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Chapter 1.4 : Unwanted Chemical Substances •We discuss the important differences between elements and chemical compounds with regard to their fate in organisms and the environment. •We describe the different possible chemical identities/structures. •We discuss the different possible origins of unwanted chemicals. 1 Unwanted Chemical Substances • Unwanted chemicals in food may come from a variety of sources and possess many different chemical structures. • Elements (distinguished by their atomic number – the number of protons in the nucleus) occur naturally e.g. iron, lead, hydrogen, nitrogen and oxygen, and may be present as solids (e.g. sodium), liquid metals (e.g. mercury), one of the different forms of carbon (e.g. graphite, diamond), or the gases (e.g. chlorine, oxygen and oxygen). • Elements may form chemical compounds (pure chemical substances consisting of two or more different elements – e.g. sodium chloride (NaCL or salt), carbon dioxide (CO2) and methylmercury (CH3Hg+ or MeHg+). • Note that not all examples in this chapter necessarily present a risk because there is little or no exposure. • EXAMPLE: Methylmercury is very hydrophobic and can bio-accumulate after long term exposure in fatty fish and other marine organisms to cause toxicity (MeHg associated with protein fraction). 2 Unwanted Chemical Substances Example : Acrylamide in food • Acrylamide is a good example of an unwanted chemical in food. Acrylamide is a chemical used primarily as a building block in making polyacrylamide and acrylamide copolymers. Polyacrylamide and acrylamide copolymers are used in many industrial processes, such as the production of paper, dyes, and plastics, and in the treatment of drinking water and wastewater, including sewage. They are also found in consumer products, such as caulking, food packaging, and some adhesives. Trace amounts of acrylamide generally remain in these products. • Acrylamide in food results from the reaction between carbohydrates and the amino acid asparagine when certain starchy foods such as potatoes are cooked at high temperatures. The chemical reaction is known as the Maillard reaction. 3 Unwanted Chemical Substances • Chemical compounds can undergo transformation/degradation. • EXAMPLE: degradation by solar light; degradation/ transformation by ionizing radiation. • Chemical reactions result in changes to molecular structure. • EXAMPLE: hydrolysis of glycosides in cassava to toxic products, and metabolism after contact with or absorption by a microorganism, plant or animal. • EXAMPLE: Ethanol is metabolised to acetaldehyde which is toxic to the liver in excess. • NOTE: The elements always stay with us as they may re-enter the food chain after excretion, • EXAMPLE: Mercury enters the food chain via a number of mechanisms; inorganic mercury is converted to its most toxic form, methymercury, by a number of metabolic processes, e.g. via bacteria and up through the food chain where it accumulates in marine organisms. 4 Unwanted Chemical Substances Why Unwanted? • Chemical compounds and elements may be unwanted in food (and feed) because: • They may affect taste and smell. • EXAMPLE: fatty products become rancid. Antioxidants added to avoid. • They may reduce intestinal absorption on important minerals such as calcium, magnesium, iron and zinc. • EXAMPLE: Phytic acid, a principle storage for of phosphorous in many plant tissues especially bran and seeds. Minerals are bound by phytic acid (or phytate in salt form) in cereals, leading to possible adverse health effects. • They may degrade vitamins before these can be absorbed. • EXAMPLE: Thiaminases are enzymes found in some plants and marine organisms (raw flesh) which, when ingested split (degrade) thiamine (vitamin B1) and render it inactive. 5 Unwanted Chemical Substances Why Unwanted? • These latter two examples are called anti-nutritional compounds or factors. • Finally, chemical compounds and elements may be unwanted in food (and feed) because: • They may be toxic (see previous section on Toxicity) 6 Unwanted Chemical Substances Elements and Inorganic Compounds Metals • Elements that have specific properties (e.g. good electrical conductivity, high density; OR can be very reactive e.g. alkali). • Metals may have positive (iron) and negative health effects (cadmium, lead, mercury, tin) • May occur in elementary state or as oxides and salts (e.g. CL-, (SO4)2-). • May be bound (chelated) to more complex counterions such as phytate (an anit-nutrient; see previous slides), or special metal-binding proteins (e.g. metallothionein which binds cadmium in the liver), or may form parts of organometallic compounds. • Some metals essential ‘dietary minerals’ e.g. calcium, copper, iron, magnesium, manganese, molybdenum, potassium, sodium and zinc. • Cobalt (as part of vitamin B12 – cyanocobalamin) and chromium may also have some role in health benefits. Excess chromium is deleterious to health and this is a problem in some countries such as China. • Rare earth metals such a silver and gold are mostly inert. 7 Unwanted Chemical Substances Elements and Inorganic Compounds Metaloids • Elements that have specific properties of both metals and non-metals (e.g. selenium and arsenic are present in the earth’s crust at different levels). • Selenium is essential for human health (although boron may also have a positive role). 8 Unwanted Chemical Substances Elements and Inorganic Compounds • Both non-essential (lead, cadmium, mercury) and essential (selenium) metals can be toxic depending on the dose. • For essential metals there is a window (upper and lower levels) between lowest intakes to prevent deficiency and highest acceptable intakes that cause toxicity. 9 Unwanted Chemical Substances Elements and Inorganic Compounds Non metallic Compounds • There a few non-metallic compounds that can affect human health including iodine, boron and fluorine. • Fluorine is a non-metallic element of concern to human health. • Forms a single bond with itself to form F2 – extremely reactive poisonous, yellow-brown gas. • Sodium fluoride (NaF) found in drinking water to help prevent dental caries, and found in black tea. 10 Unwanted Chemical Substances Organic Compounds • Carbon atoms linked to each other in chains as supplemented with hydrogen (e.g. CH2-CH2; ethane), but can incorporate oxygen and nitrogen (e.g. nitrogen in alkaloids and non-protein amino acids from plants. • Substitutions include with other elements such as sulphur, phosphorous, chlorine, fluorine and iodine, and may present important food safety problems. • A large number of organic compounds – e.g. 100,106 marketed between 1971-1981 (ELINCS). • Many more chemical compounds produced by degradations (incineration) and side products of chemical processes. 11 Unwanted Chemical Substances Organic Compounds (Cont’d) • Many chemical compounds are synthesised by industry – e.g. pesticides, medicines, cosmetics and toiletries, packaging materials, and food additives – and most of these will find their way into the food supply! • Pollution via a variety of pathways is an important source of many chemical compounds in the food supply e.g. dioxins – very toxic, carcinogenic compounds e.g. resulting from unwanted side products of the herbicide 2,4,5- trichlorophenoxyacetic acid, and burning of waste materials; widespread continuous into the environment • Persistent organic pollutants (POPs) found everywhere. 12 Unwanted Chemical Substances Organometallic Compounds • Chemical compounds such as methylmercury, selenium, tetraethyl lead (TEL) and arsenic all mentioned previously). • May have different characteristics concerning absorption and distribution leading to different toxic profiles compared to the elemental metals and metalloids or their inorganic salts. • EXAMPLE: TEL in petrol is totally man-made and can be absorbed via the skin, but is a pollutant that can end up in food and feed. • EXAMPLE: Methylmercury is synthesised by microorganisms, and was previously used as a fungicide that proved highly toxic (CNS). • EXAMPLE: Mercury vapour and readily absorbed into lungs, past the blood-brain barrier to cause CNS damage; ingested fluid metal mercury absorption is restricted; inorganic mercury compounds affect the kidneys. 13