Counting Nitrogen Atoms – Part 2: Von Liebig and Gerard Mulder’s theory of proteins

Counting Nitrogen Atoms – The History of Determining Total Meat Content
Part 2:  Von Liebig and Gerard Mulder’s theory of proteins
By Eben van Tonder
25 September 2018

Previous Installments in Counting Nitrogen Atoms

Part 1: From the start of the Chemical Revolution to Boussingault


More men and women who led us to the theory of proteins, understanding their metabolism, digestion, characteristics and how to manipulate them followed the work from the 1600s and 1700s.  Few others had such a profound impact on the progression of the concept of protein and understanding its metabolism than Justus von Liebig.  This chapter mainly deals with his contributions, but also that of DaltonWöhler, Berzelius, and Mulder.

John Dalton (1766 – 1844)  – developed the atomic theory.

Friedrich Wöhler (1800 – 1882) – In 1828 he was able to synthesise urea.

Justus von Liebig (1803 – 1873) – studied protein metabolism and placed it on the firm chemical basis;  father of agricultural chemistry.

Wöhler, in collaboration with Liebig, developed the organic chemistry concept that a common radical that would combine with other reagents, but still retain its own nature and be recoverable by further reactions.

Jöns Jacob Berzelius (1779 – 1848) coined the term nitrogen and suggested it to Mulder on 10 July 1838.

Gerard Mulder (1802–1880) – in 1839 established the basic nitrogenous component of a number of organic compounds (fibrin, egg albumin, gluten, etc.) to contain  ~16% nitrogen.   It is the basis of the calculation N x 6.25 (1/0.16 = 6.25) to convert nitrogen content into protein content.  He used the term protein to refer to a protein radical.

In 1842 Liebig also contributed to the study of protein metabolism by drawing attention to urea as an end-product of protein breakdown in the body.

Liebig at first embraces the concept, but after contradictory laboratory results, he rejected the theory in its current form in 1847 (English publication).  The concept of a protein radical disappeared from literature, but the concept of protein as the basic building block of nature and the name were retained.

Despite the fact that almost all his theories have been disproven in subsequent years, Liebig made immense contributions in advancing the study of protein metabolism.


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 (R100kg plus 20L water less smoking loss) will yield them 108kg.  Dividing the meat weight you started with by the end weight after processing is now 103.84/ 108 = 96.1% 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 by 100/16, the protein content is estimated. Therefore, nitrogen x 6.25 is the protein content.

Justus von Liebig

Justus von Liebig’s 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 him, set his career on the path it took.  Thus far, it was the French chemists who were responsible for the progression of protein metabolism.

Humboldt arranged an appointment for Liebig at the small University of Giessen in May 1824. Liebig wrote about this appointment  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. With this, the advance in our understanding of protein metabolism shifted to Germany.

“In Giessen, Liebig built up the most thriving school of organic chemistry than in existence, and he perceived that his studies could be logically extended to the chemistry of the living body. His book “Thierchemie in Ihrer Aufwendung auf Physio logie” appeared in 1840, and an English edition entitled “Animal Chemistry, or Organic Chemistry in its Applications to Physiology and Pathology” was published in 1842. Liebig’s main contribution to the study of protein metabolism was to point to its chemical basis, a contribution he was well fitted to make through his training in France and his own studies in organic chemistry. His views on various aspects of protein metabolism can be assessed by quoting some passages from his books. In “Animal Chemistry” (1842), he writes (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 fiber, 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 fiber, and muscular fiber 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 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 fiber, 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 is, 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 proceeds to recognize a vegetable fibrin, vegetable albumin and vegetable casein similar in properties to these animal products, and goes on to comment (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 summarizes his views on the role of nitrogen in nutrition 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.”

These comments do not add appreciably to the concepts which Magendie had propounded 25 years before. It will be particularly noted that Liebig had no conception of the possibility of digestion and reconstruction of proteins taken in the diet.”  (Munro and Allison, 1964)

“Liebig and his students also applied oxidizing agents such as manganese dioxide and chromic acid during acid hydrolysis of proteins, thus obtaining and identifying a series of acids and aldehydes. The idea of studying the degradation products of protein, which was to play such an important role in the next generation, stems from Liebig’s imaginative genius.”  (Sahyun, M. (Editor). 1948)

The atomic theory

“Another important advance in chemistry was taking place that would be put to use in subsequent nutritional studies. John Dalton, a poor and largely self-educated schoolmaster in the north of England, had an important idea. This was that all elements are made up of indivisible particles, or “atoms,” and that for each element every atom is identical. Chemical combination occurs when two or more different atoms form a firm union. These ideas were supported by the proportions of different elements in any compound being fixed and by the different compounds between the same two elements being in simple ratios by weight. Thus the gas we call “carbon dioxide” has exactly twice the weight of oxygen (per unit weight of carbon) that is present in the other gas called “carbon monoxide.” Finally, gases were found to combine in simple relations by volume. Thus 3 volumes of hydrogen combine with 1 volume of nitrogen to form exactly 2 volumes of ammonia gas. From this it also follows that equal volumes of different gases contain the same numbers of molecules, once one accepts that many elements, such as hydrogen, oxygen and nitrogen, have two atoms combined together to form a single molecule.

For some years there was controversy as to whether carbon and oxygen each had one-half of the atomic weights that are now assigned to them, although it is easy to correct molecular formulas obtained in that period. Thus Prout, in England, subjected urea to improved methods of analysis and obtained a molecular formula of C2H4N2O2, which agrees with the modern formula of CH4N2O when we double the atomic weights for C and O.

In the following decade, Friedrich Wöhler in Germany found that he had obtained urea by heating silver cyanate with ammonium chloride. He wrote excitedly to his former professor: “I can make urea without the use of kidneys.” Admittedly, urea was only an excretion product, but the synthesis was one small step in demonstrating that an organic compound produced in living systems could also be produced in the laboratory without the aid of any “vital force.”

Wöhler, in collaboration with Liebig, also developed an important concept in organic chemistry. This was the idea of a common radical that would combine with other reagents, but still retain its own nature and be recoverable by further reactions. The first example was the “benzoyl” radical. Starting with benzaldehyde, one could oxidize it to benzoic acid or form a chlorinated derivative, and so on, and then reproduce the original benzaldehyde by appropriate reduction.”  (Carpenter, 2003)

Gerard Mulder and the nature of animal substance

The Dutch chemist Gerard Mulder (1802–1880) had published a paper in a Dutch journal in 1838 and this was reprinted in 1839 in the Journal für praktische Chemie. Mulder had 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 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 also 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 united to 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)

“This concept was seized upon by Liebig, who elaborated it thus (p. 104): “… 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 caseine, 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)

Viewed in this light, the chief constituents of the blood and the caseine of milk may be regarded as compounds 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 invariable; and this compound may be considered as the commencement and starting-point of all other animal tissues because these are all produced from the blood. . . . Mulder further ascertained, that the insoluble nitrogenised constituent of wheat flour (vegetable fibrine), when treated with potash, yields the very same product, protein; and it has recently been proved that vegetable albumen and casein are acted on by potash as animal albumen and casein are. The true starting-point for all the tissues is, consequently albumen; 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 then (p. 131), 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 continued to explore the field of protein chemistry and eventually came to reject Mulder’s original concept of the nucleus of “protein.”

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)

“In the midst of this destructive criticism, however, Liebig is constructive enough to suggest the lines along which research has 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 ironical to think that, in using the word “protein” to denote the most important class of body constituents, we are commemorating an erroneous oversimplification of protein structure, and furthermore are using the word in a meaning different from that originally intended. It is significant that the German word for protein, as English-speaking people now use the word, is “Eiweiss.” This may well be a tribute to 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 concept of a 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?

In his book, Animal Chemistry or Organic Chemistry in its Application to Physiology and Pathology, Liebig argued 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.

If that was true, what role was left for the other constituents of the diet, and why did carbonic acid production increase so greatly during exercise? Liebig’s explanation was that increased respiration was needed to keep the heart and other tissues from overheating. However, this, unfortunately, led to more oxygen gaining access to the tissues, which could cause oxidative damage and loss of protein tissue. It was the function of the fats and carbohydrates to mop up this excess by being themselves preferentially oxidized.

Liebig’s book was at first generally regarded as a giant intellectual synthesis, and many people were converted to his ideas. For example, when the Professor of Medicine at Edinburgh University was called in to investigate a serious and unexpected outbreak of scurvy in a Scottish prison, his immediate conclusion was that it must be the result of an inadequate intake of protein. However, his calculations indicated that the average daily protein intake was an ample 135 g. But only 15 g of this quantity were from animal sources and 102 g were from gluten.

He suggested that the power of the body to convert gluten to animal protein was limited and that the level of milk in the diet should be increased so as to raise the intake of animal protein.

Another Scottish physician 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.

Another difficulty in believing that muscular work required the breakdown of protein was that the traditional diet of labourers was of lower protein content than of the less active rich. Edward Smith, a British physician and physiologist who was interested in the welfare of prisoners, and was concerned at the stressfulness of their having to work on a treadmill, measured their urea excretion in the 24 h during and after their 8 h of work, and again on their subsequent rest days, and found no difference. This was, of course, quite contrary to what Liebig would have predicted 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

“Liebig also contributed to the study of protein metabolism by drawing attention to urea as an end-product of protein breakdown in the body. Here again, however, he appears to have been the author of some misconceptions, for in his “Animal Chemistry” (1842), (p. 62) he says: “… 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 which undergoes metamorphosis. Thus, after a walk, the secretion of urine in man is invariably increased.

Later (p. 245), he says: “The amount of tissue metamorphosed in a given time may be measured by the quantity of nitrogen in the urine.” This statement reflects Liebig’s view that protein in muscle was the fuel for muscular exercise, and 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

“It may appear in these quotations from Liebig’s writings that he did not contribute much of permanent value to the study of protein metabolism. This is not so. Through his vigorous application of organic analysis to compounds of biological interest, of which he identified several, he laid the foundations of intermediary metabolism and made advances possible for his successors. Thus, although he did not resolve any major problems, he pointed the way to their ultimate solution. Of intermediary metabolism, he says prophetically (“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.”

He was also aware that some chemical reactions only occur in biological systems and suspected that these were dependent on the presence of proteins. The following passage (p. 7) from his book on food chemistry shows how close he comes to our modern 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.”

Finally, Liebig contributed to the development of protein metabolism by founding a school of biochemical studies, first in Giessen, and later in Munich, where he became professor of chemistry in 1852. From this school emerged a number of distinguished exponents of metabolism, chief among them being Carl Voit, whose researches in protein metabolism placed the concept of nitrogen balance on a firm footing.

He too had become interested in the subject of “animal chemistry,” and wrote that Dumas must be wrong because it was well known that pigs would fatten when fed on potatoes that were rich in starch, but contained only a negligible level of fat. This meant that animals must be able to convert carbohydrates to fat even though the conversion required “reduction” rather than oxidation.

This was a challenge to the French workers who had been the undisputed authorities in the field, and Boussingault put the matter to the test in another pioneering study. He killed and analyzed the carcass of a young pig, while feeding a littermate of the same starting weight on measured amounts of feed for an additional 3 months. Carcass analysis of the second pig showed that it contained an additional 13.6 kg fat, whereas the feed it had eaten had only contained 6.8 kg.

This careful work had therefore shown that the French school was in the wrong on this point. Boussingault and Dumas both retired from working with animals, and Liebig became the new authority, even though he had never actually carried out a feeding trial. He continued to push 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)


Neither Mulder nor Liebig illuminated protein or its metabolism fully, but we gain a great appreciation for the work done by these men in the early 1800s. I wonder how many of today’s researchers would do as much as these men did with the scant knowledge they had.

Nitrogen, key to the art of bacon curing takes front and centre stage in the formulation of the theory of animal proteins and nutrition. It becomes essential, not just in preserving meat, but in defining it. Its chemistry is important, not just to meat processing, but to life itself. It is astounding to recognize a man like Edward Smith as a contemporary of Liebig who would pen one of the most authoritative works on food and nutrition.

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.

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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.

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Featured Image: Venison Sausage Catalan Style, Robert Goodrick.

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