regard to the amount of the natural motions permitted. Every motion increases the amount of organised tissue that undergoes metamorphosis. Thus, after a walk, the secretion of urine in man is invariably increased.”

Later (p. 245), he wrote, “The amount of tissue metamorphosed in a given time may be measured by the quantity of nitrogen in the urine.” All this shows Von Liebig’s central thought that protein in muscle was the fuel for muscular exercise. He believed that the nitrogenous components of the diet must first be converted to living tissue before being broken down to yield urea. “There can be no greater contradiction, with regard to the nutritive process, than to suppose that the nitrogen of the food can pass into the urine as urea, without having previously become part of an organized tissue.” (p. 144).” (Munro and Allison, 1964)

Liebig’s Contribution to Protein Metabolism and the work of Carl Voit

Why this attention to Von Liebig? It appears from what we have seen that he did not contribute much permanent value to our understanding of protein metabolism. Nothing could be further from the truth. Von Liebig adhered to a vigorous application of organic analysis to compounds of biological interest, he undoubtedly laid the foundations of intermediary metabolism and much of the important work that followed Von Liebig was predicated on these findings. Besides these, Von Liebig identified many of the compounds of biological interest which subsequent researchers made their focus areas with great success.

Von Liebig’s ultimate genius was that he took on seemingly insurmountable problems and even though he did not come up with the ultimate solutions, he managed to break the issues down to such an extent that one can say he pointed the way to their ultimate solution. Look for example at his comments on intermediary metabolism.  He wrote (“Researches on the Chemistry of Food,” (1847) p. 10): “The intermediary members of the almost infinite series of compounds which must connect Urea and Uric acid with the constituents of the food, are, with the exception of a few products derived from the bile, almost entirely unknown to us; and yet each individual member of this series, considered by itself, inasmuch as it subserves certain vital purposes, must be of the utmost importance in regard to the explanation of the vital processes, or of the action of remedies.”

Another good example is the fact that he saw certain chemical reactions as only occurring in biological systems and suspected that these were dependent on the presence of proteins. Have a look at the following statement (p. 7) from his book on food chemistry where he came agonizingly close to our modern understanding of the concept of enzymes:  “… There is, probably, no fact more firmly established as to its chemical signification, than this, that the chief constituents of the animal body, albumen, fibrine, the gelatinous tissues, and caseous matter, when their elements are in a state of motion, that is, of separation, exert on all substances which serve as food for men and animals, a defined action, the visible sign of which is a chemical alteration of the substance brought in contact with them. That the elements of sugar, of sugar of milk, or starch, etc., in contact with the sulphurised and nitrogenised constituents of the body, or with analogous compounds which occur in plants, when these are in a state of decomposition, are subjected to a new arrangement and that new products are formed from them, most of which cannot be produced by chemical affinities, this is a fact, independent of all theory.”

Von Liebig’s greatest contribution to the development of protein metabolism is in the school of biochemical studies, founded by him.  This was done first in Giessen, and later in Munich, where he became professor of chemistry in 1852. From here emerged a number of very important proponents of metabolism, chief among them being Carl Voit, whose researches in protein metabolism placed the concept of nitrogen balance on a firm footing.

Voit was intensely interested in “animal chemistry.” He wrote that Dumas was wrong in his assertions since it was well known that pigs would fatten when fed on potatoes that were rich in starch, but had only a small amount of fat. Accordingly, it must be concluded that animals are able to convert carbohydrates to fat even though the conversion required “reduction” rather than oxidation.

French researchers who were regarded as the authorities on this subject challenged this view, and Boussingault put the matter to the test. He performed another groundbreaking experiment with pigs. He took a young pig and killed it and analysed its carcass. He took a littermate of this pig, of the same weight, and fed it measured amounts of feed for another 3 months. The carcass analysis of the second pig indicated that this pig had an extra 13.6kg fat but the feed it consumed only had 6.8kg.

This very clearly showed the French school to be wrong on this point. Both Boussingault and Dumas retired from working with animals. Von Liebig became the new authority, even though he had never actually carried out a feeding trial. He continued to advocate his ideas on physiology and nutrition. Most of these were gradually shown to have been completely wrong, but at least they stimulated others to do research, putting them to the test.”  (Munro and Allison, 1964)

An Inspirational Message

Kids, take note that neither Mulder nor Von Liebig illuminated protein or its metabolism fully, but we gain a great appreciation for their work in the early to mid-1800s. I wonder how many of today’s researchers would do as much as these men did with the scant knowledge they had and it is a lesson to us all. Rigour in our work will yield results, no matter how tentative at first. It reminds me of the old verse from Sunday school in the Groote Kerk in Cape Town that there is profit in all labour.

There can be no doubt that nitrogen is key to the art of bacon curing and the most important macromolecule we are working with is protein. I came to realise that bacon is nothing less than the art of manipulating it. A question of whether the nitrogen that we add to the meat in the form of nitrate or nitrite is good for us or not is in the first instance the wrong question since when you are talking about protein you are talking about nitrogen and vice versa.

Thirdly, central to the concepts of nitrogen and protein is the concept of nutrition.  Yes, we eat because we are social animals and there is nothing more sociable than a great meal.  We eat because we listen to Bach and drink Pilsner.  We enjoy it!  But most importantly, we eat because it keeps us healthy and it contains the fuel we need every day to live and breathe and have our being.  Nutrition is of the absolute greatest importance when we produce food. The development of the art of meat curing and understanding its chemistry and processes is intimately connected to our most basic understanding of life itself.

I am downstairs in the living room.  Minette passed ut on the couch – she is exhausted.  I’m finishing up and then we will all go to bed.

I love you more than life itself!

Your Dad.

Practical Applications for the Modern Bacon Curer

In bacon production, one determines the total meat content as follows.  Assume you start with 100kg of meat and inject 20L brine.

Meat weight:  100kg
Brine added:  + 20L (100kg becomes 120kg; added through injection/ tumbling)
Loose 10% in cooking/ smoking: – 12kg (120kg becomes 108kg)
Freezing loss of -1%: – 1.08kg yields total bacon ready for slicing: 106.92kg.

Divide the meat weight you started with by the end weight after processing (100/106.92) = 93.52% total meat content.

According to SA regulations, bacon must be at least 95% total meat content.

One doesn’t lose proteins during steam cooking. Only during water cooking. In the older literature on the subject, when they talk about curing, they mean salt only curing as in dry-curing and in this process there is a loss of proteins (if done in the traditional way of turning the meat every day and allowing the extracted meat juices to run off). If one, however, cooks the bacon, as in Australia, during the cooking step, fat will melt and drip off. Exactly how much fat is lost is determined through analysis. I am sure the % is small, but surprising results are obtained through analysis.

It will impact the calculation since total meat is defined as lean meat plus fat. Meat weight after the actually visible fat has been trimmed off x 0.9 is a good approximation to determine actual lean meat content. All meat contains fat that can not be seen. Without it, meat will be completely un-edible. Two further ratios we want to become familiar with are the ratio of percentage protein nitrogen to lean meat % being N x 30 = lean meat % and the nitrogen to protein factor which is 6.25 meaning N x 6.25 = total protein.

These ratios are important for meat processors.   Let’s look at our calculation again which we used above.  Note that they only achieve total meat content of 93% in their bacon and they need to have it at 95% or above.  They can now do the following:

Meat weight:  100kg
Brine added:  + 20L (100kg becomes 120kg; added through injection)

In the tumbling stage, add 1kg of pork protein (80% actual protein – the other 20% will be a filler).  Of course, various levels of functionality are commercially available and one must inquire of what the actual protein percentage is to complete the calculation.  This means that the nitrogen added in our example of a product with an 80% functionality is 80% x 1kg = 0.800kg protein / 6.25 – the nitrogen-to-protein ratio to give us the weight of the protein nitrogen x 30 – the protein-to-lean-meat factor = 3.84kg lean meat. In other words, by adding 800g functional protein, they have effectively added 3.84kg to the starting meat weight as lean meat.  There is no fat since the added functional pork proteins do not contain fat.

They can then use their starting ratio as 100kg + 3.84kg = 103.84 which, after injection and tumbling will yield them 106.92.  Dividing the meat weight you started with by the end weight after processing is now 103.84/ 106.9 = 97.1% total meat content which, if this is in SA, places you well within the legal requirements for bacon.

For those interested in having this in a live spreadsheet I include this sheet, courtesy of Dr. Francois Mellett. ED2-8 Cost op Protein, LME, and TME.  Here he compares the cost of different protein sources and uses the conversion factor of 4.8 to move between % protein and TME/ LME.  He derives his conversion factor of 4.8 to move between % protein and LME eqw as follows:  The two equations he works with are:

Protein Nitrogen x 6.25 = Proteins

Percentage Lean Meat = (Percentage Protein Nitrogen × 30 )

Let’s take TVP Soy with a protein content of 50%.  Therefore:

Protein Nitrogen x 6.25 = 50%; Protein Nitrogen % = 50%/6.25 = 8

Percentage Lean Meat = (8 × 30 ) = 240/100 = 2.4.

The same can be achieved by the factor 30/6.25 = 4.8; 4.8 x 50% = 250/100 = 2.4

A very small added benefit for the producer will be that the protein added representing 3.84kg lean meat will be cheaper than the actual meat.  There is, therefore, no financial downside for the producer.  The producer is limited in how much of the protein can be added since it will start to affect the appearance and colour of the bacon.  My suspicion is that in countries like Australia, more can be added due to the fact that the bacon is sold fully cooked which yields a paler bacon as opposed to South African producers where the bacon is sold par-cooked and have a much brighter reddish-pinkish appearance.  Adding protein, I suspect,  will, therefore, have less of an impact in Australia compared to South Africa.  I will not be surprised if some Australian producers add a lot more non-meat and meat protein alike and therefore inject more brine.

The reality is that actual food legislation in Australia and New Zealand allows for a slightly different approach which we will look at in detail in the next article. For now, it is enough that we start interacting with some of the values we encounter as we learn how they were discovered.

We continue our fascinating journey by looking at the contribution of a formidable man, Justus von Liebig during whose time, protein was identified and named.  We also encounter our first ratio when Mulder estimated that meat proteins contain 16% nitrogen (N).  By multiplying the nitrogen content with 100/16, the protein content is
estimated. Therefore, nitrogen x 6.25 is the protein content.

Further Reading

Counting Nitrogen Atoms – The History of Determining Total Meat Content


(c) eben van tonder

Bacon & the art of living” in bookform
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(1) Nitrate is the essential curing agent and in Salpeter is coupled with potassium or sodium or calcium.


Bier, D. M.; The Energy Costs of Protein Metabolism: Lean and Mean on Uncle Sam’s Team, Protein and Amino Acids, 1999, Pp. 109-119. Washington, D.C., National Academy Press

Bulletin des Sciences Physiques et Naturelles en Néerlande (1838). pg 104. SUR LA COMPOSITION DE QUELQUES SUBSTANCES ANIMALES.

Carpenter, K. J.; A Short History of Nutritional Science: Part 1 (1785–1885), The Journal of Nutrition, Volume 133, Issue 3, 1 March 2003, Pages 638–645,

Hartley, Harold (1951). “Origin of the Word ‘Protein. Nature 168(4267): 244–244. Bibcode 1951Natur.168..244Hdoi10.1038/168244a0.

Munro, H. N., and Allison, J. B..  1964.  Mammalian Protein Metabolism.   Academic Press.

Vickery, H, B; The origin of the word protein” Yale journal of biology and medicine vol. 22,5 (1950): 387-93.

“Ueber die Zusammensetzung einiger thierischen Substanzen”. Journal für Praktische Chemie (in German).16: 129–152. 1839.doi10.1002/prac.18390160137

Featured Image: Venison Sausage Catalan Style, Robert Goodrick.

One thought on “Chapter 09.08 Von Liebig and the Theory of Proteins of Gerard Mulder.

  1. This was absolutely amazing reading,all about a dream,research,teachers,adventures,people,team,family,balls,implementation,pride,believe,and a lot of hard work,satisfaction.That dream will be successful.
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    Christo Wessels

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