2. ♦Nitrosamines is a general term used to designate a vast
group of N-nitroso compounds (NOCs), bearing a common
functional >N–N=O group. The NOCs can be divided into
two classes: N-nitrosamines and N-nitrosamides and related
compounds.
♦N-Nitrosamines are N-nitroso derivatives of secondary
amines (N-nitrosamides and its related compounds are those
of substituted ureas, amides, carbamates, guanidines, and
similar compounds.
♦The interest for this group of compounds increased since
the evidence of their mutagenic and carcinogenic properties.
N-NITROSAMINE AS CARCINOGEN
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4. Among the vast group of compounds included under the term of NOC some are
of special interest.
That is the case of the simplest N-nitroso compound, N-nitrosodimethylamine
(NDMA), or the so-called tobacco-specific nitrosamines (TSNAs), which are
among the most abundant carcinogens identified in tobacco and its smoke.
Seven TSNAs have been identified both in tobacco filler and tobacco smoke.
They are nonvolatile and formed by nitrosation of nicotine and related tobacco
alkaloid precursors with nitrogen oxides.
In general, foods in most western diets have been examined more extensively
than foods from Asia, Africa, and South America.
Although approximately 20 volatile nitrosamines have been identified in a variety
of foods and beverages, NDMA and NPYR have been found most commonly.
Most of the other volatile nitrosamines which occur in foods do so infrequently.
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5. Table 1. presents a condensation of many reports of the occurrence
of volatile nitrosamines in foods from around the world. The
summary is not comprehensive; rather, it is intended to show the
foods in which nitrosamines occur most commonly.
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6. Nitrosamines form in foods because under certain circumstances the
precursors, amines and nitrosating agents, occur in foods. Amines
occur commonly in food, and are formed biosynthetically and by
microbial activity.
Nitrosating agents can be formed from certain compounds
added to foods and as a result of specific processing conditions. In
most instances, nitrosamines are found in foods in one of the three
following categories.
I)Cured meats
Nitrosamines are formed in cured meats because nitrite, and
sometimes nitrate, are added to these products during processing.
Nitrate is reduced to nitrite by the enzyme nitrate reductase, which
occurs in a number of bacteria. Nitrite is converted to nitrosating
agents which subsequently react with amines in the meat during
processing, storage, and cooking to form nitrosamines.
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7. Typically, fried bacon contains 1–20 μg kg−1 of NPYR and 1–3 μg kg−1 of
NDMA.
The formation is related to the relatively high internal temperature of bacon
during frying and the relatively low moisture content of bacon as compared to
other cured meat products.
When bacon is cooked by other methods, particularly in a microwave oven,
considerably lower amounts of nitrosamines are found.
The majority of evidence suggests that the free amino acid proline is first
nitrosated and then decarboxylated to form NPYR during frying.
Neither the precise chemical nature of the nitrosating agent nor
the amine precursor for NDMA in bacon is known with certainty. However, the
evidence suggests that the nitrosating agent is a reaction product of nitrite and
lipids in the bacon.Nitrite and nitrate have been added to cured meats for many
years to prevent outgrowth and toxin formation by Clostridium botulinum.
Nitrite, in combination with other curing ingredients such as
sodium chloride, is particularly effective in inhibiting formation of the deadly
botulism toxin. In addition, nitrite reacts with pigments in meat to impart the
desirable pink color of cured meats and it prevents the development of off-flavors.
.
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8. II)Dried foods and ingredients
I. A variety of processes and equipment are used to dry foods and food
ingredients. During the direct-fire process, air used to dry the food is
first heated by passing the air through the flames of burners.
II. As a result, products of combustion, including oxides of nitrogen, are
directly incorporated into the hot air used to dry the food.
III. The oxides of nitrogen which include nitrosating agents such as nitrous
anhydride can then react with amines in the food being dried to produce
nitrosamines.
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9. The discovery of nitrosamine formation in direct-fire-dried
malted barley has led to the investigation of other dried
foods.
In the USA, certain dried dairy products, notably nonfat dry
milk, are manufactured by the direct-fire drying process.
Nonfat dry milk manufactured by this process consistently
contains low amounts (less than 1 μg kg−1) of NDMA.
Since the direct-fire drying process is not used commonly
in Europe for manufacture of nonfat dry milk, the product
does not contain NDMA.
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10. Seafood can contain nitrosamines as a result of either cooking
or salt-drying.
When fish is broiled with a gas flame, the oxides of nitrogen
produced in the flame can cause nitrosamine formation in a
manner analogous to nitrosamine formation in direct-fire drying.
In certain areas of the world, particularly the Orient, fish is
preserved by salt.
Often sea salt is used which contains appreciable amounts of
nitrate.
The nitrate is reduced to nitrite with subsequent formation of
nitrosating agents.
In some cases, reaction with amines in the fish produces
relatively large amounts of nitrosamines, principally NDMA.
The potential for NDMA formation in fish is considerable since
high levels of NDMA precursors, such as dimethylamine and
trimethylamine, can occur in fish.
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11. III)Migration from surfaces which contact foods
Vulcanized rubber products, such as baby nursing nipples, have been
shown to contain nitrosamines.
It has been demonstrated that when baby bottle nipples are stored inverted in
milk, the nitrosamines partially migrate from the nipples to the milk.
Furthermore, studies have shown that nitrosamines migrate from rubber netting
used to hold cured meats during the smoking process.
In addition to rubber, nitrosamines in such substances as wax-treated
wrapping paper and paperboard-based materials have been shown to migrate to
foods.Nonvolatile nitrosamines in foods.
The notion that nonvolatile nitrosamines might form in foods follows
logically from the fact that the precursors, nonvolatile amine and nitrosating
agents, occur in foods.
Due to limitations in analytical methodology for nonvolatile
nitrosamines, less is known about the occurrence of nonvolatile than volatile
nitrosamines in foods. Recently developed methods for nonvolatile nitrosamines
have been limited to the detection of N-nitrosated amino acids and amino acid
derivatives. 11
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