Mechanisms of meat curing – the important nitrogen compounds
By: Eben van Tonder
4 September 2016
INTRODUCTION
In these articles, we examine the mechanics of meat curing so that we can ensure factory conditions and processing steps that favour curing.
In our previous article, we set the historical context of our understanding of curing mechanisms as it relates to colour development in cooked cured meat. Here we deal with the reactivity of nitrogen and how it changes into a form that we use in meat curing.
SUMMARY
This article follows the formation of some of the important nitrogen compounds and molecules starting with nitrogen gas in the atmosphere, the formation of nitric oxide, nitrogen dioxide, nitric acid and ammonia. We briefly discuss the history of the discovery of nitrogen and its compounds and the nitrogen cycle.
THE REACTIVITY OF NITROGEN
Since a French craftsman, Bernard Palissy made a scientific connection between food production and fertalizers in 1563, it has become common understanding among farmers that plants use substances that are in the soil and that soil must, therefore, be replenished with manure in order to aid plant growth. Palissy used the term “salts” to refer to minerals and nutrients and he understood that plants needed these salts from the soil. (, J. N, et al., 2013)
By the 1880’s, farmers universally were well informed that one of the key components in Palissy’s “salts” is nitrogen. Nitrogen was known to be a gas that forms part of the atmosphere and even in 1880, it was correctly estimated to comprise at least 4/5th. The rest, roughly 20%, is oxygen. (2) (Marion Record, 1887, p3: About Nitrogen)
An article from the Marion Record in 1887, reminded us that the role and effect of nitrogen in human and animal tissue are relevant, not just to the living but also to pork from which we make bacon. The art of bacon curing can very succinctly be stated as the art of the manipulation of the properties of meat through nitrogen, sodium and chloride.
The curing of meat revolves around nitrogen and it is helpful to know a bit more about the reactivity of this unique chemical element and how it naturally becomes part of our world in forms that we can use. Nitrogen forms the link between fertilizers, food processing and war since the same power fires bullet, provides nutrition to plants and cures meat for future consumption and its power gives life to the world we live in. By it, plants and living organisms live and breathe and have their being.
THE DISCOVERY OF NITROGEN AND IMPORTANT NITROGEN COMPOUNDS
Nitrogen was so named by the early chemists as the generator of nitre. Nitre is also called saltpeter.
Nitrogen was independently discovered by two scientists. In 1772, by the Scottish physicist, Daniel Rutherford (Marion Record, 1887, p3: About Nitrogen) and in the early 1770’s by a Swedish chemist, Carl Scheele. “Rutherford named his discovery “noxious air,” because animals were not able to breathe in it. Scheele called it “foul air.” (Farndon, J, 1999: 9)
It was Antoine Lavoisier (1743 – 1794) who realized that air was basically a mixture of two gases, oxygen, and nitrogen. He burned mercury in a closed jar and found that a 5th of the air combined with the mercury to form a red powder, mercury oxide. No matter what he did, the rest stayed a gas. Mice died in it and a candle could not burn in it. “Lavoisier decided that air is made of two gases. One, which he called oxygen, was the gas that burned with the mercury. The other he called azote from the Greek for ‘no life.’ It later came to be known as nitrogen, because it can be generated from niter, the common name for sodium or potassium nitrate or saltpeter” (Farndon, J, 1999: 9) “The name ‘nitrogen’ was not actually coined until 1790 by French chemist Jean Antoine Claude Chaptal. He originally named it ‘nitrogène’, a reference to nitre (potassium nitrate), which was known to contain nitrogen.” (, J. N, et al., 2013)
At ambient temperature, the gas, nitrogen, is an inert molecule. It is, however, one of only two elements that can occur in eight oxidation states, the second one being carbon. (Honikel, 2007)
“The outer shell of five electrons (
) can take up three additional electrons giving the nitrogen an ‘‘oxidation status’’ 
as it exists in ammonia (
) or amines or it can release five electrons forming 
as it exists in nitrate 
.” “This is the reason for the variability and wide range of carbon compounds (organic matter) but also for the complexity of nitrogen reactivity. The latter is shown below.” (Honikel, 2007) Here we show the most important nitrogen compounds in their different states of oxidation.
“In the twentieth century, a process to provide an inexhaustible supply of reactive N (Nr; all N species except N2) for agricultural, industrial and military uses was invented.” (, J. N, et al., 2013) Today we have enough nitrogen available to cure meat, fertilise our crops and create explosives. So much so that we alter the nitrogen cycle with detrimental effects on humanity and the world by the wholesale use of nitrogen which we access by converting N2 into one of the Nr -forms.
Back to the chronology of the discovery of nitrogen compounds, following the identification of nitrogen, the door was open for the discovery of the important nitrogen compounds which began in the 1700’s. (1)
“Priestley, an English scientist, and clergyman, in his book Experiments and observations on different kinds of air describes how he demonstrated the presence of nitrous oxide, hydrochloric acid and ammonia gases in air. The experiments he carried out in order to do this ultimately led him to discover ammonia on 1 August 1774. In 1784, C. L. Berthollet discovered that ammonia was made up of the elements nitrogen and hydrogen. Austin was then one of the first to try to synthesize ammonia, which he called ‘volatile alkali’, from these elements. He studied the discoveries of Berthollet and Priestley and came to the conclusion that, if ammonia is decomposed by spark, it must also be possible to form it from its original elements. Priestly describes ‘nitrous air’ (nitric oxide, NO) and developed a ‘nitrous air test’ to determine the ‘goodness of air’. Using a pneumatic trough, he would mix nitrous air with a test sample, over water or mercury, and measure the decrease in volume—the principle of eudiometry. Henry Cavendish produced HNO3 in 1785 by passing sparks through a jar of air confined over water. Between 1772 and 1780, Joseph Priestley isolated 10 different gases, among those were nitric oxide, nitrogen dioxide and nitrous oxide. He also invented the apparatus to create his experiments, and wrote and published to disseminate his findings.” (, J. N, et al., 2013)
“If the eighteenth century was known for the discovery of N and its species, the nineteenth century was noted for the discovery of how nitrogen was transformed from one species to another. Most notably, and with a hint of what was to come about 80 years later, in 1823, Johann Wolfgang Döbereiner was one of the first to produce ammonia from the elements hydrogen and nitrogen using platinum as the catalyst. This was a notable achievement for a number of reasons, not the least of which was that it demonstrated that NH3 could be synthesized from its elements by human action. However, the process was inefficient and could not be reproduced on a large scale.” (, J. N, et al., 2013)
“In 1836, over sixty centuries after it was noted that manure and legumes were beneficial to crop production, French chemist Jean-Baptiste Boussingault also identified nitrogen as an important substance for plants, recognizing that the effectiveness of a fertiliser depends on and is proportional to its nitrogen content. The revolutionary work of Von Liebig entitled ‘Die organische Chemie in ihre Anwendung auf Agrikultur und Physiologie’ (Organic Chemistry in its Application to Agriculture and Physiology) heavily criticized the old concepts of plant nutrition, such as the humus theory, and developed modern ideas. Von Liebig believed that plants only need a limited number of nutrients to grow. These nutrients are present in the soil and, having been used up following intensive cultivation, need to be added to the soil again in order to guarantee the next crop. He was, therefore, the founder of the artificial fertiliser industry. However, confusion about where the necessary nitrogen came from remained. Liebig’s incorrect belief that all nitrogen assimilated by plants came from precipitation inspired further research, including a long-running crop experiment started by John Bennet Lawes and Joseph Henry Gilbert in 1843 at Rothamsted, UK. In these experiments, plots of wheat received varying treatments; they learned that crops with added nitrogen (applied as ammonium sulfate) achieved substantially larger yields. This finding confirmed the belief that nitrogen helps plants grow, and it also proved that nitrogen comes from sources other than precipitation.” (, J. N, et al., 2013)
“About three decades after it was discovered that living plants needed nitrogen, Jules Reiset recognised in 1856 that decaying organic matter releases nitrogen. This discovery ultimately provided the basis for the nitrogen cycle because it was the first evidence of nitrogen cycling in the biological sphere.” (, J. N, et al., 2013)
It is enlightening to understand one of the ways that nitrogen change from an inert gas to a form that is used as food for plants and from our vantage point, ends up curing meat. These are parts of the natural nitrogen cycle which is responsible for fertilising the earth, the very cycle we are altering by adding massive amounts of nitrogen into.
Two of the ways in the natural nitrogen cycle that atmospheric nitrogen enters the food chain is through the power of lightning and small microorganisms. Let’s first look at nitrogen that falls from the skies.
NITRIC OXIDE (NO)
Nitrogen gas exists as two atoms, tightly bound in one molecule (N2). The bonds between the atoms are so strong that it doesn’t normally react with anything else. Lightning provides enough energy to break these strong bonds which now makes the nitrogen available to react with other elements. (Farndon, J, 1999: 10)
One of these elements is oxygen. When they react, they form nitrogen monoxide (NO). Nitrogen monoxide is a colourless gas, also called nitric oxide or nitrogen oxide. The nitric oxide is heated due to the energy from the lightning flash that created it. (Farndon, J, 1999: 10)
The reaction is written as follows:
N2 (g) + O2 (g) lightning —> 2NO (g)
NITROGEN DIOXIDE (NO2)
Other sources of nitric oxide, besides lightning, are certain bacteria and volcanos. (Air Quality Guidelines, 2000: chapter 7). As it cools down, it reacts further with the oxygen molecules around it to form nitrogen dioxide. One nitrogen atom attached to two oxygen atoms forms nitrogen dioxide. “It is a poisonous, brown, acidic, pungent gas”. (Farndon, J, 1999: 12) Nitrogen dioxide is however mainly formed in the atmosphere through it’s a reaction with ozone (O3).
Like nitrogen, oxygen occurs as two oxygen atoms, bound in one molecule. Ultra-violet light and lightning cause the two tightly bound oxygen atoms to separate and react, either with other single atom oxygen molecules or with more stable two-atom oxygen molecules. In the latter case, three oxygen atoms are bound into one molecule (O3). (3) (Wikipedia, Ozone) It is not very stable and quickly breaks down into one oxygen atom (O) and or two oxygen atom molecules or it reacts with nitric oxide to form nitrogen dioxide. (Huffman, R. E.; 1992: 210) (Air Quality Guidelines, 2000: chapter 7)
The reaction occurs as follows:
NO (g) + 1/2O2 (g) —> NO2 (g)
NITRIC ACID (HNO3)
Nitrogen Dioxide (NO2) reacts with more oxygen and rain drops to form nitric acid (HNO3) which falls to earth and enters the soil to provide nutrients for plants. (Ramakrishna, A.; 2014: 14) Nitric acid (HNO3) is also known as aqua fortis and spirit of niter. (Wikipedia, Nitric Acid)
The reaction occurs as follows:
3NO2 (g) + H2O —> 2HNO3 (aq) + NO (g)
Nitric acid is highly reactive and combines with salts in the soil, converting it to nitrates which in turn become food for the plants. (Ramakrishna, A.; 2014: 14) It is this reaction of nitric acid with salts that create sodium nitrate or calcium nitrate or potassium nitrate that are used as fertilizer or in gunpowder or to cure bacon.
It has been discovered that curing happens much faster if nitrite is used directly. Bacteria are responsible for changing nitrate to nitrite when it is injected into meat as a curing agent, just as it is done by bacteria in soil. Nitrite (NO2–) is the same as nitrate (NO3–), with one less oxygen atom. By using nitrite directly, curing is accomplished much faster since the reduction to nitrite takes time.
AMMONIA (NH3)
Nitrogen comes into our lives from the atmosphere, but despite the fact that “nitrogen oxides trapped in rocks and sediments probably represent a larger total quantity of nitrogen, this nitrogen, for the most part, is not accessible to living organisms.” (Igarashi, Y. and Seefeldt, C. L.. 2003)
BACTERIA AND ITS ROLE IN NITRIFICATION
Following the discovery of nitrogen and the nitrogen compounds, “the next advances were with respect to the role that microbes play in converting N from one species to another. In short order, the processes of nitrification, biological nitrogen fixation (BNF) and denitrification were discovered. The process of nitrification was discovered in 1877 by Theophile Schloesing and Achille Müntz through their experiments with sewage water filtered through a mixture of sand and limestone. In 1886, Ulysse Gayon and Gabriel Dupetit officially discovered the process of denitrification after isolating two strains of denitrifiers.” (, J. N, et al., 2013)
“The remaining process left to be discovered was arguably the most important—BNF. In the mid-nineteenth century, scientists wondered how legumes could grow and thrive without nitrogen additions. In 1838, Boussingault conducted a series of experiments in which he grew legumes in sterilized sand, which did not have any nitrogen. When the legumes continued to grow, the only conclusion he could reach was that they are capable of fixing their own nitrogen, but he did not yet know how. In 1880, Hermann Hellriegel and Hermann Wilfarth answered this question through their discovery of BNF, the process by which microbes can convert unreactive diatomic nitrogen into a usable reactive form. They had carried out experiments testing the response of legumes and cereals to additions of different mediums and to different treatments. They determined that the only explanation for their results was that microbes can fix nitrogen and that the nodules on the roots of legumes contain these nitrogen-fixing microbes.” (, J. N, et al., 2013)
“So by the end of the nineteenth century, the basic building blocks of the N cycle had been discovered—the species, their reactions and how the reactions connected into a cycle that converted N2 to reactive N forms and then reconverted back to N2. Or so it was thought. One century later, a new microbially mediated process was discovered. Anaerobic ammonium oxidation (anammox) was hypothesized by the Austrian theoretical chemist Engelbert Broda in 1977; about 10 years later in 1986, it was observed to account for the disappearance of NH4+and the formation of N2 in a Dutch wastewater treatment facility. Anammox organisms have the ability to combine ammonium and nitrite or nitrate to form N2. This discovery led to the realisation that a substantial part of the enormous nitrogen losses that are observed in the marine environment—up to 50 percent of the total nitrogen turnover—were due to the activity of these bacteria.” (, J. N, et al., 2013)
Most nitrogen, therefore, enters our world through special bacteria that take nitrogen from the atmosphere and combine it with another important chemical element, hydrogen, to produce ammonia. (www.eoearth.org) Many different bacteria achieve this conversion through various means, but a common denominator is that they all use the most interesting enzyme, nitrogenase. It is this enzyme that is responsible for changing N2 to ammonia. The general N2 reduction reaction catalyzed by these enzymes is typically presented as follows:
N2 + 8 e− + 16 ATP + 8 H+ → 2 NH3 + 16 ADP + 16 Pi + H2 (Igarashi, Y. and Seefeldt, C. L.. 2003)
This amazing enzyme has the ability to break apart the very strong N2 molecule and form ammonia. “Ammonia is easily manipulated by biological cells and by converting it into ammonium (NH4+) and other compounds such as nitrate and nitrites.” (Dincer, I. and Zamfirescu, C.; 2011: 706) Interestingly enough, a small amount of ammonia is also produced through pressure and energy from lightning. (Krasny, M. E.; 2003: 46)
Bacteria with this remarkable ability are found in fresh water, soil and in seawater. A few of these bacteria live in a special relationship with plants where both benefits in special ways. The bacteria live in the roots and supply the plant with nitrogen. In turn, the plant supplies the bacteria with sugars and other carbon compounds. Examples of these plants are alfalfa, clover, peas, peanuts and beans. (Krasny, M. E.; 2003: 46)
Ammonium (NH4+) is taken up by the plants and incorporated in amino acids, the building blocks of proteins. When animals or humans eat the plants, the nitrogen is taken up in their bodies in the form of amino acids and proteins. (Krasny, M. E.; 2003: 46)
“Similar to carbon, organic nitrogen is returned to the atmosphere when plants and animals die and are decomposed. Bacteria first break protein and amino acids back down into ammonium.” The process now becomes complicated as some ammonium is taken up again by plants and used by the plants to build amino acids and protein. Some are broken down further by bacteria into nitrite (NO2–)and nitrate (NO3–). Nitrate (NO3–) itself can be taken up directly by the plants. Some of the nitrates are transformed by bacteria into gaseous nitrous oxide (N2O), nitric oxide (NO), or nitrogen gas (N2), which are released into the air. Some nitrate makes its way into streams, lakes and groundwater. (Krasny, M. E.; 2003: 46)
Conclusion
Nitrate, nitrite, nitric acid and nitrous acid take centre stage in our chemical sequence development from nitrite to nitric oxide (NO) which now becomes key in the subsequent articles.
Note 1
The broader context of the discovery of nitrogen is the identification of different gases.
Johann Baptista van Helmont (c. 1580 – 1644) suggested that all substances, except air, were ultimately derived from water. Van Helmont coined the word gas in the 1650’s. He took the word from the Greek word chaos (khaos) meaning “empty space.” In Dutch, the g is roughly equivalent to the Greek kh. He wrote in Ortus Medicinae, published after his death for fear of prosecution by the Roman Catholic Inquisition, “Hunc spiritum, incognitum hactenus, novo nomine gas voco” meaning, “This vapor, hitherto unknown, I call by a new name, ‘gas.'”
He realized that gases are individual chemical entities, separate from air. The idea of a gas, as many scientific discovery, did not appear in an “eureka-moment”, but started as a very tentative idea, in many instances being only true in a very small way. This is true of his concept of a gas. He saw gas as “primal water modified by a specific ferment: each body in nature contains such a gas and under specific conditions, for example, by heating, this gas can be liberated. Van Helmont described the production of such a gas. After burning 62 pounds of charcoal, only 1 pound of ashes remained. He assumed the other 61 pounds had changed into a wild spirit or gas (he called it gas sylvestre) that could not be contained in a vessel. He obtained the same gas by burning organic matter and alcohol and by fermenting wine and beer.” (biography.yourdictionary.com)
It was Boyle (1627 – 1691) who took the old Greek concept of the atom, originated by Democritus and applied it to chemical change. The Greeks thought of atoms as the smallest particle that anything can be divided into. In quality, these atoms were thought to be the same. It was thought that their arrangements in different numbers and configurations produced the different substances found in the world. This concept Boyle referred to as the mechanical philosophy in contrast to notions of Aristotle who saw principles at work behind matter. In Boyle’s mind, the matter was not driven by laws of inertial motion, but by its inclusion into a grand universal machine, constructed by a supreme Maker or Opificer, as he referred to God and from this, the reference to his view as “mechanical.” (Cook, M. G.. 2001)
References
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Huffman, R. E.. 1992. Atmospheric Ultraviolet Remote Sensing. Academic Press, Inc.
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