Meat curing is a complex process where brine ingredients react with each other and with the meat which is made up of “water, proteins, lipids, carbohydrates and inorganic, non-protein compounds containing nitrogen and trace amounts of vitamins. (Pegg, B. R. and Shahidi, F.; 2000: 23) Each reaction is governed by many and complex factors and mechanisms, some of which are still not clearly understood by modern science.
Curing is a fascinating process. A modern understanding of the benefits of curing is that it fixes a pinkish-reddish cured meat colour. It endows the meat with unique longevity, even if left outside a refrigerator, many times longer than that of fresh meat. It is powerful enough to prevent the deadly toxin formation by clostridium botulinum. It prevents rancidity in fat. It lastly gives meat a unique cured taste.
Discovering the curing process and the mechanics behind it was a slow process that took hundreds of years. The object was the preservation of meat for future consumption. Bacon and other cured products, properly prepared, have, however, always been a delicacy as it remains to this day. Today, taste and a visual appeal probably dominates, but ask any outdoor’s person and they will tell you that preservation for future use is still a huge factor in the immense popularity of cured meats.
Before the 1600’s meat preservation was done with salt only. Vegetable dyes were used to bolster colour. (The history of curing) A few people added a little bit of saltpeter (potassium nitrate) to the salt for “cured colour development.” This practice gained momentum from the year 1700. By 1750, the trend turned into the norm, being practiced almost universally. During the 1800’s sugar was added to the mix. This, with the exception of phosphates which have been added since the mid-1900’s, is very much the same process as we follow today. (Ladislav NACHMÜLLNER vs The Griffith Laboratories)
On 7 May 1868, Dr. Arthur Gamgee from the University of Edinburgh, brother of the famous veterinarian, Professor John Gamgee (who contributed to the attempt to find ways to preserve whole carcasses during a voyage between Australia and Britain), published a groundbreaking article entitled, “On the action of nitrites on the blood.” He observed the colour change brought about by nitrite. He wrote, “The addition of … nitrites to blood … causes the red colour to return…” (Gamgee, A; 1867 – 1868; Vol. 16, 339-342) Over the next 30 years, it would be discovered that it is indeed nitrites responsible for curing and not the nitrates added as saltpeter.
It fell upon a German researcher, Dr. Ed Polenske (1849-1911), working for the Imperial Health Office in Germany, to make the first discovery that would lead to a full understanding of the curing action. He prepared a brine to cure meat and used only salt and saltpeter (nitrates). When he tested it a week later, it tested positive for nitrites. (Polenske. E. 1891)
The question is where did the nitrites come from if he did not add it to the brine to begin with. He correctly speculated that this was due to nitrate being converted by microbial action into nitrite. He published in 1891. (Polenske. E. 1891)
Karl Bernhard Lehmann and Karl Kißkalt discovered in 1899 that nitrite is responsible for the reddish color of dry cured meat. It was John Scott Haldane who showed in a 1901 article that the cured meat colour is due to a nitrosylheme complex. (Concerning the direct addition of nitrite to curing brine) (Hoagland, Ralph. 1914)  The heme part of the meat protein is where the colour is generated through the presence of an Fe ion and nitrolsyl refers to a non-organic compounds containing the NO group. In the protein nitric oxide is bound to the Fe ion through the Nitrogen atom. Therefore term, nitrosylheme complex.
The change of nitrate into nitrite through bacterial action takes weeks. If a salt, like sodium nitrite, is used instead of saltpeter, curing is accomplished in days or even hours (if a heating step is applied to the meat before it is smoked).
The only aspect in curing that is time-consuming is however not the bacterial reduction of nitrates to nitrites. The change from nitrite into a form that reacts with the meat protein and produce the nitric oxide coupling with the Fe ion is also not instantaneous. The rate of reaction is slow. It is not like mixing sugar into coffee. An analogy is if you put sugar in your coffee and have to wait twelve hours and reheat it in the microwave before you can taste sweetness.
When the brine enters the meat, the anion is formed and a very small amount of nitrite (less than 1% of the total nitrite) forms the neutral nitrous acid (). It is nitrous acid that is responsible for the formation of nitrosating compounds which is the ultimate reaction of joining nitric oxide to an organic compound, in this case, the myoglobin protein (resulting in a nitroso derivative). (Sebranek, J. and Fox, J. B. Jn.. 1985)
The first step in the reaction sequence of creating such a link between myoglobin and niric oxide is the formation of nitrous acid (). From nitrous acid, the neutral radical, nitric oxide is formed directly as well as a variety of nitrosating species or molecules that create such a nitrite-oxygen pair of atoms to link to an organic structure like a protein.   (Sebranek, J. and Fox, J. B. Jn.. 1985)
This reaction takes time and its rate is dependent on the pH of the meat it is injected into, the temperature of the meat and the brine and the concentration of nitrite. Curing then happens when the nitric oxide reacts with iron which is part of the meat proteins, myoglobin. 
One such reducing agent, introduced to brine cures in the 1800’s, is sugar.  Sugar was added originally to reduce the salty taste of the meat. Curers noticed that if sugar is added with saltpeter to the brine mix, the meat cures slightly faster and with better colour development. (The history of curing) In the 1920’s, ascorbate or its isomer, erythorbate became the magical reducing agent  , but this too is the subject for another article.
If saltpeter is used as principal curing ingredient, adding sugar favours the proliferation of bacteria that reduces nitrate to nitrite. It, therefore, speeds up the curing process.
Better colour development is due to the action of reducing sugars (such as brown sugar) to create a reducing environment in the meat which encourages the reduction of nitrous acid to nitric oxide (Kim-Shapiro, D. B. et al. 2006).  (The history of curing)
This was then the understanding of meat curing by the beginning of the 1900’s. Scientists knew that adding nitrite directly to the meat would dramatically speed up the curing process, but working out how to do it and navigating through the complex maze of public perception and legal restraints would be another matter altogether.
The meat colour generally “changes” (either red, purple or brown), based on how many electrons are spinning around the iron atom which is part of myoglobin. Nitric oxide stabilizes or fixes the myoglobin colour through a reversible chemical bond. It does not colours the meat. (Pegg, B. R. and Shahidi, F.; 2000: 23 – 45) This is an important point to remember because, in the consideration of the use of nitrite in meat, nitrite can not be viewed as a meat colourant. BACK TO POST
The reaction of nitrite in meat is slow, in part due to the very small quantity used in the curing brine. The rate of reaction, as always, depends on the concentration of the reactants, the pH and temperature. BACK TO POST
“The first step in the reaction sequence beginning with nitrous acid is the generation of either a nitrosating species or the neutral radical nitric oxide (NO).” (Sebranek, J. and Fox, J. B. Jn.. 1985)
The following list gives the relative reactivities of various nitrosating species, species 1 being the strongest and species 5 being the weakest.
Source: “From smoke which has many other phenolic compounds”
Source: From curing salt
Source: Found in the air.
Source: Nitrous acid anhydride
Nitrose derivatives of citrate, acetate, sulphate, phosphate.
Sources: Cure ingredients, weakly reactive under certain conditions.
I excluded those found under very acidic conditions. (Comparison by Sebranek, J. and Fox, J. B. Jn.. 1985) BACK TO POST
“The term nitrite is used generically to denote both the anion, , and the neutral nitrous acid , but it is the latter which forms nitrosating compounds.” (Comparison by Sebranek, J. and Fox, J. B. Jn.. 1985) BACK TO POST
One such mechanism for the conversion of “nitrite to nitric oxide in meat is by oxidation of myoglobin to metmyoglobin (brown coloured meat; Fe3+). This oxidation-reduction coupling produces both nitric oxide and metmyoglobin. It has been suggested by Kim et al. (2006) that metmyoglobin can be converted back to deoxymyoglobin through metmyoglobin reducing activity (MRA), a reaction facilitated by lactate. It is the enzyme activity of LDH that helps convert lactate to pyruvate and produce more NADH. Hendgen-Cotta et al. (2008) suggested that deoxymyoglobin can convert nitrite to nitric oxide and the generation of more deoxymyoglobin is likely to result in more nitric oxide (NO) from nitrite and less residual nitrite.” (Mcclure, B. N.; 2009: 28)
Several specific biochemical reducing systems have been the subject of intense investigation as far as their importance in the development of cured meat colour are concerned.
“Endogenous compounds such as cysteine, reduced nicotinamide adenine dinucleotide, cytochromes and quinones are capable of acting as reductants for NOMb formation (Fox 1987). These reductants form nitroso-reductant intermediates with NO and then release the NO to Mb, forming a NOmetMb complex that is then reduced to NOMb. In model systems, the rate limiting step in the production of NOMb was the release of NO from the reductant-NO complex (Fox and Ackerman 1968). Several researchers have investigated the effects of endogenous muscle metabolites including peptides, amino acids, and carbohydrates on the formation of NOMb. Tinbergen (1974) concluded that low-molecular-weight peptides such as glutathione and amino acids with free sulfhydryl groups were responsible for the reduction of nitrite to NO, wich is subsequinrtly complexed with Mb to produce NOMb. Similar work by Ando (1974) also suggested that glutathione and glutamate are involved in cured-meat colour formation. Depletion of these compounds in meat via oxidation occurs with time, but reductants such as sodium ascorbate or erythorbate are added to nitrite-cured meats before processing to ensure good colour development (Alley et al. 1992) The role of reductants in heme-pigment chemistry is somewhat ambiguous, but they can promote oxidation and even ring rupture under certain conditions. Thus to form cured meat pigment, two reduction steps are necessary. The first reduction of nitrite to NO and the second is conversion of NOmetMB to NOMb.” (Pegg, B. R. and Shahidi, F.; 2000: 44, 45) BACK TO POST
“Sugars itself does not reduce dinitrogen trioxide in the way that ascorbate or erythorbate does, but it contributes to “maintaining acid and reducing conditions favorable” for the formation of nitric oxide.” (Kraybill, H. R.. 2009)”Under certain conditions reducing sugars are more effective than nonreducing sugars, but this difference is not due to the reducing sugar itself. The exact mechanism of the action of the sugars is not known. It may be dependent upon their utilization by microorganisms or the enzymatic systems of the meat tissues.” (Kraybill, H. R.. 2009) BACK TO POST
An excellent reducing agent was discovered in the 1920’s when ascorbate was isolated. As early as 1927, two German chemists, J. Tillmans and P. Hirsch (1927) observed that there is a correlation between the reducing capacity of animal tissue and their vitamin C content. (Concerning the Discovery of Ascorbate) . It reacts so aggressively (effectively) with nitrite, that a less effective, but more manageable cousin (an isomer of ascorbate), erythorbate turned out to be the most practical to use in curing brines along with nitrite and salt.
Ascorbate (vitamin C) reacts so aggressively (effectively) with nitrite, that a less effective, but more manageable cousin (an isomer of ascorbate), erythorbate turned out to be the most practical to use in curing brines along with nitrite and salt.
The old curing brines of the 1800’s consisting of saltpeter (nitrate), sugar (create reducing conditions) (6) and salt are, therefore, equivalent to the current curing brines of nitrite (being added directly), erythorbate (reducing agent) and salt. The same general functionality at vastly reduced curing time.
Today, nitrate is still being added to many curing brines as a reservoir for future nitrite as bacteria continues to change nitrate into nitrite. This bolsters the residual nitrite levels in cured meat which is important since it was found that nitrite has a unique anti-microbial function in cured meat, in addition to its function of fixing the cured colour and contributing to the cured taste. It is unique in the sense that it is the most effective chemical control against a highly lethal pathogen, clostridium botulinum. (Concerning Nitrate and Nitrite’s antimicrobial efficacy – chronology of scientific inquiry)
Table salt remains the most important curing agent, but salt alone will not give the cured colour or taste and will not, on its own, be effective against clostridium botulinum. Sugar is still being used in many brines today, mostly to enrich the taste profile and to create browning during frying, especially in bacon. Its contribution to reducing conditions is now secondary and since the addition of ascorbate or erythorbate. Saltpeter has been replaced by sodium nitrite. BACK TO POST