xResearch notes – nitrosating agents

random research notes

 

DIFFERENT NITROSATING SPECIES OR NITRIC OXIDE

Beginning with nitrous acid, the next step in the reaction sequence  “is the generation of either a nitrosating species or the neutral radical, nitric oxide (NO).”  (Sebranek, J. and Fox, J. B. Jn.; 1985:  1170)  A nitrosating species is a molecular entity that is responsible for the process of converting organic compounds into nitroso (NO) derivatives, i.e. compounds containing the R-NO functionality.

->  Nitrosonium and nitrous acidium ions

We first look at a nitrosating species that do not exist in any relevant quantities in meat since the pH of meat is generally to low for its formation.  It is important however, since it is the strongest nitrosating species.  These species are forms of a positively charged (electrophilic) nitrogen oxide, either in its simplest form, the nitrosonium ion, CodeCogsEqn (58), or as part oa larger molecule, the ion, nitrous acidium (CodeCogsEqn (59)).  (Sebranek, J. and Fox, J. B. Jn.; 1985:  1170)

CodeCogsEqn (60)

It is not certain if nitrosonium ion exist free in solution.  It possibly only exists as nitrous acidium ions.  At the pH of meat, however, the principal reactive species is dinitrogen trioxide (CodeCogsEqn (22)).  (Sebranek, J. and Fox, J. B. Jn.; 1985:  1170)

->  Dinitrogen trioxide

We again begin our discussion with nitrous acid which, in aqueous solutions, exists in equilibrium with its anhydrate, dinitrogen tridioxide   CodeCogsEqn (12)

The equilibrium equation is:

CodeCogsEqn (11)

(Williams, D. L. H..  2004: 1, 2)

In certain chemical structures, electrons are able to move around to help stabilize the molecule, called resonance structures.  Dinitrogen trioxide is such a structure where, in aqueous solutions, the molecule is stabilized through resonance. (Sebranek, J. and Fox, J. B. Jn.; 1985:  1170)

dinitrogen trioxide resonance

An electron poor site electron poor siteis created by the charge shift which is strongly electrophilic.  This means that it is strongly attracted to electrons.  This site will, in other words, nitrosate a nucleophilic site.  A nucleaphilic site donates electrons to an electrophile to form a chemical bond where a CodeCogsEqn (13) group will be attached.  (Sebranek, J. and Fox, J. B. Jn.; 1985:  1170)

In general, a lower pH accelerates the formation of CodeCogsEqn (12). (Dikeman, M. and Devine, C..  2014: 417)

->  Nitric oxide

Nitric oxide may be formed through either a dismutation or a reduction reaction.  The following reaction takes place in a strong acid and is a good example of a rare reaction involving three molecules due to the low probability of three molecules colliding to create the reaction.  (Sebranek, J. and Fox, J. B. Jn.; 1985:  1171)

CodeCogsEqn (61)

The following two sequences are the major sources of nitric oxide in meat, especially in light of the fact that meat is a rich source of reductants. (Sebranek, J. and Fox, J. B. Jn.; 1985:  1171)

CodeCogsEqn (62)

CodeCogsEqn (63)

where Rd is such reductants as ascorbate, sufhydryl groups, hydroquinones, etc. (Sebranek, J. and Fox, J. B. Jn.; 1985:  1171)

CodeCogsEqn (64)

“Nitric acid is an electron pair donor and forms a very stable complex with transition metals.”  (Dikeman, M. and Devine, C..  2014: 417) Such a complex of a central metallic atom or ion, especially with a transition metal, is called a coordinate-covalent complexe.  The metal centre is called the coordination centre, and is surrounded by an array of bound molecules or ions, that are in turn known as ligands or complexing agents.

“The coordinate-covalent complexes of nitric oxide with the haem pigment of meat – nitrosylmyoglobin, nitrosylhaemoglobin, and dinitrosylhaemochrome – form the pink and red colours ocured meats.”  (Sebranek, J. and Fox, J. B. Jn.; 1985:  1171)

“Nitric oxide is readily oxidised, which accounts in part for the instability ocured meat colourin air.” (Sebranek, J. and Fox, J. B. Jn.; 1985:  1171)
CodeCogsEqn (65)
or with water to form nitrous or nitric acid.
CodeCogsEqn (66)
“These are both backwards reactions, regenerating previous reactants in the sequence (CodeCogsEqn (22)CodeCogsEqn (19)).  It will be noted that nitrate is produced in this recycling (the last reaction)
and, since it is relatively unreactive, it acts as a sink to remove nitrite from the system. This constant recyclingresults in a semi-stable equilibrium oreactants, intermediates, and products.While these reactions are the major characterised chemical reaction sequences, there are many other reactions that can and do take place. Even nitrous oxide (CodeCogsEqn (67), laughing gas) has been identified in the gases above curing mixes.   (Sebranek, J. and Fox, J. B. Jn.; 1985:  1171)

 

The one pathway to NO formation is then through the anhydrate of CodeCogsEqn (19), dinitrogen trioxide, CodeCogsEqn (22)”  (Pegg, R. B. and Shahidi, F.;2000: 39), which exists in equilibrium with the two oxides, of NO and NO2. (Toldrá, F., 2015:  21)  The hydradion of nitrous acid is an important time-consuming reaction.   Dinitrogen trioxide reacts with reductants found naturally in muscle tissue as well as added reducants, such as ascorbate, to form nitric oxide.  (Krause, B. L.; 2009: 9)

There are however other additives or conditions that influence the reduction of nitrite to nitric oxide.  The most important additive that influences nitric oxide formation is salt, due to the formation of nitrosyl chloride (NOCl), which is a more powerful nitrosating agent than dinitrogen trioxid. (Dikeman, M. and Devine, C..  2014: 417)

Both dinitrogen trioxide and nitrosyl chloride starts from  nitrous acid.  The reaction formation of nitrosyl chloride from nitrous acid can be viewed as follows:

CodeCogsEqn (54)

Note the reaction between the two anions  CodeCogsEqn (13) and CodeCogsEqn (56) to form CodeCogsEqn (55).  The reaction is due to the ability of electronegative anions to form resonance stabilised, charge separated molecules from nitrous acid.  They are more reactive than dinitrogen trioxide,  and less reactive than the nitrous acidium ion. (Sebranek, J. and Fox, J. B. Jn..  1985)

There are five nitrosating species that has been identified from literature that are of interest to us related to meat curing.  Species 1 being the strongest and species 5 being the weakest.

Species 1:

NO smoke

Source:  “From smoke which has many other phenolic compounds”

Species 2:

CodeCogsEqn (55)

Source:  From curing salt

Species 3:

CodeCogsEqn (57)

Source:  Found in the air.

Species 4:

CodeCogsEqn (22)

Source:  Nitrous acid anhydride

Species 5:

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)

In general, a lower pH accelerates the formation of CodeCogsEqn (12) and CodeCogsEqn (55).  (Dikeman, M. and Devine, C..  2014: 417)

Despite the fact that nitrosyl chloride (CodeCogsEqn (55)) is responsible for most of the nitric oxide in meat curing, considerable attention has been given to nitrous oxide formation.  There are many instances in the curing environment when it is advantageous to reduce the pH of the meat.

One example is in legislation that is being applied around the world as a measure to prevent the spread of the PRRS virus through imported meat.  In South Africa, for example, the requirement is that certain cuts must either undergo thermal processing of > 56 deg C for 20 minutes or pH must be reduced to < 5.  When uncooked sausages are made, the thermal processing requirement does not suite the nature of the product and pH control is favoured.

There is a concern that reducing the pH will lead to nitrous acid formation which will result in its released as a toxic gas.    Varnam A. and Sutherland J. P., for example, warns in their book,  Meat and Meat Products: Technology, Chemistry and Microbiology that ascorbate should be used in cured meats and not ascorbic acid.

By A. Varnam, Jane P. Sutherland

“As early as 1777 it was realised that there was more than one “acid of nitre.” Scheele distinguished ” phlogisticated acid of nitre ” from nitric acid as being a weaker volatile acid produced by the reduction of nitric acid. He also showed that nitre, when strongly heated, lost oxygen, and left a deliquescent salt which readily decomposed into a volatile acid when treated with acid. Priestley had previously described the brown fumes produced by oxidising nitric oxide as “nitrous acid vapour,” while the terms “nitrous acid gas ” and ” nitrous acid ” were used later by Davy and Gay-Lussac respectively. Confusion of terms, however, existed, due to the fact that both nitric and nitrous acids were present, and the means of distinguishing the two were not available at that time. It was understood clearly, however, that there were two distinct salts, nitrates and nitrites, and Cavendish showed that silver nitrite was precipitated when potassium nitrite was added to a solution of silver nitrate. Gay-Lussac was the first to prepare nitric and nitrous acids by the careful oxidation of nitric oxide with oxygen in the presence of water.”   http://nitrogen.atomistry.com/

Nitrous acid is an unstable compound, and all the methods of preparation yield an aqueous solution of the acid.

Antioxidative mechanism of Nitrite

“NO2 readily reacts with water.

The second of Hoagland’s important reactions is therefore:

(Williams, D. L. H..  2004: 1, 2)

HNO2 is known only in solution which exists in a solid state at temperatures < – 102 deg C.  Above this temperature, the liquid is a mixture of:

h reaction 4

HNO2 is a mildly strong acid with a pKa of 3.37.

(Toldrá, F., 2015:  21)

It is nitric oxide which reacts with the iron- and oxygen-binding protein found in the muscle tissue of almost all mammals, myoglobin, to form NO-myoglobin and the cured coulor.

H reaction 5

(Honikel, K.-O.; 2008: 68–76)

The two important reactions that Hoagland’s pathway runs through are the formation of nitrous acid (HNO2 )and the formation of nitric oxide (NO) from dinitrogen trioxide.

Let’s familiarise ourselves a bit with the basic chemistry of Nitrous Acid.

Nitrous Acid (HNO2 )

Formation

Hoaglands first curing step is nitrous acid formation.  This is how it happens in water.

In water, potassium nitrite (KNO2)dissolves and undergoes the following reaction.

line 4

Water is H+ and OH.  The H+ reacts with NO2- to form a weak acid.  The K+ reacts with the OH- to form a strong base.  The K+ therefore does not affect the pH of the solution since it is a weak conjugate acid, but the NO2- will.  It will act as a base.  It is a conjugate base since it came from the acid, HNO2 . We write the reaction of NO2- with water as follows.

line5

The equilibrium law for this reaction is:

line 6

South African regulations set the maximum nitrite limit in the final product as 160 ppm (R965 of 1977 (18)).  In the brine mixing tank, we will therefore not have more than 1066 ppm or mg/L so that a 15% injection yield will give the required 160ppm after injection, tumbling, drying, smoking and freezing.

Now we need to calculate the M of the initial concentration of 1066 ppm NO2-.

The definition of parts per million is 1g (part) solute per 1,000,000 g (per million) solution.

Now, divide both values by 1000 to get a new definition for ppm,

ppm = 0.001 g per 1,000 g solution

or

ppm = 1 mg solute per 1 kg solution

Then, for an aqueous solution:

ppm = 1 mg solute per liter of solution

We can equate 1L  to 1kg because the solution concentration is so low that we can assume the solution density to be 1.00 g/mL.

Also, it’s this last modification of ppm (the mg/L one) that allows us to go to molarity (which has units of mol/L).

So, by the last definition of ppm just above:

1066 ppm = 1066 mg NO2- / L of solution = 1.066 g/L

Now, we divide by the atomic weight for the nitrite ion:

1.066 g/L divided by 46.006 Da g/mol = 0.0232 mol/L

The initial concentration NO2- in the brine mixing tank is therefore 0.0232 M and a small amount of this, represented by x, reacts with water.  We then find the following:

[NO2-] = 0.0232 – x  and [HNO2] = [OH-] = x

We simplify the calculation and assumes that x is very small so that [NO2-] = 0.0232.

We now use the free proton or hydronium concentration, or, Ka(1) the acid ionization constant.  We look this up for HNO2 and it is 7.1 x 10-4.

From this, the Kb (2) = 1.4 x 10-11 (obtained by dividing Kw by Ka).  We now enter these variables into the equilibrium law and solve for x.

line 1

line 2

line 3   (3)

The pH = 14 – pOH = 14 – 4.25 = 9.75

(Brady, J. E. and Senese, F.; 2009: 370)

Remember Hoaglands first step after salpeter was changed to nitrite through bacteria reduction.

CodeCogsEqn (26)

What is the effect of CodeCogsEqn (17) of  CodeCogsEqn (19) on pH?

CodeCogsEqn (19)  is a weak acid and if CodeCogsEqn (20) is added to CodeCogsEqn (21), we get KOH which is recognised as a strong base. CodeCogsEqn (21)will therefore not affect the pH of the solution because it is a weak conjugate acid, but CodeCogsEqn (17) will and it will act as a base (it must be a conjugate base since it comes from an acid, CodeCogsEqn (19)). (Brady, J. E., Sense, E.; 2009:  370)

The question comes up to what level it will effect the pH in  water.

“The equation is written as follows: CodeCogsEqn (23)

Lets assume a random small amount of CodeCogsEqn (17) of 0.100 M. in this concentration.

The equilibrium law for the reaction is:

CodeCogsEqn (27)

The initial concentration of CodeCogsEqn (17) is 0.100M, and a small amount, x, reacts with water.  We then conclude that CodeCogsEqn (28) = 0.100 – x  and  CodeCogsEqn (29) = CodeCogsEqn (31)  = x

To simplify the calculation, we assume that x is very small so that CodeCogsEqn (28) = 0.100.

Next we look up the CodeCogsEqn (32) for CodeCogsEqn (29) and it is CodeCogsEqn (33).  From this, the CodeCogsEqn (34).gif (obtained by dividing CodeCogsEqn (35).gif by CodeCogsEqn (36).gif).  We enter these variables into the equilibrium law and solve for  x to get:

CodeCogsEqn (37).gif and pOH = 5.93

the pH is 14.00 – pOH = 14.00 – 5.93 = 8.07″

(Brady, J. E., Sense, E.; 2009:  370)

(Include p 13 from http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=3128&context=etd into the discussion on what happens when salt is added.)

Nitric Oxide (NO) Formation

Pure nitrous acid (HNO2 )has never been isolated since it decomposes into various oxides of nitrogen as final product.  This decomposition is normally represented as follows:

Nitric Oxide formation

This is however not giving the full picture of what happens.

The decomposition is relatively slow at room temperature and at low  CodeCogsEqn (4).  In the absence of oxygen, the reaction follows two equilibria.

CodeCogsEqn (5)

CodeCogsEqn (6)

In the presence of oxygen, the reaction is much faster and happens as follows:

CodeCogsEqn (7)

Nitrous acid, is a weak acid.  Its acid dissociation constant (CodeCogsEqn (8)) has been measured many times by various methods and is 3.148 at 25 deg C and at zero ionic strength.  The equilibrium equation is

CodeCogsEqn (9)

In aqueous solutions, nitrous acid exists in equilibrium with dinitrogen tridioxide   CodeCogsEqn (12)

The equilibrium equation is:

CodeCogsEqn (11)

(Williams, D. L. H..  2004: 1, 2)

There is a strong tendency for to dinitrogen tridioxide  (CodeCogsEqn (12)) to dissociate to CodeCogsEqn (13)  and CodeCogsEqn (14) as the temperature rises.

The rate equation of  the nitrosation of a large number of nucleophilic substrates (S) in a relative dilute acid aqueous solution (typically 0.1 M) and at relatively low [HNO2 ] (typically CodeCogsEqn (15)), is as follows:

CodeCogsEqn (16)

(Williams, D. L. H..  2004: 6)

Triggers of NO formation

“When ascorbate and nitrite are in solution and pH drops from 7.4 to values near 6.5, NO production occurs.”

(Pereira, C., et al,  2013:  281)

THE CHEMISTRY IN MEAT

The reaction of NO in meat

In 1865, Hermann observed that nitric oxide changed the colour and spectral characteristics of hemoglobin in the absence of oxygen.   Gamgee observed in 1868 that hemoglobin which had reacted with nitrite would not combine with oxygen and he considered that there were changes in hemoglobin due to the combination of nitrite with oxidized hemoglobin.  (Cole, Morton Sylvan, 1961: 3)

The Colour of Meat

“Lean Meat contains approximately 75% water, 19% protein, 2.5% lipid, 1.2% carbohydrate, 2.3% inorganics, non-protein compounds containing nitrogen and trace amounts of vitamins.” (Pegg, R. B. and Shahidi, F; 2000: 23, 24)

So, 1kg of lean meat contains:

750mL of water,
190g of proteins
25g lipid
12g carbohydrate
23g inorganic compounds

The concentration of myoglobin  (Mb) is the determining factor for overall redness of meat and to a lesser extent by hemoglobin.  The main pigment in meat is myoglobin (Mb).  The greater the Mb level, the more intense the colour of the meat.  (Pegg, R. B. and Shahidi, F; 2000: 23, 24)

“Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. In humans, myoglobin is only found in the bloodstream after muscle injury.”  (Wikipedia. Myoglobin)  It is the structure and chemistry of the iron atom that is the key to understand the reactions and colour changes that Mb undergoes.  (Pegg, R. B. and Shahidi, F; 2000: 26)

“The concentration of myoglobin in meats is dependent upon the age and activity of the animal (Millikan, 1939; Poel, 1949; lawrie, 1950, 1953)• Certain muscles appear to contain more myoglobin than do others. The myoglobin concentration was found to be higher in skeletal muscles than in cardiac muscles of dogs, horses (Drabkin, 1950), cattle and pigs (Watson, 1935; Lawrie, 1950). The average myoglobin concentration of beef muscle is 3*7 mg. per gram while pork muscle averages 0.79 mg. per gram of meat for light muscle and 1.44 mg. per gram for dark muscle (Ginger et al., 1954).”  (Cole, Morton Sylvan, 1961: 2)

Early on in studying of meat colour, hemoglobin, rather than myoglobin was used.  “This was mostly a matter of convenience and, in early work, a matter of necessity since myoglobin was not isolated and purified until 1932 (Theorell, 1932). Shenk et al. (1934) found only 10 percent of the residual pigment in beef to be hemoglobin. In spite of the differences between hemoglobin and myoglobin, Urbain and Jensen (1940) considered the properties of hemoglobin and its derivatives sufficiently like those of myoglobin to allow the use of hemoglobin in studies of meat pigments.”  (Cole, Morton Sylvan, 1961: 2)

The bright red colour of fresh meat is due to oxymyoglobin (MbO2).  Mb has a great affinity for O2.  It is the reaction of myoglobin with O2 that results in oxymyoglobin (MbO2) and produce the red colour which the consumer associate with freshness.  (Pegg, R. B. and Shahidi, F; 2000: 31)

Myoglobin is able to store O2 in the blood to transport it through the body and to release it where it is needed.  This means that it is able to react rapidly with O2 and reversibly.  The surface of the meat “blooms to a bright red colour within minutes of exposure to air.”  (Pegg, R. B. and Shahidi, F; 2000: 31)

We, however, know that this a short-lived phenomenon. “MbO2’s stability depend on a continuing supply of O2 because the enzymes involved in oxidative metabolism rapidly use the available O2.”  With time, the small layer of MbO2 present on the surface of the meat propagates downward, but the depth to which O2 diffuses depends on several factors, such as the activity of oxygen-utilizing enzymes (i.e. O2 consumption rate of the meat), temperature, pH, and external O2 pressure.  Consequently, maintaining the temperature of meat near freezing point minimizes the rate of  enzyme activity and O2 utilization and helps maintaining a bright red colour for the maximum possible time.”  (Pegg, R. B. and Shahidi, F; 2000: 31)

“In contrast, the interior tissue of meat is purple-red in colour.  This the colour of Mb, sometimes called deoxy-Mb, and the colour persists as long as reductants generated within the cells by enzyme activity are available.  When these substances are depleted, the hem iron is oxidized to the ferric state (Fe 3+ from ferrous, Fe 2+).”  (Pegg, R. B. and Shahidi, F; 2000: 31)

“The brown pigment formed, which is characteristic of the colour of meat left standing for a period of time, is called metmyoglobin (metMb).  It is generated by removal of a superoxide anion from the hematin and its replacement by a water molecule gives a high-spin ferric hematin.    The ferric ion, unlike its ferrous counterpart, has a high nuclear charge and does not engage in strong pi bonding.    Therefore, metMb is unable to form an oxygen adduct.”  (Pegg, R. B. and Shahidi, F; 2000: 31)

“In fresh meat there is a dynamic cycle such that in the presence of O2, the three pigments Mb, MbO2, and metMb are constantly interconverted; all forms are in equilibrium with one another.  Care is exercised by the retailer to reduce the likelihood of metMb formation, as its presence downgrades the quality of fresh meat.  When metMb is denatured by thermal processing, meat remains brown in colour, but this denatured pigment can be oxidized further to form yellow, green or colourless porphyrin-derived substances by bacterial action or photochemical oxidation.”  (Pegg, R. B. and Shahidi, F; 2000: 32)

The best way to view this is to see it represented in a diagram.

FullSizeRender
Interrelationship between Pigments of Fresh Meat (Pegg, R. B. and Shahidi, F; 2000: 33)

In other words, it turns into oxymyoglobin and a reductant in the meat or in the brine reduces it to either myoglobin or oxymyoglobin.  When the reducing agents have been depleted, the meat colour remains brown.

Notes:
1. The larger the Ka the greater the ionization of the acid, the stronger it is, the lower the pH. In the other words, pH inversely proportional to Ka. (utdallas.edu)

2. Kb – the base ionization constant is the equilibrium constant for ionization of a base in an aqueous solution. Chemical reaction:

kb(utdallas.edu)

3. pOH: The pOH of an aqueous solution, which is related to the pH, can be determined by the following equation: pOH=log[OH] (chemwiki.ucdavis.edu)

 References:

Honikel, K-O.  31 May 2007.  The use and control of nitrate and nitrite for the processing of meat products.  Science Direct.  Meat Science 78 (2008) 68–76. Elsevier Ltd.

Pereira, C, et al.  10 February 2013.  The redox interplay between nitrite and nitric oxide: From the gut to the brain.  Department of Pharmacy and Center for Neurosciences and Cell Biology, University of Coimbra, Health Sciences Campus, Azinhaga de Santa Comba, 3000-548 Coimbra, Portugal.  Elsevier.  Redox Biology 1 (2013) 276–284

The molarity of NO2-:  http://www.chemteam.info/Solutions/ppm2.html

http://www.utdallas.edu/dept/abp/Chemistry-II/Ka%20Kb%20Quiz/Ka%20%20Kb.pdf

pH calculation of KNO2 from Brady, J. E. and Senese, F..  2009.  Chemistry, Student Study Guide: The Study of Matter and Its Changes.  John Wiley & Sons, Inc. page 370.  https://books.google.co.za/books?id=zS1EX-e7kPQC&pg=PA370&lpg=PA370&dq=KNO2+reaction+in+water&source=bl&ots=Yotnusa6JT&sig=4I52PZ_qlIg8vVThpx40CSzPI6Q&hl=en&sa=X&ved=0CCoQ6AEwAmoVChMIgZD-1uiPyQIVgdYaCh33tQur#v=onepage&q=372&f=false

http://chemwiki.ucdavis.edu/Physical_Chemistry/Acids_and_Bases/Aqueous_Solutions/The_pH_Scale/Determining_and_Calculating_pH

Hoagland, R.  1914.  Cloring matter of raw and cooked salted meats.  Laboratory Inspector, Biochemie Division, Bureau of Animal Industry.  Journal of Agricultural Research, Vol. Ill, No. 3 Dept. of Agriculture, Washington, D. C. Dec. 15, 1914.

Williams, D. L. H..  2004.  Nitrosation Reactions and the Chemistry of Nitric Oxide.  Elsevier.

Krause, B. L..  2009.  Incubation of curing brines for the production of ready-to-eat uncured ham.  Iowa State University.

Writing Chemistry:  https://www.codecogs.com/latex/eqneditor.php

Cole, Morton Sylvan, “Relation of sulfhydryl groups to the fading of cured meat ” (1961). Retrospective Theses and Dissertations. Paper 2402

Pegg, R. B. and Shahidi, F.  2000.  Nitrite Curing of Meat.  Food & Nutrition Press, Inc.

https://en.wikipedia.org/wiki/Myoglobin

The Bismarck Tribune (Bismarck, North Dakota); 10 July 1912; page 2.

Brady, J. E., Sense, E.  2009.  Chemistry, Student Study Guide: The Study of Matter and Its Changes.  John Wiley & Sons, Inc.

Toldrá, F..  2015.  Handbook of Fermented Meat and Poultry.  Second edition.  John Wiley & Sons Ltd.

Dikeman, M. and Devine, C..  2014.  Encyclopedia of Meat Sciences.  Second edition. Academic Press.

http://nitrogen.atomistry.com/

http://www.medicinenet.com

Sebranek, J. and Fox, J. B. Jn..  1985.    A review of nitrite and chloride chemistry: Interactions and implications for cured meats.  J. Sci. Food. Agric. 1985, 36, 1169 – 1182.

http://cbs.umn.edu/academics/departments/bmbb/about/history/timeline

Cole, Morton Sylvan, “Relation of sulfhydryl groups to the fading of cured meat ” (1961). Retrospective Theses and Dissertations. Paper 2402

Pegg, R. B and Shahidi, F..  Nitrite Curing of Meat. 2000.  Food & Nutrition Press, Inc.

ICMSF.  1980. Microbial Ecology of Foods Volume 1.  Academic Press

Lemberg, R. and Legge, J. W..  1949.  Hematin Compounds and Bile Pigments.  Interscience Publishers, Inc.

Haldane, J. S..  1901.  The Red Colour of Salted Meat.  Journal of Hygiene 1: 115 – 122

Soltanizadeh, N., Kadivar, M..  2012.  A new, simple method for the production of meat-curing pigment under optimised conditions using response surface methodology.   Meat Science 92 (2012) 538–547  Elsevier Ltd.

http://www.foodscience.caes.uga.edu/documents/Newsletter2016June16colorforweb.pdf

https://en.wikipedia.org/wiki/Fereidoon_Shahidi

Images:

Image 1:  Ralph Hoagland.  Oakland Tribune, 5 July 1927

Image 2:  Ralp Hoagland:  Popular Science Month, March 1912; 481 Fourth Avenue, New York, page 40.