Nitrates as Contaminants in Dairy Products
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Provided for non-commercial research and educational use only. Not for reproduction, distribution or commercial use. This article was originally published in Encyclopedia of Dairy Sciences, Second Edition, published by Elsevier, and the attached copy is provided by Elsevier for the author’s benefit and for the benefit of the author’s institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution’s administrator.
All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier’s permissions site at: http://www.elsevier.com/locate/permissionusematerial Indyk HE and Woollard DC (2011) Contaminants of Milk and Dairy Products | Nitrates and Nitrites as Contaminants. In: Fuquay JW, Fox PF and McSweeney PLH (eds.), Encyclopedia of Dairy Sciences, Second Edition, vol. 1, pp. 906–911. San Diego: Academic Press. ª 2011 Elsevier Ltd. All rights reserved.
Author's personal copy Nitrates and Nitrites as Contaminants H E Indyk, Fonterra Co-operative Group Ltd., Waitoa, New Zealand D C Woollard, NZ Laboratory Services, Auckland, New Zealand ª 2011 Elsevier Ltd. All rights reserved.
Occurrence of Nitrogen and Its Oxides Nitrogen (atomic number 7) exists predominantly as isotope 14N and makes up a substantial part of the known universe. On Earth, nitrogen occurs mainly as inert gaseous N2, making 78% of the atmosphere and as such is not available for organic life unless converted to oxide form. This was achieved primordially by electrical storms and in the current biosphere by nitrogen-fixing bacteria. Thus, nitrogen exists commonly in the stable þ5 oxidation state as NO3 as part of the well-studied ‘nitrogen cycle’, one of the most significant nutrient cycles in the terrestrial ecosystem (Figure 1). Nitrate (NO3) exists commonly throughout the geological world in sediments or dissolved in the oceans and waterways. Significant nitrate reserves are stored in soil, of which the majority is bound in dead biomass and is largely unavailable to plants. The remaining nitrogen pool is available for plant growth in the form of ammonium, both free and clay-bound. Within the soil environment, nitrogen is available for use by many lifeforms for incorporation into nucleic acids, amino acids, and other essential compounds, and as such, about 16% of living organisms is nitrogen. Other inorganic compounds are also produced chemically and biochemically, containing nitrogen in a wide variety of oxidation states from 3 to +5, both gaseous and ionic (Table 1). Nitrate is a requirement of modern industry and is made in significant quantities via ammonia by the Haber and Ostwald processes. This was instigated by its historic importance in gunpowder and explosives. More recently, the higher oxides are used as propellants in rockets. Nitrate and ammonium salts are most commonly used in modern agriculture as fertilizers to replenish soil nitrogen for pasture and crops and thus nitrogen salts enter the environment where the potential for environmental pollution is high. It should be noted that atmospheric sources of aquatic nitrate have also become an increasing concern in view of anthropogenic-induced emissions of nitrogen compounds. The widespread use of nitrogen-based fertilizers, combined with domestic, agricultural, and industrial wastes, has indeed resulted in increases in the nitrate content of surface ground-water globally, since these nitrogen inputs have frequently exceeded the capacity of the biosphere. The extent of such nitrate leaching is dependent on the
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type of land use, and increases in the order forestry, agriculture, and livestock. In general, nitrate is largely unbound within the soil environment and has a much higher potential to leach compared to ammonium. The principal concern related to nitrate loss is its capacity to encourage excessive plant and algal growth in waterways and thereby interfere with air supplies to other flora and fauna. Although considered stable and not inherently toxic, nitrate is reduced to the reactive nitrite ion, NO2. While agricultural use of nitrate is dominant in terms of volumes, nitrite in the form of its sodium salt is used as a food preservative, most commonly in cured meats, with nitrate salts also included in processed foods to provide a nitrite reservoir.
Physiological Role While dietary (exogenous) intakes of nitrate and nitrite are commonly and justifiably regarded with concern, it must be noted that endogenous tissue generation of these ions is an alternative and normal consequence of the critical physiological functions of the highly transient nitric oxide. In humans, nitric oxide is endogenously produced from a range of sources, the most notable of which is L-arginine, under the action of a tissue-specific synthase system. Nitric oxide is involved in multiple metabolic functions, including vascular regulation, platelet aggregation, neurotransmission, inflammation, and immunity. Given its physiological significance, plasma nitrite levels are now considered a reliable diagnostic indicator of nitric oxide status in a range of human pathologies. Only recently is evidence available to suggest that, apart from the endogenous L-arginine route, dietary nitrate and nitrite may account for approximately half of systemic nitric oxide, a pathway that is dependent on endogenous mammalian nitrate reductases to provide the nitrite substrate required for nitric oxide production via several potential reductive mechanisms (Figure 2). In fact, it is also becoming recognized that exogenous nitrite may have physiological properties that are independent of its role as a precursor of nitric oxide. It is likely that recent developments related to the in vivo conversion of dietary nitrate and nitrite to systemic nitric oxide will have novel implications beyond the traditional consensus that regards these nitrogen species solely as contaminants.
Encyclopedia of Dairy Sciences, Second Edition, 2011, Vol. 1, 906-911
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Figure 1 Schematic illustration of the natural nitrogen cycle.
Table 1 Nitrogen oxidation states and representative compounds Oxidation state
Species
Name
3 2 1 0 þ1 þ2 þ3 þ4 þ5
NH3, NH4þ N2H4 NH2OH N2 N2O NO HNO2, NO2 – NO2 HNO3, NO3 –
Ammonia, ammonium ion Hydrazine Hydroxylamine Nitrogen Nitrous oxide Nitric oxide Nitrous acid, nitrite ion Nitrogen dioxide Nitric acid, nitrate ion
While dietary nitrates and nitrites have long been associated with increased risk of adverse health effects, recent clinical studies have indicated that exogenous nitrate and nitrite may increasingly be considered as part of normal nitrogen consumption, rather than a toxin to be avoided at all costs. Evidence that suggests that health benefits may be derived from consumption of dietary nitrate and nitrite, through maintenance of a sufficient pool of systemic nitrogen oxides for tissue defense and nitric oxide homeostasis during disease and stress, is accumulating. In addition, there have been recommendations to Codex that the acceptable daily intake (ADI) for nitrite is too low. This
Figure 2 Schematic of the sources and physiological cycling of NO, NO2, and NO3.
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means that the significance of nitrate and nitrite in dairy products with respect to toxicological implications may diminish even further.
Toxicity of Nitrite and Nitrosamines In view of poisoning events with humans and animals, particularly from contaminated drinking water, toxicological studies on nitrate and nitrite have been quite extensive. The current World Health Organization guideline for nitrate in drinking water is 50 mg l1 and the provisional guideline for nitrite is 0.2 mg l1 for longterm exposure, although individual water samples can contain up to 3 mg l1 of nitrite. These drinking water levels are rarely exceeded in Australia, New Zealand, and other developed countries. However, concentrations well above these levels have been reported in countries noted for severe environmental pollution. While nitrate itself is not reported as a significantly toxic compound, the principal toxicological problem related to dietary intake is with nitrite. This is largely attributed to oxidation of the ferrous ion in hemoglobin in a disease state known as methemoglobinemia. The oxygen-carrying capacity is thus reduced to a point that it can cause cyanosis (blue coloration) and minor or serious hypoxia (asphyxiation). Nitrite-forming bacteria in healthy adult humans are limited by the low pH of the stomach, so NO3–NO2 conversion in vivo is minimal. However, this is not the case with both the aged and neonates, who, in general, are characterized by reduced stomach acidity. Such individuals can also have inefficient enzyme systems to reverse the hemoglobin–methemoglobin conversion, notably diaphorase I (NADH methemoglobin reductase). Genetic disorders can also make certain individuals susceptible to clinical or subclinical methemoglobinemia through shortage of the enzyme or coenzyme. A further commonly reported health issue related to dietary nitrite is its propensity to form nitrosamines in the acidic environment of the stomach. Nitrite forms nitrous acid with any mineral acid such as stomach HCl (NO2 þ Hþ ! HNO2). While unstable to dissociation (2HNO2 ! NO2 þ NO þ H2O), it will also form the nitrosonium cation (HNO2 þ Hþ ! H2NO2þ ! H2O þ NOþ). This strongly nucleophilic agent (NXOþ) will react with secondary amines to form a potentially wide array of carcinogenic nitrosamines. The suspected pathway for carcinogenesis involves alkylation of DNA, primarily via formation of methylguanine, resulting in GC to AT point mutation. Modern analytical techniques have identified over 300 nitrosamines, most of which have been shown to have organ-specific carcinogenicity in animals. Epidemiological evidence in human populations remains inconclusive, but toxicity is likely to be similar to animals.
While the most frequent exogenous nitrosamines are associated with the use of tobacco and occupational exposure in chemical factories, dietary sources are also well established. Since the formation of nitrosamines is associated with cooking regimens, fried and smoked foods can contain significant concentrations, particularly when cured with sodium nitrite to reduce the potential for botulism poisoning. Fish contain high levels of amines, making them the first identified food type containing nitrosamine in the form of N-nitrosodimethylamine. All foods pickled in nitrate/nitrite (bacon, ham, etc.) will also yield high levels of nitrosamines, particularly dimethylnitrosamine, diisobutylnitrosamine, and nitrosopyrrolidine. Ascorbic acid (vitamin C) is known to limit nitrosamine formation, so it is added to modern meat-curing solutions. In the past, a significant source of dietary nitrosamines was barley used in beer production, although this problem has been largely remedied. Avoiding dietary levels of nitrite is clearly a way of minimizing the risk of gastric and liver cancers associated with nitrosamines, and US and EU regulations are currently implemented to control the use of sodium (and potassium) nitrate and nitrites. This has helped reduce the ingestion of nitrosamines significantly in the past few decades from levels of approximately 1 mg day1 to less than 0.1 mg day1.
Daily Intake The ADI for adults is 3.7 mg of nitrate per kg body weight and 0.07 mg of nitrite per kg body weight. Many official studies have been undertaken to trace the sources of nitrate and nitrite as part of regulatory decision making within Europe, the United States, Australia, and New Zealand. These generally report that selected vegetables, such as spinach and lettuce, can contain over 5000 mg kg1 of nitrate, thus making them the principal contributor to the human diet. As importantly, nitrate and nitrite are used as food additives, particularly as microbial inhibitors and to impart flavor and color in processed meats. Thus, Western diets involving bacon and other cured meat products are responsible for nitrite regularly exceeding the ADI. The opportunity for large-scale poisoning is reduced by strict control of nitrate and nitrite salts used during food manufacturing. Quite clearly, the impact of dietary nitrate and nitrite depends on three factors: (1) their concentration in food products; (2) the amount of such food in the diet; and (3) the body weight of the consumer. British studies in 1994 and 1997 concluded that the total intake of dietary nitrate from all sources can exceed 140 mg day1, with vegetables such as spinach and lettuce contributing approximately 80% of the average daily dietary intake. This level is well short of the permissible ADI of 220 mg day1 for a 60 kg
Encyclopedia of Dairy Sciences, Second Edition, 2011, Vol. 1, 906-911
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European adult. These studies also confirmed that dairy products are a measurable but not overwhelming source of these compounds. In Asian countries, the average body weight of the population is considerably less (55 kg) and they consume larger portions of high-nitrate vegetables. The absence of widespread toxicity symptoms also indicates that nitrate is not a serious health risk to the general population. Individuals can, however, be at risk depending on unique circumstances, but these are not evident in epidemiological studies. In contrast, infants are high on the list of potential victims of nitrate/nitrite toxicity. While babies raised on mother’s milk will not experience such problems, the reconstitution of milk powder or infant formula with contaminated water is a recognized causative factor of clinical manifestations, notably methemoglobinemia, leading to possible morbidity and mortality.
Sources in Dairy Products As early as c. 7000 BC, man learned to domesticate selected animals, including the cow, buffalo, sheep, goat, and camel, for the provision of milk for human consumption, with milk from the dairy cow dominant by volume. The current availability of dairy products in the modern world combines the traditional with the application of modern science and technology. Total regional dairy consumption per person varies widely, although through globalization and migration, these trends are continuously evolving. Whereas the majority of nitrate and nitrite in the human diet is derived from potable water, vegetables, and cured meats, commodity dairy products, as staple food ingredients, are traded internationally. They are, therefore, well monitored for the presence of all contaminants, including nitrate and nitrite. The regular use of milk and processed dairy ingredients in infant nutriture is a major reason to minimize such contamination, particularly that of nitrite. Within the dairy industry, cows ingest nitrate and nitrite during grazing and drinking, and milk can be contaminated by means of either secretory or post-secretory processes. It is, however, well established that the level of nitrate in the diet of dairy cows has little effect on secretory milk composition and does not lead to significant accumulation in the milk. Indeed, it has been estimated that the expression of nitrate in bovine milk is on the order of 103 of an orally administered dose. Thus, raw milk typically contains 15 mg l1 of nitrate and
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