Nitrite and Meat Curing
• Cured meats, i.e. ham, bacon, frankfurters etc.
are highly distinctive and unique
-curing process creates distinctive properties that
cannot be duplicated
-dramatic quality, shelf life and safety
improvement thereby providing highly effective
preservative
-duplication of these properties is not
straightforward and is difficult to achieve
• The net effect of curing involves a very
complex series of reactions between curing
ingredients and the meat components
-many are well understood, many are not
• To further complicate matters, some chemical
reactions that involve curing ingredients are
potentially harmful, while some of the curing
agents have more recently been suggested as
positive contributors to human health
The Saga of Sodium Nitrite
• Nitrite and nitrate have been used for some 5000 years or
more to cure and preserve meat, mostly as a salt
contaminant (saltpeter=nitrate)
• 1891 – first publication showing that nitrate-to-nitrite
conversion by bacteria was the source of preservative
effects
• 1899 – first suggestion of nitrite as the source of cured
meat color
• 1901 – discovery that nitric oxide from nitrite was source of
cured meat color
• 1925 – first regulatory (USDA) limits for nitrate and nitrite
were established
• 1920’s-1950’s – Industry and university
researchers refined curing processes
- role of nitrite in cured meat clarified
-potent antimicrobial agent
-powerful antioxidant
-distinctive cured meat flavor
-unique, attractive color that is stable
• 1956 – a problem emerges…; first report of
carcinogenic nitrosamines found in fish meal cured
with very high nitrite concentrations
• 1960’s – intensive studies of the relationship between
nitrite, nitrosamines and human cancer…
…nitrosamines were found in some cured meats
…the perceptions of cured meats as a contributor to
human cancer became firmly entrenched
• Mid 1970’s – Extensive research in meat
curing showed that nitrosamines were not an
issue with proper control of nitrite
concentrations
- use of reductants and improved
understanding of time/temperature
relationships in meat curing reduced
residual nitrite to very low concentrations
and eliminated nitrosamines
• Late 1970’s – discovery of endogenous
nitrate/nitrite in the human intestine and in
saliva begins to suggest a biological role for
these compounds
-leads to discovery of nitric oxide as a very
important biological molecule
-beginning of the turnaround in scientific
perceptions of nitrite
• 1986 – Nitric oxide found to be “the body’s
messenger” resulting in explosion of research
in biology and physiology
Nitric oxide…
-regulates blood pressure and blood flow
-provides neurotransmission/brain function
-kills bacteria and other immune system
challengers
-promotes healing
(also provides critical role in the function of
the drug Seldinafil)
2002 – Importance of nitric oxide for blood transfusions
suggested
-25% greater heart attacks/strokes observed
following transfusions
- collected blood loses NO within hours of
collection to 30% of initial after one day; 17%
after 7 days and virtually none at 42 days
- work with dogs showed decreased heart attacks
with NO-treated transfusions
Reynolds et al., Proc. Nat. Acad. Sci. 104:17058(2007).
• 2005 – Protective role of nitric oxide in
endothelial function, cardiovascular disease
and heart attacks suggested
- first reports of nitrite as major storage
form of NO in blood and tissue
- role for dietary nitrate and nitrite
suggested for cardiovascular health
• Source of nitric oxide (NO) in biology is nitric oxide
synthase (NOS) conversion of arginine to citrulline
L-arginine + O2 → L-citrulline + NO + H2O
• Nitrite serves as a low-oxygen source of NO
thus nitrite may be a critical protective component
for increasing blood flow when oxygen is limited
• Current questions
– role of blood and tissue nitrite concentrations?
– role of dietary nitrite?
Dietary nitrite in mice
• Insufficiency of dietary nitrite (7 days) decreased steady
state nitric oxide and increased injury from heart attacks
• Dietary supplementation with nitrite restored NO
homeostasis within 7 days, reversed injury and improved
survival from heart attacks
• Dietary nitrite intake can restore NO homeostasis and
blood and tissue nitrite to a greater extent (emphasis mine)
than change in NOS activity
• Dietary supplementation reduced plaque formation in
genetically modified, atherosclerotic-susceptible mice
Bryan et al., Proc. Nat.
Acad. Sci. 104:19144(2007)
“Most provocatively, these studies suggest that the
cardioprotective effects of leafy green vegetables could
derive from nitrite, in addition to the often cited
antioxidant effects of these food groups.”
Gladwin et al. Nat. Chem. Biol. 1:308-314(2005)
“The solution to these problems should…lead to the
potential consideration of nitrite as an “essential
nutrient”…”
Gladwin et al. Nat. Chem. Biol. 1:308- 314(2005)
“Could it be that the stringent regulations on
nitrite/nitrate in drinking water and in foods
contribute to the contemporary diseases of today
due to inadequate nitrite or nitrate in the diet
and NOS activity unable to supply enough nitrite
to maintain cellular homeostasis?”
Bryan, Free Rad. Biol. and Med. 41:691-701(2006)
“Therefore, nitrite and nitrate may serve as
essential nutrients for optimal cardiovascular
health and may provide a treatment modality for
cardiovascular disease.”
Bryan et al., Proc. Nat. Acad. Sci. Early Ed. (2007).
www.pnas.org/cgi/doi/10.1073/pnas.0706579104
Nitrite:
• Has profound effects on human physiology
• Dietary intake may be protective and maintain/restore
endothelial function
• Dietary supplementation may inhibit onset and
progression of atherosclerosis
• Dietary levels of nitrite and nitrate should be
reconsidered for optimal health
• Increasing steady state NOx concentrations is a natural
physiological response in humans and highlights the
physiological justification and safety of such
intervention
from Bryan, MIRC (2007)
Nitrite…
• …derives it’s many advantages in cured meats
from the unique chemistry of nitric oxide.
• …participates in many different chemical
reactions during meat curing involving a
variety of meat components, but most, if not
all, of those reactions produce nitric oxide
• …is still not completely understood in some
aspects of meat curing and several issues and
questions concerning cured meats remain
To understand the issues…
…it is necessary to understand and appreciate
the complexity of conventionally cured meats
“Cured” meat
• Characterized by addition of nitrate
and/or nitrite (and salt)
– unequivocally defines cured meat (IMHO)
– “uncured” otherwise
-USDA-FSIS has ruled that any product that has
not had nitrate or nitrite added is uncured,
however, the USDA recognizes salted (only)
products as cured also
Nitrate (NO3-)
• Insignificant by itself
• Contributes cured meat properties only after
reduction to nitrite
• Reduction is not easily done by chemical
means in meat systems; typically requiring a
bacterial culture with nitrate reductase
activity
• May be important in dried products (hard
salami, hams, etc.)
For cured products…
“…heated within hours after adding the curing
agent, nitrate is useless and superfluous.”
Karl Honikel,
Federal Research Center for Nutrition and Food,
Kulmbach, Germany.
(Encyclopedia of Meat Sciences, 2004, p. 196.)
Nitrite (NO2-)
• Highly reactive compound
• Combines with many different substrates in meat
products to provide several highly useful functions =
Magic!
• Proportional distribution among reactions is likely
dependent on product conditions
• Understanding nitrite complexity =
understanding the issues associated with
conventional curing, “natural” curing and the
negative consumer image of nitrite and cured
meat
Nitrite reactions in meats
•
•
•
•
•
•
•
•
•
Hydrogen ions
Reductants
Meat pigments
Salt
Proteins
Radicals/pro-oxidants
Bacterial cells/cell walls/cell components
Flavor precursors
Lipids
Most obvious effect of nitrite
• Color development/change (most often desireable,
sometimes not)
- addition of nitrite directly affects myoglobin
– necessary prerequisite for desirable cured color is
nitric oxide (NO)
– several reactive intermediates play critical roles
for desirable color development by producing
nitric oxide
Hydrogen ions
1. Acidic conditions (hydrogen ions)
NO2- + H+
→
HNO2 (nitrous acid)
2HNO2 →
N2O3 + H2O
(highly reactive intermediate)
N2O3
NO2 + NO
(nitrogen dioxide + nitric oxide)
→
(2 NO2 + H2O
→
HNO3 + HNO2)
(Note: nitrate + nitrous acid)
Effects of salt, nitrite and 0.2% GDL on
residual nitrite, pH and cured color
Treatments
Residual nitrite
20.6b
156 ppm
46.7a
Salt ( with 75 ppm nitrite)
0%
35.4a
0.5%
34.9
1.0%
35.4a
2.0%
32.5b
4.0%
26.0c
GDL (156 ppm nitrite)
0%
40.0a
0.2%
26.7b
Nitrite 75 ppm
% cured pigment
60.3b
62.3a
pH
5.89b
5.93a
59.8b
60.6b
60.8b
61.1b
64.2a
5.79e
5.87d
5.91c
5.96b
6.02a
58.7b
63.9a
6.03a
5.79b
2. Reductants
– Natural reductants (NAD, cysteine, cytochromes)
– Ascorbate/erythorbate
N2O3 + 2 H-Asc ↔ 2 dehydro-Asc + H2O + 2 NO
3. Myoglobin
Mb-Fe+2 + NO2- → MMb-Fe+3 + NO + OH(purple)
(brown)
reductant
MMb-Fe+3 + NO →MMb-Fe+3–NO →Mb-Fe+2–NO
(brown)
(brown)
(red)
Nitrite concentration effects on raw meat color
0 ppm
0.5 ppm
1.0 ppm
2.5 ppm
5.0 ppm
10.0 ppm
15.0 ppm
20.0 ppm
25.0 ppm
50.0 ppm
Sebranek and Turner, 2002.
Meat Process. 41(9):52-55.
4. Salt (NaCl)
HNO2 + H+ + Cl-
↔
NOCl + H2O
(highly reactive)
NOCl + Mb-Fe+2
(purple)
→
Mb-Fe+2– NO + Cl(red)
NaCl and ascorbate effects on
measurable nitrite in cooked meat
Salt (mM)
Ascorbate
Nitrite (% of initial)
0
0.5
1.0
0
0
0
91
91
84
0
0.5
1.0
6 mM
6 mM
6 mM
73
59
49
0
0.5
1.0
12 mM
12 mM
12 mM
55
44
35
Fox et al., 1981.
J. Assoc. Off. Analyt. Chem. 64:1397-1402.
5. Proteins/sulfhydryl groups
R-SH + 2 HNO2 →
R-S-S-R + 2 H2O + 2 NO
sulfhydryl
disulfide
-besides generating nitric oxide, this reaction has been
suggested as a contributor to increased firmness on cured
meat by crosslinking proteins with the disulfide bond
In addition to cured color development from the
various sources of nitric oxide
• Nitrite is strongly inhibitory of anaerobic
bacteria, particularly Clostridium botulinum
– dependent on pH, salt, reductants, iron content
• Interestingly, these are many of the same factors that
favor nitrous acid and nitric oxide production
Is ingoing or residual nitrite most
important to bacterial control?
• USDA requires minimum of 120 ppm ingoing for cured
products that are refrigerated
• Research suggests that residual nitrite is critical to
antibotulinal effects
• Most commercial cured meats at retail have less than 15
ppm residual nitrite
Probably…a certain minimum ingoing amount is necessary to
achieve initial inhibition, given competitive chemical
reactions, and a certain residual amount is necessary for
sustained inhibition but the the amounts of each are
unclear and probably dependent on several other inherent
conditions in the product
Nitrite provides potent
antioxidant effectiveness
• Stabilizes heme iron
• Chelates metal ions and radicals
• Reacts with unsaturated lipids
Fish, chicken,
pork and beef
-measured
48 hours
post-cook
Morrissey and Tichivangana, 1985.
Meat Sci. 14: 175-190.
Role in cured flavor?
• Over 100 volatiles identified in cured ham
• Not well understood
-part of antioxidant effect?
-more reaction products of nitrite/nitric
oxide?
Nitrite safety issues
• Toxicity
-highly toxic (~70 mg/kg body weight, i.e. 4-5 gram dose)
-inconsequential in normal cured meat applications because
of extremely low concentration
• Nitrosamines issue
-secondary amines (-NH) may react with nitrite (or nitrous
acid (nitrosating intermediates) to form -N-NO (nitrosamine)
-most are definite carcinogens, some are quite potent
- epidemiological studies based on nitrosamine hypothesis
• “Natural” curing
-significantly reduced nitrite concentration, indistinguishable
appearance
There are a number of processed meat
products, mostly natural and organic
cured meats, being introduced in the
market that are labeled “Uncured” and
“No nitrites or nitrates added”.
Ingredients: Organic grass-fed beef, water, sea
salt, organic spices, organic garlic powder, organic
paprika, celery powder, organic onion, lactic acid
starter culture
Even though labeled “uncured”, almost all of
these products display all the typical cured
meat properties that are derived from
addition of sodium nitrite, and that can be
achieved ONLY with sodium nitrite…
Further, chemical analyses of the products
confirm the presence of nitrite and nitrate.
Thus…the oxymoron in labeling;
uncured = cured
Why the oxymoron…?
…why is “Uncured” and “No nitrites or
nitrates added” used on the label of
products that are unequivocally cured in
terms of both product properties and the
presence of curing agents?
USDA has required that uncured versions
of “typical” cured meats be
“… labeled with such standard name
when immediately preceded with
the term “Uncured” in the same
size and style of lettering as the rest
of such standard name…”
Code of Federal regulations 9 CFR 319.2 (2006)
“Natural” (processed meats)
definition:
• “…does not contain any artificial
flavor or flavoring, coloring
ingredient, or chemical
preservative…”
(USDA Food Standards and Labeling
Policy Book, 2005)
“Organic”
Defined and controlled by Organic Foods
Production Act (1990)
-established National Organic Program
Standards
-National List of Allowed and Prohibited
Substances
-nitrite, nitrate not permitted
Thus, nitrite or nitrate, by themselves,
cannot be directly added to any natural
or organic processed meat
product…and…any “typically” cured meat
product that does not have added nitrite
or nitrate must be labeled “uncured”.
Critical point:
Cured meats, i.e. ham, bacon, frankfurters are
highly distinctive and unique
-nitrite/nitrate create distinctive cured meat
properties that cannot be duplicated
-dramatic quality, shelf life and safety
improvement that consumers expect for
these products
Alternative Production Methods
• Two types of “Uncured” Meat Products
– No intention of replacing nitrite
– Intention of replacing nitrite with
natural sources of nitrite
-Nitrite between 0.9ppm to
9.2ppm
-Nitrite introduced indirectly as
component of other ingredients
Manufacture of cured product without
added nitrite/nitrate
-utilize natural sources of nitrate
-0.2%-0.4% celery juice/powder (natural flavor,
celery powder) plus nitrate reducing culture
-hold at 30C-37C - time/temperature
considerations (franks preheated at 37C for 30
min. result in acceptable color)
-or…utilize pre-converted celery juice/powder as a
natural source of nitrite
Examples of commercially available
ingredients for natural curing
-VegStable™ 502 * (celery powder, sea salt)-30,00033,000 ppm nitrate; recommended use level of
0.2% to 0.4%
-VegStable™ 504 * (celery powder, sea salt)-10,00012,000 ppm nitrite (pre-converted, starter and
holding time not required); recommended use level
of 0.2% to 0.4%
*from Florida Food Products, Inc.
Concerns for cured products without
added nitrite/nitrate
-Variable nitrate sources
-Nitrate reduction rate/consistency
-Nitrite concentration produced-both extremes
-issues will depend on product properties
-Shift in nitrite dispersion
-will rate of production affect distribution?
-Product quality/consistency
-Microbiological impacts
-ingoing vs. residual nitrite?
Clostridium botulinum inoculations
Time until botulism toxin was detected during vacuum-packed storage
of inoculated frankfurters at various storage temperatures
Inoculated frankfurters
Products stored at:
cured control (156 ppm)
22ºC
14 days
10ºC
none/84 days
4ºC
none/84 days
63 days
uncured control (no nitrite)
5 days
14 days
natural cure
5 days
56 days
natural cure-VS 507
5 days
natural cure-VD 55
none/84 days
none/84 days
none/84 days
none/84 days
none/84 days
none/84 days
-data courtesy of B. Wanless and K. Glass
Clostridium botulinum
Time until botulism toxin was detected during vacuum-packed
storage of inoculated ham at various storage temperatures
Inoculated ham
Products stored at:
cured control (156 ppm)
22ºC
none/28 days
10ºC
none/84 days
4ºC
none/140 days
uncured control (no nitrite)
3 days
14 days
none/140 days
natural cure
5 days
21 days
none/140 days
natural cure-VS 507
5 days
none/84 days
none/140 days
natural cure-VD 55
21 days
none/84 days
none/140 days
-data courtesy of B. Wanless and K. Glass
Formulated nitrite
• Conventional cure
• Natural cure 504 (converted)
156 ppm
30 ppm
• Residual nitrite after cooking (ppm)
frankfurters
Cured control
115
Natural cure
43
Natural cure-VG517 30
Natural cure-VD55
29
ham
142
36
38
37
-data courtesy of B. Wanless and K. Glass
It appears that natural and organic “uncured”
RTE processed meat products will permit
greater growth of L. monocytogenes and Cl.
perfringens and earlier toxin production by Cl.
botulinum than conventionally cured
products.
Some natural antimicrobials provide
potentially suitable adjuncts for improved
safety.
Despite the recent good news, negative past
perceptions of nitrite persist
• Epidemiological studies continue to report
associations between nitrite consumption and
various forms of human cancer
• One of the most significant was a very large
summary released on Halloween, 2007.
“The evidence that red meats and processed
meats are a cause of colorectal cancer is
convincing” (emphasis mine)
“…nitrite, from processed meats… may be
involved in carcinogenesis… Dietary nitrates
and nitrites are probable human carcinogens
because they are converted in the body to
N-nitroso compounds”
WCRF/AICR Report, 2007
It is critical to remember:
•
These conclusions are based on epidemiology
•
A causative link for meat consumption is still
missing
-NIH report (2001) found no relationship
between nitrite consumption and cancer.
On the Toxicology and Carcinogenesis Studies of Sodium Nitrite.
2001. U.S. Dept. of Health and Human Services, 276 p.
- review and report in 2007 by Exponent, an
independent consulting firm found no
relationship
Even if nitrite is assumed to be a risk for
carcinogenesis, less than 5% of human exposure to
nitrite and nitrate come from processed meat
There is now evidence of some very positive health
effects of dietary nitrite.
Meat may well be associated with diets high in fat
from other sources, high in caloric content from other
sources and many other confounding factors (body
weight, exercise, smoking, alcohol consumption,
stress, etc.)
“…there is an easy solution to every human problem…
… neat, plausible, and wrong.”
- H. L. Mencken
Writer, editor, social critic
(1880-1956)