selenium se in foods
DESCRIPTION
Course material - Food chemistry CHEM 1083TRANSCRIPT
Selenium (Se) in Food
By
Elis Cen
Dr. Darryl M. Small
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The Element Se
• One of the rarest elements• Once known only for its toxicity• Plays a key role in all animal life• Essential component of the human diet (minute
amount required)
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Se Chemistry
• Atomic weight: 78.96• Atomic number: 34• Group 16/VIA (with oxygen, sulfur and Tellurium)• A metalloid (neither fully metallic nor
nonmetallic, shares chemical and physical properties of both)
• Boils at 684C• Very stable and highly insoluble
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Common inorganic Se compounds
• Ferro- (FeSe) and nickel-selenide (NiSe)
• Selenium dioxide (SeO2)
• Cadmium sulfoselenide
O
S
O
-O
SeO
SO
-O
Cd++
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Common inorganic Se compounds
• Selenium diethyldithiocarbamate
N S-
S N
-S SSeH2
++
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Common inorganic Se compounds
• Sodium selenite (Na2O3Se)
-O Se
O
O-
Na+
Na+
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Common inorganic Se compounds
• Selenate (SeO42−)
• Selenide sulfide (SeS and SeS2)
O
SeO
O-
O-
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Common organic Se compounds
• Selenomethionine
NH2
Se
O
HO
NH2
S
O
HO
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Common organic Se compounds
• Selenocysteine
NH2
SH
O
HO
NH2
SeH
O
HO
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Isotopes of Se
• 74Se, 76Se, 77Se, 78Se, 80Se, and 82Se• Relative abundance:
80Se (49.8%) > 78Se (23.5%) > 76Se (9.4%) > 82Se (8.7%) > 77Se (7.6%) > 74Se (0.87%)
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Other applications of Se
• Glass industry: to decolorise and to color glass• Component of lubrication oils and greases used
at high temperatures• Metallurgy: added to improve the machinability
of wrought iron and steel castings• Used in photocopier and laser printers (when
light strikes Se, it becomes charged electrically)
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Applications of Se (cont)
• Agricultural and horticultural: sodium selenite and selenate as– Additives and dietary supplements in animal feeds– Fertilizers– Top dressings for soil deficiency
• Pharmaceutical industry: sodium selenite and selenate, selenomethionine and selenium-enriched yeast are used as over-the-counter dietary supplements
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Se analysis
Challenges:• Low levels in biological materials• Very volatile and easily lost during sample
preparation
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Se analysis
Sample preparation:• Removal of organic matter • Brings the element into the mineralized state in
solution• Involves some forms of oxidation
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Se analysis
Various approaches:• Dry procedure: sample is incinerated at high
temperature in a furnace or other type of heating apparatus
• Wet digestion: sample is heated with various acids or mixtures of them
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Se analysis (comparison)
FactorsDry procedure/ dry ashing
Wet digestion/ wet ashing
Risk of loss Higher Lower
Sample size Wide range Small
Amount of digestion fluid
MinimumRelatively large volume
PracticalityMore convenient
Relatively more complex
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Wet ashing
• Using open vessels– on a temperature controlled heating block– plate heater– under reflux
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Wet ashing
• Using closed digestion systems – an oxygen bomb or– microwave-heated sealed polytetrafluoroethylene
(PTFE) tubes
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End-determination methods for Se analysis
1. Spectrofluorimetry• Can measure down to nanogram quantities in
many different biological matrices
2. Atomic Absorption Spectrophotometry (AAS)• Can measure down to microgram per gram range
with ease in biological samples• Commonly used: Hydride Generation AAS
(HGAAS)
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End-Determination Methods for Se Analysis3. Inductively Coupled Plasma Atomic Emission
Spectrophotometry (ICP-AES)• Can measure down to microgram per gram
4. Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
• Capable of determining Se down to 10 pg/g (where p indicates pico which is 10-12)
Both allow simultaneous multi-element analysis
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ICP-MS
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Speciation analysis
• To detect the different Se compounds• Among the separation techniques found to be
effective for selenium speciation are:• High Performance Liquid Chromatography (HPLC),
including– Ion Exchange (IE)– Size Exclusion Chromatography (SEC)
• Capillary Electrophoresis (CE)
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Speciation analysis
• Detectors used include:• Mass Spectrometry (MS)• ICP-MS• Neutron Activation Analysis (NAA) • Atomic Fluorescence Spectrometry (AFS)
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Se in health
• A constituent of the antioxidant enzyme glutathione peroxidase (GPX):
• has diverse roles in the animal body in addition to its antioxidant function.
• involved in thyroid hormone homeostasis, immunity and fertility, has anticancer properties, acts as a growth factor, plays important roles in metabolism.
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Se in health
• The selenoproteins found in mammals include:• several different GPXs• 3 thyroid hormone deiodinases• 3 thioredoxin reductases (TRs)
• Each has selenocysteine (Sec) at the active site
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Se toxicity (selenosis)
Selenosis in farm animals • Characterized by general dullness, lack of vitality,
emaciation, stiffness, lameness, loss of hair, hoofs to drop off, and reduction in reproductive performance.
• Acute selenosis can be caused by an intake of 400 mg/kg; a lower dose between 5 to 40 mg/kg over a period of several weeks or months can result in chronic poisoning.
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Se Toxicity (cont)
Selenosis in humans • Characterised by dermatitis, loose hair, damaged
nails, accompanied by elevated serum and urine Se levels. More seriously may lead to abnormalities of the nervous system.
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Se deficiency
In farm animals
– White muscle disease (WMD) > affects heart muscle leading to heart failure and death.
– Exudative Diathesis > growth rate is slowed, lose condition, develop leg weakness and eventually die.
– Liver necrosis– Pancreatic degeneration > poor growth and feathering– Ill thrift > loss of weight and death, poor wool quality
and quantity in sheep– Impaired reproduction– Impaired immune response
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Se deficiency (cont)
In humans Se deficiency is one of the contributing factors to various conditions:• Keshan disease > a sudden onset of precardial
oppression and pain, nausea and vomiting, in some cases ending in death.
• Kashin-Beck disease > an osteoarthropathy, characterized by chronic disabling degeneration and necrosis of the joints and the epiphysial-plate cartilages of the arms and legs.
• Goiter or “Derbyshire neck”
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Se supplementation
• Supplementation strategies for animals:• Application of the element directly to pastures• Free choice supplementation, i.e. incorporated into
salt blocks• Direct administration to animals
• Combined Se and vitamin E and other micronutrients >> improve the symptoms due to Se deficiency in many cases
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Se levels (µg/kg) in foods from different countries
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Enhancing Se content of foods
• Adding selenium to fertilizers, used effectively to increase the Se content of a variety of food crops, including rice, wheat, broccoli, onion, tea.
• Examples of food enrichment:• Se-enriched milk (Korea) • the “Mega egg” which has added vitamin E as well as
Se (Ireland), • Se-enriched bread and flour (UK), • Se-fortified breakfast cereals, table salt, margarine
and sports drink (various countries)
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Enhancing Se content of foods (cont)
Chemical forms of Se used to fortify foods• Inorganic compounds
sodium selenite, sodium hydrogenselenite, sodium selenate
• Organic forms
selenomethionine, selenocysteine and Se-enriched yeast
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Dietary Reference Values for Se
Australia1 UK2 USA3 Adults
EAR RDI UL LRNI RNI RDA UL
Men (19 to >70 yr)
60 µg/day
70 µg/day
400 µg/day
40 µg/day
75 µg/day
55 µg/day
400 µg/day
Women (19 to >70 yr)
50 µg/day
60 µg/day
400 µg/day
40 µg/day
60 µg/day
55 µg/day
400 µg/day
EAR – Estimated Average Requirement; RDI – Recommended Dietary IntakeLRNI – Lower Reference Nutrient Intake; RNI – Reference Nutrient IntakeRDA – Recommended Dietary Allowance; UL – Upper Limit
Sources:1Department of Health and Aging (2005) Nutrient Reference Values for Australia and New Zealand. http://www.nhmrc.gov.au2Department of Health (1991) Dietary Reference Value for Food Energy and Nutrients for the UK, COMA, HMSO, London3Food and Nutrition Board National Academy of Sciences (2002) US Dietary Reference Intakes: Elements. http://www.nationalacademies.org
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Additional references
• Reilly, C. (2006). Selenium in Food and Health, 2nd ed. USA
• CEM Corporation (2006) MarsXtraction. http://www.cem.com/analytical/extraction.asp
• Reilly, C. (1998). Selenium: A new entrant into the functional food arena. Trends in Food Science and Technology, 9, 114-118.