Chemical hazards - an overview

SESSION 4. Chemical hazards and control measures

Abstract

Chemical hazards are toxic substances and any other compounds that may render a food unsafe for consumption. Some chemical contaminants are harmless and others are hazardous because of the toxicological risks from their intake to the consumer. Harmless contaminants may still have the disadvantage of interfering with food processing and causing interactions during storage. Examples are metal ions and plant pigments [1].

The origin of chemical hazards are various, but may be introduced into the milk through feed & water administered to the animals, and/or insufficient control of equipment, animal environment, and milk storage facilities.

Milk and dairy products are remarkably susceptible to contamination for many reasons. Dairy animals while grazing ingest contaminants which are present such as lead in soil or are deposited on grass from industrial emissions (e.g. PCBs and dioxins) or are present in soil as a result of the use of persistent pesticides in former years. Another major route of intake of contaminants is through compounded animal feeds (e.g. mycotoxins and heavy metals). A range of veterinary drugs is in use including antibiotics, sulfa drugs. Residues of anti-infective, anthelmintics and other drugs may occur in milk and milk products [2].

During processing, food can be contaminated with processing aids, such as cleaning agents, and with metals coming from the equipment. Finally, contaminants can mitigate to foods during packaging and storage, e.g. from plastics, coatings and tins [3].

1. MYCOTOXINS

Mycotoxins are metabolites of moulds, which evoke pathological changes in man and animals. Many mycotoxins are known and have been shown to cause different biological effects in laboratory animals: acute toxic, mutagenic, carcinogenic, teratogenic, hallucinogenic, emetic and estrogenic. The presence of mycotoxins in dairy products may be result of:

The aflatoxins have a very high toxicity and carcinogenicity and more than 75 countries have introduced or propose regulations for control and analysis of aflatoxins in foodstuffs. Aflatoxin M1 levels in dairy products are regulated in at least 22 countries and the maximum tolerated levels range between 0 and 1 ug/kg (0-0.1 ug/kg for infant/children’s food). EU maximum level of aflatoxin M1 in milk is 0.5 ug/kg [5]. Aflatoxin M1 appears in milk and milk products as the direct result of the intake of aflatoxin B1 - contaminated feed by dairy cows. Aflatoxin B1 can be produced by the fungi Aspergillus flavus and Aspergillus parasiticus, under certain conditions of temperature, water activity and availability of nutrients. Mycotoxins produced by fungal species other than Aspergillus and Penicillium are of minor concern for dairy products.

In recent years concern has been expressed about the presence of aflatoxin M1 in milk and milk products. In underdeveloped countries there have been reports of very young children being exposed to the effects of this toxin through their mothers’ milk before weaning [6]. In developed countries it is cows’ milk and products derived from it which have been under the close scrutiny and evidence has accumulated that milk often contains extremely low levels of aflatoxin M1 [7]. The efficiency of aflatoxin conversion in cows is poor; in 1986, Frobish et al. [8] reported that less than 2% of aflatoxin B1, deliberately added to gain fed to lactating animals was converted to the hydroxylated form (M1). Again, although many different types of cheeses can contain aflatoxin M1, it is almost invariably in such low levels that no risk to human health is insignificant . Detailed studies have been performed to  determine the fate of aflatoxin M1, in different cheeses but similar results have been reported.

2. POLYCHLORINATED BIPHENYLS (PCBS)

Polychlorinated biphenyls (PCBs) are mixtures of chlorinated biphenyls with varying percentages of chlorine by weight (e.g. the mixtures of PCBs denoted as Aroclor 1242, 1254 and 1260 contain 42, 54 and 60 % chlorine by weight, respectively). The differences in physical/chemical properties of the various PCBs are reflected in their distribution and mobility in the environment. Although generalizations may not be appropriate for a group of chemicals with such widely varying properties, it has been observed that, as the chlorine content of PCBs increases their rate of degradation by environmental systems decreases, resulting in increased persistence and vice versa.

It is estimated that, the industrial production of PCBs and the release into the environment differ widely and range up to 2.4 million metric tons for production and up to 200.000 metric tons released into the environment from human activities. These large figures in combination with the chemical and physical properties of PCBs led to an appreciable worldwide contamination of the environment, biota and food-produced ecosystems [9].

PCB mixtures manufactured by different companies were sold under different trade names (e.g. Aroclors in the US, Clophens in Europe and Kanaclors in Japan). Due to the difference in chlorine content, commercial PCB mixtures had markedly different physical/chemical properties, as well as, different use patterns. Chemically, there are 209 chlorinated biphenyls ranging in degree of chlorination from the 3 monochlorinated congeners to the fully chlorinated decachlor-biphenyl congener. There is also a wide range in the physical/chemical properties of PCBs both within isomeric groups and between congener families. Commercial PCBs are stable, non-flammable and heat-resistant synthetic compounds that have been used historically in various industrial products. Since the use, storage and disposal of PCBs are now closely regulated; future emissions of PCBs to the environment will be largely caused by accidents, improper disposal procedures, and release from poorly maintained electrical equipment, clandestine dumping and environmental dissipation from sites with elevated concentrations of PCBs. By the 1960s, PCBs became ubiquitous in the environment, and their toxicity came under closer scrutiny after several accidents and poising incidents. Although the manufacture and use of PCBs was phased out from the mind-1970s onwards, low levels of persistent PCBs can still be detected in the environment and via bioaccumulation in certain fatty foods. Stricter environmental controls have led to decreased levels in foods and subsequently lower human exposure over the last decade [10].

Originating from environmental sources such as soil, water and air, the PCBs are distributed mainly to feed material by atmospheric transport, either in the gaseous phase or condensed as aerosols. The water solubility is extremely low for the stable congeners, so that a direct root uptake by forage plants will practically play no role in the contamination of the forage plants for the dairy cow. In fact, the diffuse background contamination originates from the atmospheric deposition on the structure of forage plants or absorption of vapour, thus depending on factors like wind, rain, temperature, air particulates, distance from source, typical features of the plant surface itself. Levels in milk and animal fat are normally under 5 ug/kg (ppb) on a fat basis. Variations in reported levels in foods as well as in human intake estimates are due to analytical differences (number of PCB congeners analysed) and dietary habits.

PCBs are not a significant hazard in milk and milk products. However, they are perceived as being of general significant concern as these substances can accumulate in human tissue and in the long term perspective cause a variety of adverse health effects Accurate scientific assessment, especially of the potency, is difficult because PCBs occur only as complex mixtures and frequency together with other potent toxins such as dioxins and chlorinated pesticides. PCBs have been classified as probable human carcinogens. In animal studies, PCBs exhibit reproductive, developmental and immunotoxic effects.

The mean daily intake has been estimated as 7-70 ug/ person/day. At current background exposure levels there is no real evidence of adverse effects in humans. Of greatest concern in this context are the coplanar PCB congeners (dioxin-like) because of their similar mode of action to dioxins. Furthermore, no safety standards, such as Tolerable Daily Intake (TDI) are established for PCBs. However, ‘dioxin- like’ PCBs are included in the new WHO-TDI for dioxins and ‘dioxin-like’ compounds, set at 1-4 pg Toxic Equivalents (TEQ)/kg body weight/day. Many countries have set maximum residue limits for PCBs in dairy products, mainly based on the seven most abundant congeners. These are: PCB 28, -52, -101, -118, -138, -153 and -180 [10].

3. POLYCHLORINATED DIBENZO-PDIOXINS (PCDDS) AND POLYCHLORINATED DIBENZOFURANS (PCDFS)

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are environmental contaminants which are lipophilic, chemically stable, of low volatility, and which are known to be present, even if only at very low concentrations, in the fatty tissues of animals and humans. For some time the term ‘dioxin’ was associated with 2,3,7,8 - tetrachlorodibenzo-p-dioxin (TCDD), but this is only one of a large number of PCDD and PCDF congeners. Since both PCDDs and PCDFs can have from one to eight chlorine substituents there is much scope for positional isomers and the number of PCDD and PCDF congeners is, respectively, 75 and 135 [11]. Dioxins are formed as inadvertent by-products in many chemical processes involving chlorine and in any combustion process. The main sources for PCDDs/PCDFs are (municipal) waste incinerators and steel sintering plants, also metal recycling plants and cement kilns. Dioxins, bound to  particulate matter, are deposited via the atmosphere on any surface. Elevated contamination levels can be found in milk from farmland in the vicinity of these industries. The meal background levels in dairy products expressed as toxic equivalents (TEQ) are 0.6-3 pg TEQ/g milk fat (10- 12 g/g fat; ppt) in industrialised countries. Slightly higher levels are possible in urban and industrial areas [10]. The EU Commission has adopted a maximum level for dioxin of 3 picograms/g fat for milk that might lead to global areas being excluded from milk production.

Dioxins are very potent toxicants, and TCCD is one of the most potent animal carcinogens, recently classified as a human carcinogen [12]. Apart from carcinogenicity, various effects have been demonstrated in animal models and suspected in humans, for example on the immune system, reproduction and development, and neurobehavioral alterations. The main source of human exposure are foods of animal origin, and the estimated average daily intake of dioxins in industrialised countries 1s 1-3 pg TEQ/kg body weight/day. Dairy products contribute approximate ¼ to ½ to the dietary intake of total dioxins, included ‘dioxin-like’ PCBs.

Dioxins occur as complex mixtures. They act through a common mechanism, but vary in their toxic potency. The compounds are assessed and regulated together as a group by the sum of the potency of the congeners relative to TCDD. The result is thus expressed as TEQ. ‘Dioxin-like’ PCBs, which act in a similar fashion, are included in the TEQ concept.

The Tolerable Daily Intake (TDI) for dioxins, furans and ‘dioxin-like’ PCBs is 1-4 pg TEQ/kg body weight/day (WHO, 1998). It appears that part of the population in industrialised countries exceed this save level of intake, therefore dioxin in food is a health concern and efforts are undertaken to further reduce human exposure.

4. OTHER PERSISTENT HALOGENATED HYDROCARBONS AND ORGANOCHLORINE PESTICIDES (OCS)

There are other persistent halogenated hydrocarbons which persist in the environment and although not of practical concern, can be detected in milk, such as polybrominated flame retardants (e.g. polybrominated diphenylethers), toxaphene (mixture of chlorinated boranes), chlorinated paraffin’s, and polychlorinated naphthalene’s. While analysis is difficult in many cases not well developed, only limited data are available. It is generally thought that these compounds are of less health concern compared to dioxins and PCBs. However, data on exposure and toxicity are scarce and efforts are underway to decrease environmental levels.

Organochlorine Pesticides (OCs), which include compounds such as DDT, HCB, lindane, and aldrin, are addressed under “Contaminants of Milk and dairy products: contaminants resulting from agricultural and dairy pesticides”

5. ANTIMICROBIALS

Antimicrobials are administered to treat bacterial infections or employed prophylactically to prevent spread of disease. All antimicrobial drugs administered to dairy cows enter the milk to a certain degree, and each drug is given a certain withdrawal period, during which time the concentration in the tissues declines and the drug is estimated from the animal. The most frequently and commonly used antimicrobials are antibiotics, employed to combat mastitis- causing pathogens. Other infectious diseases such as laminitis, respiratory diseases, and meritis, are also treated with antimicrobial agents, but are of minor comparative importance [13].

National surveys in developed countries show that contamination of bovine milk (residue - positive samples using a bacterial inhibition screening test) at tanker level is generally at 0.1 - 0.5% Such a range may not necessarily equated with the failure rate since it could also encompass the socalled “false positives” (e.g. still within legislative limits but positive in the rapid test), thus depending on the sensitivity of the test towards certain individual antibiotics.

The occurrence of residues of antimicrobials in milk has both economical and technological impact on the dairy industry. Antimicrobial residues can lead to partial or complete inhibition of acid production by starter cultures, inadequate ripening and ageing of cheese and cause defects of flavour and texture of these products [14].

A general concern linked to the widespread usage of antimicrobials at the farm level is the potential development of antibiotic-resistance pathogens, particularly if treatment is not diagnostically targeted. This may complicate human treatment and possibly cause selection of antibiotic-resistant strains in the gut. Further concern was raised that sensitive individuals may exhibit allergic reactions to residues of antibiotics and/or their metabolites, mainly beta-lactam antibiotics. However, the allergic risk is very low. Only the individuals sensitised through previous therapeutic exposure can react with mild and transient symptoms around the tolerance levels (Codex MRL for penicillin G is4 ug/kg). National surveys on residues in milk in developed countries only very seldom reveal positive samples exceeding these levels. Al permitted veterinary drugs should have MRLs by the end of 1998 in EU. As a consequence, there are no antimicrobials in use which have no MRLs or safe/tolerance figures established [14].

6. PARASITICIDES

Parasiticides are agents which have a destructive or at least banishing effect on the parasite.

Parasites are organisms (protozoic to metazoic), which are living in or on another organism (the host) in parasitism, thus sharing the nutrients taken up by the host (including internal or external blood sucking) without giving an own contribution to the mutual benefit.

From the large abundance of endoparasites (a parasite in the host animal) capable of invading farm animals including the lactating cow, practically only two tribes from the helmints are of certain importance for the adult animal. The one are the plathelminths with the class of the trematodes (worms with a suction apparatus) and the nemathelmints (threat-shaped worms) with the most important family of the trichostrongylids [15].

According to chemical structure the parasiticides are divided into three main groups: (1) endoparasiticides: the dose is administered either orally or parenterally and is in the range of 1,500 - 6,000 mg/cow, (2) nematicides: are given in a higher dose, up to 10,000 mg/cow and (3) ectoparasiticides: the dosage on the dairy cow is different and includes oral administration, spraying on the body surface, pour-on application, especially on the back of the animal, or using dust bags.

7. HORMONES

The employment of hormones in animal husbandry serves a number of purposes, which include increased food production, medical treatment, or improved reproductive performance. Some Hormones that impact on food production are classified as growth promoters, anabolics or performance enhancers, with the prime goal of enhancing economic competitiveness. However, this use is only acceptable if no potential risks are known to the health of consumers and the animals involved. The use of hormones as growth promoters is approved in some countries, e.g. in the US and Canada where the natural steroid hormones oestradiol, testosterone, progesterone and the (semi)synthetic hormones melengestrol acetate, trenbolone acetate and zeranol are approved for use only in meat-production animals. Safety concerns have been raised by other countries, e.g. EU member states, regarding hormone residues in meat. However, elevated levels of residues in milk are not expected if these hormones are administered appropriately [13].

Natural Hormones (Steroids, Peptide/Protein): The endogenous steroids 17-estradiol, progesterone, testosterone and derivatives are the main sex hormones present in all mammals. They can be used for anabolic purposes, and the two female sex hormones 17-estradiol and progesterone are also used to induce lactation, control/ improve fertility and oestrus cycle synchronization. The natural hormone content of milk will fluctuate depending on the physiological status of the animal, nutritional status and probably other factors. Hormone levels, mainly estrogens and progesterone, are also used for diagnostic purpose (oestrus, pregnancy). Reported levels in the literature for the whole milk are e.g. total estrogens 50- 70 ng/l, for progesterone 10-13 mg/l. Steroids are soluble in lipids, therefore daily products with lower fat content contain comparatively lower concentrations of steroids.

Semi-Synthetic and Synthetic Hormones: In mastitis treatment, synthetic corticosteroids, e.g. dexamethasone, prednisolone and derivatives are administered systematically or into the mammary gland to relieve inflammatory conditions. The (semi)synthetic hormones melengestrol acetate, trenbolone acetate and zeranol are approved in some countries as growth promoters in meat producing animals.

8. PESTICIDES

Modern pesticides are used for an abundance of applications in agriculture on food and forage plants and practically bear no risk of significant residues in milk products, when the rules of Good Agricultural Practice are strictly followed. Up to now, 191 pesticides are under discussion in the Codex Alimentarious Commission, 85 of them having a maximum residue Limit for milk and milk products between 0.0008 and 0.1 mg/kg product base. Estimates of the different daily intake figures yield of the daily consumption of 1.5 liters of milk, just a few percent contribution of milk to the total Average Daily Intake (ADI), for the adult [16].

9. HEAVY METALS

Metals are present in the environment either naturally or as a consequence of industrial and/or agricultural activities. They find their way to milk through several routes. Elements such as chromium and nickel from the stainless steel dairy equipment or tin from soldered cans may enter milk through direct contact. Heavy metals such as cadmium, lead, mercury and arsenic are not expected to have any direct contact with milk and milk products, except in accidental cases. For these elements, the main pathway to milk is not the ingestion of contaminated feeds by milkproducing animals as transfer of heavy metals from feed to milk is very low. The main route is direct contact and/or absorption from air. Data on buffalo and goat milk indicate that levels of heavy metals are likely to be similar to those encountered in bovine milk. With respect to milk products, contamination reflects the levels found in fresh milk, taking into account concentration factors. Metals may be associated with particular milk fractions. E.g. lead and cadmium bind strongly to casein. The use of specific milk fractions may thus concentrate or remove metals. The Tolerable Weekly Intake (TWI) for lead through food in US is 3 mg for adults and 25 mg/kg body weight for children [3]. Milk and milk products is not a major contributor to the daily intake of heavy metals.

10. NITRATE AND NITRITE

Nitrate and nitrite are anions, which contain nitrogen in its five- or trivalent configuration bound to oxygen. The cation of nitrate and nitrite is usually sodium or potassium. Concentrations of the nitrite ion in milk and milk products are usually very low, even after the use  of potassium nitrate in cheesemaking. The nitrate level in milk is naturally relatively constant in the range of <1-12 mg nitrate/kg raw milk, while nitrite is practically absent in milk and fermented milk products.

11. FOOD CONTAMINANTS FROM PACKAGING MATERIALS

Contact of packaging material with food may result in the transfer of trace quantities of particular chemicals, such as monomers and plasticizers. Well-known chemicals used in the production of polymers are vinyl chloride and styrene. Vinyl chloride is the monomer of polyvinyl chloride, and styrene is used in the manufacturing of a number of plastics. Important plasticizers in polyvinyl chloride are phthalic acid esters di(2-ethylhexyl) phthalate (DEHP) and di-n-butyl phthalate (DBP). Vinyl chloride has been identified as a liver carcinogen in animal models as well as in humans. Styrene-induced toxic effects include renal and hepatic damage, pulmonary edema, and cardiac arrhythmia [3]. These contaminants are not of concern if appropriate materials are  used.

12. SANITISERS / DISINFECTANTS

Cleaning and disinfection are critical aspects of Good Manufacture Practice in the food production and dairy sector to ensure removal of bacteria and residual milk from surfaces of equipment. Residues of detergents and disinfectants/sanitizers can be introduced into milk on the farm and at the dairy plant level, particularly if cleaning, disinfection, draining and rinsing procedures of milking equipment and containers are improperly conducted. Sanitiser contaminants occur in milk and dairy products at very low concentrations and are present as indirect and incidental food contaminants [13].

REFERENCES

1. Tsaknis, J. Quality assurance, edited by the Technological Educational Institutions of Athens, Athens (2002).

2. Heeschen, W.H., Burt, R. & Biuthgen, A. Introduction and background information. In: International Dairy Federation - Special Issue 9701, Monograph on residues and contaminants in milk and milk products - Brussels (1997).

3. Jansen, M.M.T. Contaminants. In: J. de Vries (Editor), Food safety and
toxicity - CRC Press, London (1996).

4. van Egmond, H.P., Svensson, U.K. & Fremy, J.M. Mycotoxins.In: International Dairy Federation - Special Issue 9701, Monograph on residues and contaminants in milk and milk products - Brussels (1997).

5. van Egmond, H.P. & Dekker, W.H. World-wide regulations for mycotoxins in 1995 - a compendium, FAO food an nutrition paper, Food and Agriculture Organization, Rome, Italy (1996).

6. Forsythe, S.J. and Hayes, P.R. Food hygiene, microbiology and HACCP, A Chapman & Hall (Editors), Aspen Publishers, Inc., Gaitherburg, Maryland (1998).

7. Blanco, J.L. et al. Presence of aflatoxin M1, in commercial ultra-high-temperature treated milk. Applied and Environmental Microbiology, 54:1622-1623 (1988).

8. Frobish, R.A., et al. Aflatoxin residues in milk and dairy cows after ingestion of naturally contaminated grain. J. of Food Prot. 49:781-785 (1986).

9. Biuthgen, A., Burt, R. & Heeschen, W.H. Persistent polyhalogenated environmental chemicals. In: International Dairy Federation - Special Issue 9701, Monograph on residues  and contaminants in milk and milk products - Brussels (1997).

10. Fisher, W.J., Trischer, A.M., Standler, R.J. and Schiter, B. Contaminants of milk and dairy products. In: H. Roginski, J.W. Fuquay & P.F. Fox (Editors), Encyclopaedia of Dairy Sciences - Academic Press, Oxford (2003).

11. Startin, J.R. Polychlorinated Dibenzo-p-dioxins, Polychlorinated Dibenzofurans, and the food chain. In: C.H. Creaser and R. Purchase (Editors), Food Contaminants: Sources and Surveillance - the Royal Society of Chemistry, London (1991).

12. IARC. Polychlorinated dibenzo-para-dioxins and polychlorinated dibenzofurans. In: Monographs on the Evaluation of the Carcinogenic Risk of Chemicals to Humans, Vol. 69, IARC, Lyon (1997).

13. Fisher, W.J., Trischer, A.M., Schiter, B. and Standler, R.J. Contaminants of milk and dairy products (A) Contaminants resulting from agricultural and dairy practices. In: H. Roginski, J.W. Fuquay & P.F. Fox (Editors), Encyclopaedia of Dairy Sciences - Academic Press, Oxford (2003).

14. Honkanen-Buzalski, T. and Reybroeck, W. Antimicrobials. In: International Dairy Federation - Special Issue 9701, Monograph on residues and contaminants in milk and milk products - Brussels (1997).

15. Bluthgen, A. and Heeschen, W.H. Parasiticides. In: International Dairy Federation - Special Issue 9701, Monograph on residues and contaminants in milk and milk products - Brussels (1997).

16. Bluthgen, A. and Tuinsta, L.G.M.Th. Pesticides. In: International Dairy Federation - Special Issue 9701, Monograph on residues and contaminants in milk and milk products - Brussels (1997).  

IDF/FAO international symposium on dairy safety and hygiene Cape Town, March 2–5, 2004, South Africa