The nitrogen cycle and meat curing

**  article still being written**

Until the end of the 1800’s, we used saltpeter to cure meat or potassium nitrate.  Dr. Ed Polenske (1849-1911), working for the Imperial Health Office in Germany, observed that nitrites were formed in curing brine and speculated that this was due to the action of bacteria, removing the one loosely bound oxygen atom (reducing) from nitrate (CodeCogsEqn (24)) to create nitrite (CodeCogsEqn (17)).  He published in 1891.  The link between nitrite and the reddening effect upon meat was finally established in 1899 by the German scientist, K. B. Lehmann.  (Fathers of Modern Meat Curing)

His correct speculation of the mechanism that transforms nitrate to nitrite is very interesting.  Where did he get this information from and since when has this been known?

Answering this question takes us to the discovery of one of the greatest invisible forces on earth which make life possible namely the discovery of the nitrogen cycle.  It was this knowledge that was necessary for Polenski to make this remarkable conclusion in 1891 which echoed in the era of modern meat curing.

The most abundant gas on earth is nitrogen, making up 78% of the volume of our atmosphere.  The three cousins of the chemical gas, nitrogen are ammonia, nitrite and nitrates.  These three cousins are key to all life and exist almost everywhere.  It occurs naturally in sea salt, in the ground, in salt beds.  They are pervasive.  Without them, we won’t be able to shoot a gun, fertilize our fields or cure bacon.

The chemical element of nitrogen becomes one of the most important chemicals for us to understand when we learn about bacon curing.  Nitrogen by itself is a gas and it becomes useful to plants, other animals, and humans when it combines in the chemical compounds of nitrate, nitrite and ammonia and ammonium.

I wrote to Tristan about nitrate when I introduced him to saltpeter and nitrite, when I told him about Dr. Ed Polenski’s insight and experiment showing that in bacon cures, nitrate is converted to nitrite before curing takes place.  It has recently been shown that there is a conversion of each of these compounds into the other through the action of small organisms, called bacteria in soil and water.  It was these discoveries that gave Dr Ed the insight that it may be bacteria in brine, changing the nitrate to nitrite.

In order to get the full picture then of how nitrite is formed, we also have to understand ammonia.

The name, ammonia, came from the ancient Egyption god,  Amun.  The Greek form of Amun is Ammon.  At the temple dedicated to Ammon and Zeus near the Siva Oasis in Lybia, priests and travellers would burn soil rich in ammonium chloride.  The ammonium chloride is formed from the soil, being drenched with nitrogen waste from animal dung and urine.  The ammonia salts were called sal ammoniac or “salt of ammonia” by the Romans because the salt deposits were found in the area.  During the middle ages ammonia was made through human endeavour through the distilling of animal dung, hooves and horns.   (Myers, RL.  2007:  27)

The first step in understanding the compound was to identify its constituent chemical parts.  In Part II of his work, Experiments and Observations, the English natural philosopher and chemist, Joseph Priestley described work that he had done between the years 1773 and the beginning of 1774.  In this document he gives a a reprint of an earlier publication on effluvia from putrid marshes.  Here he identifies ammonia and nitrous oxide.  (Schofield, RE.  2004:  98)

His discovery of the composition of ammonia was the result of a consistent application of the English scientist, Stephen Hales’s (1677 – 1761) technique for distilling and fermenting every substance he could get his hands on or capture over mercury rather than over customary water so that the air would “release.”  He heated ammonia water and collected a vapour.  When it cooled down, it did not condense, proving it was air.  He called it alkaline air.  (Schofield, RE.  2004: 98, 103, 104)

More experiments showed him that alkaline air was heavier than common inflammable air but lighter than acid air.   It dissolved easily in water, producing heat and it was slightly inflammable in the sense that a candle burned in it with an enlarged colour flame before going out.  In the end he not only described ammonia chemically, but also its mode of production, and its characteristic’s.   (Schofield, RE.  2004: 98, 103, 104)

In 1781 the French Chemist, Claude Louis Bertholett became aware that something joined with hydrogen to form ammonia.  Three years later, Claude joined Lavoiciere who was responsible for unraveling the composition of saltpeter along with de Morveau and de Fourcroy, in naming the substance azote.  (Smil, V.  2001:  61, 62)  Lavoiciere named it from ancient Greek, ἀ- (without) and zoe (life).  He saw it as the part of air that can not sustain life.  (Wikipedia,  Azote)

The modern name, ammonia, was given in 1782 by the Swedish chemist Torbern Bergman.  (Myers, RL.  2007:  27)

So Jeppe introduced me to the three cousins of nitrogen that are important for our consideration of the curing agent, nitrite and the key scientists in history who helped to unlock their secrets.

The next step would be the discovery of how nitrogen change into its cousins and enters the earth and living plants and animals.

A science class uses microscopes in a lab in 1908. (University Archives Photo)

The afternoons with Jeppe became challenging as I tried to keep up with his lectures.  He seemed to remember the names and formulations off by heart and I was not always sure who or what we were talking about.  It was nevertheless engaging and I tried to keep up.

People knew that nitrogen is all around us in the air, but how did its family members end up in decaying animals, urine, in salt crystals in the Indian deserts, in bat caves in America  and grow on walls around the world.

HB de Saussure (1740 – 1799) discovered that the nitrogen in plants does not come directly from the atmosphere.  (Bynum, WF, et al, 1981:  300)  He was born in Switzerland and became interested in biology and geography.  Most of his discoveries he made while scaling some of the highest mountain peaks and passes in the world.  He regarded the Alps as central to understand the geology of the world and spend much time there. (Wikipedia, Horace-Benedict _de_Saussure)

He’s idea was that nitrogen must be taken up through the roots of plant, through the decomposition of humus (4).    (Bynum, WF, et al, 1981:  300)  Not everybody agreed with him and a debate developed that raged for almost 50 years.

The German chemist, Justics von Liebig (1803 – 73), was the first to see nitrogen as an essential plant nutrient.  This discovery gave him the honour of being regarded as the father of the fertilizer industry.  (Wikipedia, Justice_von_Liebig)

Justice was also an important man in the meat processing industry.  He developed the manufacturing process for beef extract and founded a company, Liebig Extract of Meat Company, and later trademarked the Oxo brand beef bouillon cube. (5)  (Wikipedia, Justice_von_Liebig)

This question of how nitrogen was absorbed by plants remained very controversial (6).  Justice believed it being taken directly from ammonia gas in the air.  (Craine, JM,  2008:  70)  This was the state of affairs until a French chemist, JBJD Boussingault (1802 – 87) demonstrated that plants are incapable of absorbing free nitrogen but was able to flourish even without humus as long as alternative sources of nitrates or ammoniacal salts are supplied.  (Bynum, WF, et al, 1981:  300)

Boussingault and his contemporaries saw the uptake of ammonia as purely chemical.  (Bynum, WF, et al, 1981:  300)  What other way could there be?

The great German physiologist, Theodor Schwann, born in 1810, took a step closer to the solution.  He discovered that alcoholic fermentation and the fermentation that causes putrefaction were carried out by microbes. (7)  (Barnett, JA)

Louis Pasteur, born in 1822 grew up to become very important in the field of science.  He was the first one to suggest that microorganisms may be involved in the nitrogen absorption process of plant.  (Bynum, WF, et al, 1981:  300)

He studied the breakdown and reorganization of material that contained nitrogen by soil bacteria, fungi and algae.  It seemed that nitrogen was not used up but was circulated.  Decaying humus gave ammonia, from which micro organisms constructed nitric acid and its compounds.  These were then absorbed by plants and turned into proteins and incorporated into living substance.  The cycle was completed by the death and natural decay of the plant and the animal. (Bynum, WF, et al, 1981:  300)

The German agricultural chemist, Hermann Hellriegel (1831-1895), discovered that certain plants (leguminous) take atmospheric nitrogen and “replenished the ammonium in the soil through the process now known as nitrogen fixation. He found that the nodules on the roots of legumes are the location where nitrogen fixation takes place.”  (Boundless, 2014)

Hermann did not discover how this is done. Martinus Willem Beijerinck (March 16, 1851 – January 1, 1931), a Dutch microbiologist and botanist, discovered that the small growth areas on the roots contained bacteria.  He called it rhizobia.  It is the rhizobia that is responsible for changing the nitrogen to ammonium.  Ammonia is NH3 and ammonium is NH4.  (Boundless, 2014)

Soon more ways were discovered that changed nitrogen in the air into a form that plants can absorb.

Berthelot described in 1885 how lightning was responsible for nitrogen fixation before he too turned his attention to microscopic organisms in the ground that is responsible for nitrogen fixation. (Elmerich, C, Newton, WE.  2007:  3)

The energy of a lightning strike disrupts the nitrogen (N2) and oxigen (O2) molecules in the air producing highly reactive nitrogen and oxygen atoms that attract other nitrogen (N2) and oxigen (O2) molecules that form nitrogen oxides that eventually become nitrates. (Zumbal, 2000:  924)  Alternatively, Beijerinck’s rhzobia bacteria fixes the atmospheric nitrogen directly (Boundless, 2014)  in small growths on plant roots such as beans, peas and alfalfa (Zumbal, 2000: 924), or  animal droppings and urea or dead animal or plants provide saprobiotic bacteria, nitrogen or nitrogen-family members that can be changed. (Wikipedia, Nitrogen Cycle)

Nitrogen is turned directly into either ammonia (NH3) or amonium (NH4) or into nitrate (NO4).  Nitrifying bacteria turns the ammonia into nitrite.  Nitrite is toxic and nitrifying bacteria change the nitrites into nitrates that either becomes plant food along with nitrate’s that is formed during lightning strikes or is changed back into nitrogen by denitrifying bacteria.  (Wikipedia.  Nitrogen Cycle).

Chemical Engineering at MIT
Chemical Engineering at MIT

A friend of Jeppe, Dr. Ed Polenski found in 1891, months before I arrived in Denmark, that when he mixed curing brine for bacon with Saltpeter and tested it, that he found nitrate to be present.

After a week, when he tested it again, there was only nitrites. The same with the meat that he cured.  At the beginning of the week, there was nitrate present in the meat and later he found only nitrites. (8)

The notion that bacteria is responsible for changing the nitrate to nitrite was well established by the time he did the experiment and so, his conclusion that what had happened in the brine was the result of bacteria, was reasonable.  He suggested that nitrite is the curing agent at work in meat. (8)

I realised that saltpeter was a key part of the world we live in.  The energy of the acid in the air, harnessed by an entire world of microorganisms that probably occur in every environment on earth and changed into a format that plants and then humans and animals can absorb.    An acid, coupled with a salt, helping us to preserve meat and change pork meat into bacon, grow plants, feed oceans and drive the processes of earth.  By it we fight wars, we grow crops and we eat and live!

food 2

Edward Smith wrote on bacon and said, “bacon is the poor man’s food, having a value to the masses which is appreciated in proportion to their poverty, and it is a duty to offer every facility for its production in the homes of the poor.” (Smith, Edward, 1876:  65)

The reason why it is good for the poor is that it can be cooked in water and the liquid part can be given to the children and the solid part consumed by the parents and “thus both be in a degree pleased, if not satisfied.” (Smith, Edward, 1876:  65)

He continues to say that “it is also the rich man’s food, for the flavour, which is naturally or artificially acquired by drying (and curing), is highly prized, and although it may be taken as a necessary by the rich, it is in universal request as a luxury”  (Smith, Edward, 1876:  65)

This is our business plan.  To produce the best bacon on earth.  Uncle Cornelius passed away after a full life and I can not help to see our current quest as a necessary evolution of time as young and new thoughts replace older methods.

The evolution must in the first place be predicated on sound science as well as common sense.

I miss you my little girl.  There is not a single day that I dont think off you!  Its late.  I am sure that you are fast asleep by this time and that you are holding your bear and dream of the cumming summer.

I learn so much and still, you are my biggest lesson in life.  Your love and your spirit has taught me how to live myself!

I count the days till I see you guys again!  I miss you all so much and love you!

Your Dad.


(c) eben van tonder

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(1)  After a short service in the Woodstock house, the procession moved to the Groote Kerk where Jacobus has been an elder.  The coffin was carried into the church by the Cape premier, Cecil John Rhodes, Sir John Henry de Villiers (subsequent chief justice of the Union), JW Sauer, Onze Jan Hofmeyer, Sir Gordon Sprigg, Colonel F. Schermbrucker, ML Neetling and DC de Waal.

After the service the funeral procession moved to the Cape Town station, where a special train took the mourners to the Maitland Cemetery.  The coffin, of Cape teak, was lowered into the ground which Jacobus picked himself.

The grave was filled up and wreaths were laid on top.  One from David and Johanna Graaff, a second from John and Rosetta Graaff and a third from Jacobus and Susan Graaff. (Dommisse, E, 2011:  48, 49)

Jacobus Combrinck’s grave in the Mailtland Cemetery.
The funeral procession would have walked along this path from the train tracks at the far top side of the picture to Jacobus’s grave on the right, under the tree, on the right.



The affection from the Graaff brothers who were responsible for erecting the grave stone is evident.  At the top, the words, “Ter Dierbare Herinnering aan Jacobus A. Combrinck,” “For affectionate remembrance of Jacobus A. Combrinck.”

Under Jacobus’s birth date and date of passing, the inscription in Dutch reads, “Ik weet op wien te vertrouen,” “I know in whom to trust.”

Underneath is written in Dutch,”Opgericht door zyne dankbare neven de broeders Graaff,” “Erected by your grateful nephews, the brothers Graaff.”

David took over Jacobus’s position in the Legislative Council of the Cape Colony soon after his passing.

The following notice appeared in a colonial newspaper.

A notice published on page 11 in The Colonies and Indian under the heading “Colonial, Indian and American News Items” on 10 Oct 1891.

(2) The Woodstock house was previously owned by a highly respected judge, Henry Cloete in the suburb of Papendorp (later to be renamed, Woodstock).  He enlarged it greatly.  The house was built on an estate where Jacobus planted trees, erected a water mill of his own design, cultivated a splendid flower garden.  (Simons, PB, 2000:  14)

(3)  Sir Gordon Sprigg, prime minister before Rhodes ousted him, was moved when he heard the news of Combrink’s death.  He said, “A good man has gone from among us.”  Rhodes apparently only slipped a posy of white and purple violets into his coffin and said nothing.  These two powerful men were never the best of friends. (Simons, PB, 2000:  27)

(4)  When doing trials at the then Vion Factory in Malton, Ken Pickles was the NPD (New Product Development) manager.  A young intern from Brazil would walk behind him and every time we went to the curing tanks, he would ask the young man this question.  It’s an image that I will never forget.

(4)  Humus is decaying organic matter.  (Bynum, WF, et al, 1981:  300)

(5)  The trademark was granted in 1899 for Oxo.

(6)  The German chemist, Justice von Niebig (1803 – 73), continued to believe that plants got their nitrogen from the air (in the form of ammonia).  (Wikipedia, Justice_von_Liebig)  He was popularised a principle developed in agriculture science by Charl Sprengel (1828) and was called Liebig’s law of the minimum, often simply called Liebig’s law or the law of the minimum. It states that growth is controlled not by the total amount of resources available, but by the scarcest resource (limiting factor)  (Wikipedia, Law_of_the_Minimum)

(7)  He also attributed fermentation to microorganisms.

“Schwann  is famous for developing a ‘cell theory’, namely, that living structures come from formation and differentiation of units (the cells), which then constitute the bodies of organisms (Schwann, 1839). His paper on fermentation (Schwann, 1837) was entitled ‘A preliminary communication concerning experiments on fermentation of wine and putrefaction’. Using a microscope, Schwann examined beer yeast and described it as resembling many articulated fungi and ‘without doubt a plant’. His conclusions from his observations and experiments were unequivocal, revolutionary and correct:The connection between wine fermentation and the development of the sugar fungus is not to be underestimated; it is very probable that, by means of the development of the fungus, fermentation is started. Since, however, in addition to sugar, a nitrogenous compound is necessary for fermentation, it seems that such a compound is also necessary for the life of this plant, as probably every fungus contains nitrogen. Wine fermentation must be a decomposition that occurs when the sugar-fungus uses sugar and nitrogenous substances for growth, during which, those elements not so used are preferentially converted to alcohol.

In one of his experiments, Schwann boiled some yeast in a solution of cane sugar in four stoppered flasks. After cooling, he admitted air into the flasks: for two flasks, the air was first passed through a thin red-hot glass tube (analysis showed this air still to contain 19·4 % oxygen); the other two flasks received unheated air. Fermentation occurred only in the latter two flasks. Schwann’s conclusion was important:Thus, in alcoholic fermentation as in putrefaction, it is not the oxygen of the air which causes this to occur, as previously suggested by Gay-Lussac, but something in the air which is destroyed by heat.

In this notable 1837 paper, Schwann anticipated observations made by Pasteur over twenty years later, writing:Alcoholic fermentation must be regarded as the decomposition effected by the sugar fungus, which extracts from the sugar and a nitrogenous substance the materials necessary for its own nutrition and growth; and substances not taken up by the plant form alcohol.

(Barnett, JA.   1998, 2000)

(8)  The chemist, Eduard Polenske (1849-1911) (Wikipedia. Pökeln), was born in Ratzebuhr, Neustettin, Pommern, Germany on 27 Aug 1849 to Samuel G Polenski and Rosina Schultz. Eduard Reinhold married Möller. He passed away on 1911 in Berlin, Germany. (Ancestry.  Polenske)  He was working for the German Imperial Health Office when he made the discovery about nitrite in curing brine. (Wikipedia.  Eduard_Polenske)

The Imperial Health Office was established on 16 July 1876 as a focal point for the medical and veterinary in Berlin. First it was the division of the Reich Chancellery and since 1879 the Ministry of the Interior assumed. 1879, the “Law concerning the marketing of food, luxury foods and commodities” was adopted, including the Imperial Health Office was responsible for its monitoring.  Erected in 1900 Reichsgesundheitsrat supported the Imperial Health Office in its tasks.  (Original text:  “1879 wurde das „Gesetz betreffend den Verkehr mit Lebensmitteln, Genußmitteln und Gebrauchsgegenständen“ verabschiedet, für dessen Überwachung unter anderem das Kaiserliche Gesundheitsamt zuständig war.”) (Wikipedia.  Kaiserliches Gesundheitsamt)

Spelling of his surname varies between Polenski and Polenske.


Barnett, JA.   1998, 2000.  Extracts from lectures.  Beginnings of microbiology and biochemistry: the contribution of yeast research.

Bynum, WF, Browne, EJ, Porter, R.  1981.  Dictionary of the History of Science.  Princeton Legacy Library.  Macmillan Press.

Craine, JM.  2008.  Resource Strategies of Wild Plants.  Princeton University Press.

Danchin, A.  From Lamarck to Semmelweis, Transformation of chemical biology1800 – 1849:

Dommisse, E. 2011.  First baronet of De Grendel.  Tafelberg.

Associative and Endophytic Nitrogen-fixing Bacteria and Cyanobacterial …

Elmerich, C, Newton, WE.  2007.  Associative and Endophytic Nitrogen-fixing Bacteria and Cyanobacterial Associations.  Springer.

Myers, RL.  2007.  The 100 most important chemical compounds.  Greenwood Press, Westport.

Schofield, RE.  2004.  The Enlightened Joseph Priestly.  The Pennsylvanian Sate University

Smith, Edward.  1876. Foods. D. Appleton and Company, New York.

Simons, Phillida Brooke. 2000. Ice Cold In Africa. Fernwood Press

Smil, V.  2001.  Enriching the Earth.  Massachusetts Institute of technology.

Zumbal. 2000. Chemistry, 5th edition.  Houghton Mifflin Company. (Early Discoveries Nitrogen Fixation)


Figure 1:  From Simons, Phillida Brooke. 2000. Ice Cold In Africa. Fernwood Press, page 9.

Figure 2:

Figure 3:

Figure 4:

Figure 5:  From

Figure 7 – 9:  Photos of Combrinck’s grave by Eben.

Figure 10:  The Colonies and Indian, 10 Oct 1891, p 11.