Transcript Unwanted Chemical Substances

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.
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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).
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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.
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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.
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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.
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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)
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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.
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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).
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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.
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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.
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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.
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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.
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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.
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