Abstracts
Chemical speciation allows knowing the the different chemical forms of metals in a sample. Sequential extraction procedures are used to evaluate the fractionation of the metals in solid matrices. Few studies are described in the literature related to chemical speciation in food. Residues from fruits and vegetables generated in industry processes results in significant nutritional losses. The nutritional value of a food containing a given mineral depends not only on its content, but also on its bioavailability. The present paper emphasizes the importance of chemical speciation in food, considering the amount of loss of fruits and vegetables, since they are great sources of minerals.
chemical speciation; micronutrients; food.
A especiação química permite conhecer as diferentes formas químicas de minerais em uma matriz. A extração sequencial identifica as diferentes frações químicas que um metal está na amostra. Há pouco relato na literatura da aplicação da extração sequencial em alimentos. Muitos resíduos de frutas e hortaliças são gerados em processos industriais caracterizando perda nutricional significativa. O valor nutricional dos minerais em um alimento está relacionado à quantidade deste e à sua biodisponibilidade. O presente estudo enfatiza a aplicação da extração sequencial em alimentos para melhor aproveitamento de resíduos de frutas e hortaliças, que são fontes de minerais.
especiação química; micronutrientes; alimentos.
INTRODUCTION:
URE (1991URE, A.M. Trace element speciation in soils, soil extracts and
solutions. Microchimica Acta, Austria, v.104, p.49-57, 1991. Available from:
<http://link.springer.com/article/10.1007%2FBF01245495#page-1>. Accessed: Nov.
19, 2013. doi: 10.1007/BF01245495 pp.49-57.
http://link.springer.com/article/10.1007...
) defined chemical speciation as "the
process of identification and quantification of the different forms or phases in which
one element is present in a material" or as "the description of the amounts, types of
species, forms or phases present in a material "(LIMA et
al., 2001 MARIN, B. et al. Reproducibility testing of a sequential extraction
scheme for the determination of trace metal speciation in a marine reference sediment
by inductively coupled plasma-mass spectrometry. Analytica Chimica Acta, Netherlands,
v.342, n.2, p.91-112, Apr., 1997. Available from:
<http://www.ingentaconnect.com/content/els/00032670/1997/00000342/00000002/art00580>.
Accessed: Nov. 19, 2013. doi: 10.1016/S0003-2670(96)00580-6.
http://www.ingentaconnect.com/content/el...
). Recent studies define the chemical speciation as the individual
chemical forms of an element, which together, constitute the total concentration of the
element in the sample. The aim of sequential extraction method is to identify and
quantify the various forms of an element in a sample (BARRA et al., 2000BARRA, C.M. et al. Especiação de arsênio - Uma revisão. Química Nova,
São Paulo, v.23, p.58-70, Apr., 2000. Available from:
<http://www.scielo.br/pdf/qn/v23n1/2145.pdf>. Accessed: Nov. 19, 2013. Doi:
10.1590/S0100-40422000000100012.
http://www.scielo.br/pdf/qn/v23n1/2145.p...
).
Over the past 20 years, studies of chemical speciation have been presenting great
relevance, leading to the development of various methods of analysis used in the areas
of health, food quality control and environment. There are many studies related to the
deposition of metals in soils and waters that under reactive chemical forms, change the
ecosystem and cause adverse health effects, which have attracted the attention of the
scientific community (GLEYZES; TELLIER & ASTRUC,
2002GONÇALVES, E.C.B.A.; TAKASE, I. Determinação dos teores de cobre em
diferentes extratos de hortaliças do tipo A e B. Ciênc. Tecnol. Aliment. vol.24 no.2
Campinas Apr./June 2004. Available from:
<http://www.scielo.br/scielo.php?pid=S0101-20612004000200020&script=sci_arttext>.
Accessed: Nov. 19, 2013. doi: 10.1590/S0101-2061200400020002.
http://www.scielo.br/scielo.php?pid=S010...
; JAIN, 2004 JAIN, C.K. Metal fractionation study on bed sediments of River Yamuna,
India. Water Research, United States, v.38, n.3, p.569-578, Oct., 2004. Available
from: <http://www.sciencedirect.com/science/article/pii/S0043135403006213>.
Accessed: Nov. 19, 2013. doi: 10.1007/BF00398602.
http://www.sciencedirect.com/science/art...
; YAO & GAO, 2007YAO, Z.; GAO, P. Heavy metal research in lacustrine sediment: a review.
Chinese Journal of Oceanology and Limnology, China, v.25, n.4, p.444-454, Oct., 2007.
Available from:
<http://link.springer.com/article/10.1007%2Fs00343-007-0444-7#page-1>.
Accessed: Nov. 19, 2013. doi: 10.1007/s00343-007-0444-7.
http://link.springer.com/article/10.1007...
; GARCÍA-BARRERA
et al., 2012GLEYZES, C. et al. Fractionation studies of trace elements in
contaminated soils and sediments: a review of sequential extraction procedures.
Trends in Analytical Chemistry, Netherlands, v.21, n.6, p.451-467, June, 2002.
Available from:
<http://www.sciencedirect.com/science/article/pii/S0165993602006039>. Accessed:
Nov. 19, 2013. doi :10.1016/S0165-9936(02)00603-9.
http://www.sciencedirect.com/science/art...
; ).
Single and sequential extraction procedures are widely used for the investigation of
soil, water, sediment, and very important, but hardly mentioned, foods, providing
information on the mobility and availability of metals and other elements. Values of the
total concentrations of the metals are insufficient to be evaluated, so, simple
extractions can be used to estimate the fraction of the element that is potentially
mobile (GLEYZES; TELLIER & ASTRUC, 2002GONÇALVES, E.C.B.A.; TAKASE, I. Determinação dos teores de cobre em
diferentes extratos de hortaliças do tipo A e B. Ciênc. Tecnol. Aliment. vol.24 no.2
Campinas Apr./June 2004. Available from:
<http://www.scielo.br/scielo.php?pid=S0101-20612004000200020&script=sci_arttext>.
Accessed: Nov. 19, 2013. doi: 10.1590/S0101-2061200400020002.
http://www.scielo.br/scielo.php?pid=S010...
;
BACON & DAVIDSON, 2008BACON, J.R.; DAVIDSON, C.M. Is there a future for sequential chemical
extraction? Analyst, United Kingdom, v.133, p.25-46, Sept., 2008. Available from:
<http://pubs.rsc.org/en/content/articlelanding/2008/an/b711896a#!divAbstract>.
Accessed: Nov. 19, 2013. doi: 10.4236/fns.2013.48A024.
http://pubs.rsc.org/en/content/articlela...
; ). A single
extraction reagent (usually a binder, acid or dilute salt) is used to treat the sample
and the measurement is made on the number of elements of interest that are released from
the matrix (RAO et al., 2008RAO, C.R.M. et al. A review of the different methods applied in
environmental geochemistry for single and sequential extraction of trace elements in
soils and related materials. Water Air & Soil Pollution, Germany, v.189,
p.291-333, Jan., 2008. Available from:
<http://link.springer.com/article/10.1007%2Fs11270-007-9564-0#page-1>.
Accessed: Nov. 19, 2013. doi: 10.1007/s11270-007-9564-0.
http://link.springer.com/article/10.1007...
).
In sequential extraction, a detailed insight into the properties and behavior of the
elements can be accessed. Reagents with different chemical properties are used so that
the chemical forms of an element can be leached out by various mechanisms, like
complexation or acidification (RAO et al., 2008RAO, C.R.M. et al. A review of the different methods applied in
environmental geochemistry for single and sequential extraction of trace elements in
soils and related materials. Water Air & Soil Pollution, Germany, v.189,
p.291-333, Jan., 2008. Available from:
<http://link.springer.com/article/10.1007%2Fs11270-007-9564-0#page-1>.
Accessed: Nov. 19, 2013. doi: 10.1007/s11270-007-9564-0.
http://link.springer.com/article/10.1007...
;
ABOLLINO, et al., 2011ABOLLINO, O. et al. The role of chemometrics in single and sequential
extraction assays: a review: Part I. Extraction procedures, uni-and bivariate
techniques and multivariate variable reduction techniques for pattern recognition.
Analytica Chimica Acta, Netherlands, v.688, n.2, p.104-121, Jan., 2011. Available
from: <http://www.ncbi.nlm.nih.gov/pubmed/21334476>. Accessed: Nov. 19, 2013.
doi: 10.1016/j.aca.2010.12.020.
http://www.ncbi.nlm.nih.gov/pubmed/21334...
;). This results in a
process that it is more time consuming than the simple extraction because provides a
separation of the total content of an element in fractions with different
bioavailability: while the first fraction is unstable and thus readily available for
absorption, the last fraction is characterized by its reduced mobility (BACON & DAVIDSON, 2008BACON, J.R.; DAVIDSON, C.M. Is there a future for sequential chemical
extraction? Analyst, United Kingdom, v.133, p.25-46, Sept., 2008. Available from:
<http://pubs.rsc.org/en/content/articlelanding/2008/an/b711896a#!divAbstract>.
Accessed: Nov. 19, 2013. doi: 10.4236/fns.2013.48A024.
http://pubs.rsc.org/en/content/articlela...
; RAO et al., 2008RAO, C.R.M. et al. A review of the different methods applied in
environmental geochemistry for single and sequential extraction of trace elements in
soils and related materials. Water Air & Soil Pollution, Germany, v.189,
p.291-333, Jan., 2008. Available from:
<http://link.springer.com/article/10.1007%2Fs11270-007-9564-0#page-1>.
Accessed: Nov. 19, 2013. doi: 10.1007/s11270-007-9564-0.
http://link.springer.com/article/10.1007...
; ABOLLINO, et al.,
2011ABOLLINO, O. et al. The role of chemometrics in single and sequential
extraction assays: a review: Part I. Extraction procedures, uni-and bivariate
techniques and multivariate variable reduction techniques for pattern recognition.
Analytica Chimica Acta, Netherlands, v.688, n.2, p.104-121, Jan., 2011. Available
from: <http://www.ncbi.nlm.nih.gov/pubmed/21334476>. Accessed: Nov. 19, 2013.
doi: 10.1016/j.aca.2010.12.020.
http://www.ncbi.nlm.nih.gov/pubmed/21334...
;).
There are several factors that influence the availability of minerals: speciation,
molecular bonding and the matrix. The mobility and availability of metals depend on the
reactivity and behavior of chemical bonds with the components of the matrix elements,
making it important to determine the accuracy and precision of each matrix studied
separately (BACON & DAVIDSON, 2008BACON, J.R.; DAVIDSON, C.M. Is there a future for sequential chemical
extraction? Analyst, United Kingdom, v.133, p.25-46, Sept., 2008. Available from:
<http://pubs.rsc.org/en/content/articlelanding/2008/an/b711896a#!divAbstract>.
Accessed: Nov. 19, 2013. doi: 10.4236/fns.2013.48A024.
http://pubs.rsc.org/en/content/articlela...
; KHOUZAM et al., 2012KRISHNAMURTI, G.S.R. et al. A new soil test method for the determination
of plant-available cadmium in soils. Communications in Soil Science & Plant
Analysis, United Kigdom, v.16, n.17-18, p.2857-2867, Nov., 1995. Available from:
<http://www.tandfonline.com/doi/abs/10.1080/00103629509369493#.Uowwo8SThqU>.
Accessed: Nov. 19, 2013. doi: 10.1080/00103629509369493.
http://www.tandfonline.com/doi/abs/10.10...
; RAO et al., 2008RAO, C.R.M. et al. A review of the different methods applied in
environmental geochemistry for single and sequential extraction of trace elements in
soils and related materials. Water Air & Soil Pollution, Germany, v.189,
p.291-333, Jan., 2008. Available from:
<http://link.springer.com/article/10.1007%2Fs11270-007-9564-0#page-1>.
Accessed: Nov. 19, 2013. doi: 10.1007/s11270-007-9564-0.
http://link.springer.com/article/10.1007...
).
Chemical speciation by sequential extraction
Sequential extraction consists in the use of a number of extractors with different
chemical properties that are progressively applied in a sample. Most of sequential
extraction schemes include different groups of extractors: extractors with ion exchange
properties; extractors with dissolution of carbonates properties; extractors with acid
reducing properties; extractors that weaken the metal with organic compounds and
sulfides extractors, which promote the dissolution of silicates and minerals (ANDRADE et al., 2005ANDRADE, E.C.B. et al. Determinação dos teores de zinco em diferentes
extratos de hortaliças dos tipos A e B. Ciência e Tecnologia de Alimentos, Campinas,
v.25, n.2, p.591-596, Apr./Jun., 2005. Available from:
<http://www.scielo.br/pdf/cta/v25n2/25022>. Accessed: Nov. 19, 2013. doi:
10.1590/S0101-20612005000200014.
http://www.scielo.br/pdf/cta/v25n2/25022...
; RAO et al., 2008RAO, C.R.M. et al. A review of the different methods applied in
environmental geochemistry for single and sequential extraction of trace elements in
soils and related materials. Water Air & Soil Pollution, Germany, v.189,
p.291-333, Jan., 2008. Available from:
<http://link.springer.com/article/10.1007%2Fs11270-007-9564-0#page-1>.
Accessed: Nov. 19, 2013. doi: 10.1007/s11270-007-9564-0.
http://link.springer.com/article/10.1007...
).
The growing interest in sequential extraction can be justified by the classic works of
TESSIER et al. (1979TESSIER, A. et al. Sequential extraction procedure for the speciation of
particulate trace metals. Analytical chemistry, United States, v.51, n.7, p.844-851,
Jun., 1979. Available from: <http://pubs.acs.org/doi/abs/10.1021/ac50043a017>.
Accessed: Nov. 19, 2013. doi: 10.1021/ac50043a017.
http://pubs.acs.org/doi/abs/10.1021/ac50...
). They used a five-stage
extraction: five extractants to fractionate cadmium, cobalt, copper, iron, lead, nickel,
manganese and zinc in river sediments containing low levels of potentially toxic
elements. These stages were: metals exchange (exchangeable fraction); extractors linked
to carbonates (bound to carbonate fraction); iron and manganese oxides (reducible /
bound to Fe and Mn oxides / oxidic fraction); the organic compounds and sulfides (bound
to organic matter / Sulfuric acid and organic) and, finally, residual extractors. The
extractors were chosen based on the ability to remove specific substances, in this case,
sediments.
There are still other methods in the literature about sequential extraction, which is a
specific criteria used for the matrix examined, wrapped profile, metals of interest,
etc. Among the procedures for sequential extraction, besides the above, one of the most
used is the KERSTEN & FORSTNER (1986KHOUZAM, R.B. et al. Trace element speciation in food: State of the art
of analytical techniques and methods. Pure Applied Chemistry, United States, v.84,
n.2, p.169-179, Jan., 2012. Available from:
<http://www.iupac.org/publications/pac/84/2/0169/>. Accessed: Nov. 19, 2013.
doi: 10.1351/PAC-CON-11-08-14.
http://www.iupac.org/publications/pac/84...
) method.
The difference between these two methods is the inclusion of a specific reagent for the
extraction of the iron oxidic fraction. Table 1
shows the different methods proposed for sequential extraction in solid matrices.
The nutritional value of a food containing a given mineral depends not only on its
content, but also on its bioavailability (RUZIK,
2012SUTHERLAND, R.A. et al. Operationally defined metal fractions in road
deposited sediment, Honolulu, Hawaii. Journal of Environmental Quality, Madison,
v.29, n.5, p.1431-1439, Oct., 2000. Available from:
<https://dl.sciencesocieties.org/publications/jeq/abstracts/29/5/JEQ0290051431?access=0&view=pdf>.
Accessed: Nov. 21, 2013. doi: 10.2134/jeq2000.00472425002900050009x.
https://dl.sciencesocieties.org/publicat...
). Bioavailability is a term used to indicate the proportion of elements
that are absorbed and utilized by the body. The processes of absorption of
micronutrients are complex and include several functions, such as osmosis, filtration,
diffusion, active and selective participation and selective of the intestinal villi
cells (SANTOS et al., 2004SANTOS, H.B. et al. Estudos bioquímicos e hematológicos em ratos sobre
biodisponibilidade de minerais numa dieta enriquecida com multimistura. Ciência e
Tecnologia de Alimentos, Campinas, v.24, n.4, p.613-618, Oct./Dec., 2004. Available
from:
<http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0101-20612004000400023&lng=en&nrm=iso>.
Accessed: Oct. 07, 2014. doi: 10.1590/S0101-20612004000400023.
http://www.scielo.br/scielo.php?script=s...
).
To understand the mechanisms involved between the chemical forms and the extractors, such as the mobility and bioavailability associated to these elements, it is necessary to know each phase corresponding to the sequential extraction.
Exchangeable fraction
This fraction includes weakly adsorbed metals retained on the solid surface by
electrostatic interactions relatively weak, metals that can be released through ion
exchange processes. Changes in the ionic composition influence the adsorption-desorption
reactions, or by reducing the pH, which may cause the remobilization of metals from this
fraction (KRISHNAMURTI et al., 1995LI, X. et al. Chemical forms of Pb, Zn and Cu in the sediment profiles
of the Pearl River Estuary. Marine Pollution Bulletin, Netherlands, v.42, n.3,
p.215-223, Mar., 2001. Available from:
<http://www.ncbi.nlm.nih.gov/pubmed/11381876>. Accessed: Nov. 19, 2013. doi:
10.1016/S0025-326X(00)00145-4.
http://www.ncbi.nlm.nih.gov/pubmed/11381...
; AHNSTROM & PARKER, 2001AHNSTROM, Z.A.S.; PARKER, D.R. Cadmium reactivity in metal-contaminated
soils using a coupled stable isotope dilution-sequential extraction procedure.
Environmental Science & Technology, United States, v.35, n.1, p.121-126, Nov.,
2001. Available from: <http://www.ncbi.nlm.nih.gov/pubmed/11351995>. Accessed:
Nov. 19, 2013. doi: 10.1021/es001350o.
http://www.ncbi.nlm.nih.gov/pubmed/11351...
).
The reagents used for this purpose are salts of strong acids, bases or salts of weak
acids and bases, pH 7.0. Thus, some salts such as MgCl2, CaCl2,
KNO3 or NH4OAc, are normally chosen to extract metals, for
displacement of the adsorption sites (URE &
DAVIDSON, 2001URE, A.M.; DAVIDSON, C.M. Chemical Speciation in the Environment. 2.ed.
Glasgow: University of Strathdyde, Blackwell Science, 480 p. 2001. Available from:
<http://onlinelibrary.wiley.com/book/10.1002/9780470988312>. Acessed, Nov., 19,
2013. doi: 10.1002/9780470988312.
http://onlinelibrary.wiley.com/book/10.1...
). These reagents do not attack organic matter, silicates and
metal sulfides, but can dissolve carbonates, except when the extraction time is short
(GLEYZES et al., 2002GONÇALVES, E.C.B.A.; TAKASE, I. Determinação dos teores de cobre em
diferentes extratos de hortaliças do tipo A e B. Ciênc. Tecnol. Aliment. vol.24 no.2
Campinas Apr./June 2004. Available from:
<http://www.scielo.br/scielo.php?pid=S0101-20612004000200020&script=sci_arttext>.
Accessed: Nov. 19, 2013. doi: 10.1590/S0101-2061200400020002.
http://www.scielo.br/scielo.php?pid=S010...
).
The most popular is the extractor MgCl2 1mol l-1, which combines the high
capacity of Mg+2 with the weak complexation of Cl-. However,
Ca2+ has been the most effective at removing Mg2+ ions related
to exchangeable fraction (GLEYZES et al.,
2002GONÇALVES, E.C.B.A.; TAKASE, I. Determinação dos teores de cobre em
diferentes extratos de hortaliças do tipo A e B. Ciênc. Tecnol. Aliment. vol.24 no.2
Campinas Apr./June 2004. Available from:
<http://www.scielo.br/scielo.php?pid=S0101-20612004000200020&script=sci_arttext>.
Accessed: Nov. 19, 2013. doi: 10.1590/S0101-2061200400020002.
http://www.scielo.br/scielo.php?pid=S010...
).
Bound to carbonates / Soluble fraction in acid
This fraction is bound by covalent forces, and therefore there is no easy displacement
of metal as in the previous fraction. The metals are precipitated or co-precipitated
with carbonates, which can be an important adsorbent for many metals when the organic
matter and the oxides of iron and manganese are less abundant (RAO et al., 2008RAO, C.R.M. et al. A review of the different methods applied in
environmental geochemistry for single and sequential extraction of trace elements in
soils and related materials. Water Air & Soil Pollution, Germany, v.189,
p.291-333, Jan., 2008. Available from:
<http://link.springer.com/article/10.1007%2Fs11270-007-9564-0#page-1>.
Accessed: Nov. 19, 2013. doi: 10.1007/s11270-007-9564-0.
http://link.springer.com/article/10.1007...
).
This phase is susceptible to changes in pH by the use of a moderate acid. The use of
solutions buffered at pH 5.0 have long been used at this stage, for example, the buffer
acetic acid / sodium acetate or acetic acid / ammonium acetate at pH 5.0 allowing the
release occurrence of organic and inorganic metal substrates (AHNSTROM & PARKER, 2001AHNSTROM, Z.A.S.; PARKER, D.R. Cadmium reactivity in metal-contaminated
soils using a coupled stable isotope dilution-sequential extraction procedure.
Environmental Science & Technology, United States, v.35, n.1, p.121-126, Nov.,
2001. Available from: <http://www.ncbi.nlm.nih.gov/pubmed/11351995>. Accessed:
Nov. 19, 2013. doi: 10.1021/es001350o.
http://www.ncbi.nlm.nih.gov/pubmed/11351...
; LI et
al., 2001LI, X. et al. Chemical forms of Pb, Zn and Cu in the sediment profiles
of the Pearl River Estuary. Marine Pollution Bulletin, Netherlands, v.42, n.3,
p.215-223, Mar., 2001. Available from:
<http://www.ncbi.nlm.nih.gov/pubmed/11381876>. Accessed: Nov. 19, 2013. doi:
10.1016/S0025-326X(00)00145-4.
http://www.ncbi.nlm.nih.gov/pubmed/11381...
).
The reduction of pH 7.0 (the exchangeable phase) to pH 5.0 at this stage allows release
of the remaining adsorbed ions, more specifically, trace elements which were not
extracted in the previous fraction. High proportions of manganese are found in the
extracts of acetic acid / sodium acetate (TESSIER et
al., 1979TESSIER, A. et al. Sequential extraction procedure for the speciation of
particulate trace metals. Analytical chemistry, United States, v.51, n.7, p.844-851,
Jun., 1979. Available from: <http://pubs.acs.org/doi/abs/10.1021/ac50043a017>.
Accessed: Nov. 19, 2013. doi: 10.1021/ac50043a017.
http://pubs.acs.org/doi/abs/10.1021/ac50...
).
Reducible / bound to Fe and Mn oxides / oxidic fraction
The reducible fraction or also called oxidic dissolve oxides and hydroxides of iron and
manganese. The extraction of these secondary oxides present superficial layers of
minerals as well as discrete particles or may occur by the following mechanisms or
combination thereof: adsorption; ion exchange; complex formation surface;
co-precipitation and penetration crystal lattice (HALL
et al., 1996HALL, A.C. et al. Intestinal metallothionein and the mutual antagonism
between copper and zinc in the rat. Journal of inorganic biochemistry, United States,
v.11, p.57-66, 1979. Available from:
<http://www.ncbi.nlm.nih.gov/pubmed/479878>. Accessed: Nov. 19, 2013. doi:
10.1016/S0162-0134(00)80054-9.
http://www.ncbi.nlm.nih.gov/pubmed/47987...
).
The reducible fraction may be divided into three: fraction easily reducible (manganese
oxides), moderately reducible fraction (amorphous oxides of iron); and poorly reducible
fraction (crystalline iron oxides). This classification is discussed in some sequential
extraction schemes (KRISHNAMURTI et al., 1995LI, X. et al. Chemical forms of Pb, Zn and Cu in the sediment profiles
of the Pearl River Estuary. Marine Pollution Bulletin, Netherlands, v.42, n.3,
p.215-223, Mar., 2001. Available from:
<http://www.ncbi.nlm.nih.gov/pubmed/11381876>. Accessed: Nov. 19, 2013. doi:
10.1016/S0025-326X(00)00145-4.
http://www.ncbi.nlm.nih.gov/pubmed/11381...
).
The control of the reduction potential and the pH of this fraction can dissolve reagents
in some or all of the phases of oxides and hydroxides of iron and manganese (GLEYZES et al., 2002GONÇALVES, E.C.B.A.; TAKASE, I. Determinação dos teores de cobre em
diferentes extratos de hortaliças do tipo A e B. Ciênc. Tecnol. Aliment. vol.24 no.2
Campinas Apr./June 2004. Available from:
<http://www.scielo.br/scielo.php?pid=S0101-20612004000200020&script=sci_arttext>.
Accessed: Nov. 19, 2013. doi: 10.1590/S0101-2061200400020002.
http://www.scielo.br/scielo.php?pid=S010...
).
Bound to organic matter / Sulfuric acid and organic fraction
In this fraction occurs organic matter degradation in oxidizing conditions leading to a
release of soluble metals associated with these components. The metals bound to sulfides
can also be extracted during this phase (MARIN et al.,
1997 MARIN, B. et al. Reproducibility testing of a sequential extraction
scheme for the determination of trace metal speciation in a marine reference sediment
by inductively coupled plasma-mass spectrometry. Analytica Chimica Acta, Netherlands,
v.342, n.2, p.91-112, Apr., 1997. Available from:
<http://www.ingentaconnect.com/content/els/00032670/1997/00000342/00000002/art00580>.
Accessed: Nov. 19, 2013. doi: 10.1016/S0003-2670(96)00580-6.
http://www.ingentaconnect.com/content/el...
). The most common oxidants are hydrogen peroxide in moderate acid, NaOCl
in pH = 9,5, Na4P2O7 in pH = 9,5 and
K4P2O7. In general, hydrogen peroxide heated is more
used to dissolve the organic matter, because it causes an effective attack therein and
produces minimal change in silicates. Perhaps the most important disadvantage of this
reagent is supplied by the resorption of metals in the residual fraction, which requires
an additional extraction with ammonium acetate at pH = 2 (SUTHERLAND et al., 2000SUTHERLAND, R.A. et al. Operationally defined metal fractions in road
deposited sediment, Honolulu, Hawaii. Journal of Environmental Quality, Madison,
v.29, n.5, p.1431-1439, Oct., 2000. Available from:
<https://dl.sciencesocieties.org/publications/jeq/abstracts/29/5/JEQ0290051431?access=0&view=pdf>.
Accessed: Nov. 21, 2013. doi: 10.2134/jeq2000.00472425002900050009x.
https://dl.sciencesocieties.org/publicat...
; FERREIRA
& SANTANA, 2012GARCÍA-BARRERA, T. et al. Biological responses related to agonistic,
antagonistic and synergistic interactions of chemical species. Analytical and
Bioanalytical Chemistry, Germany, v.403, p.2237-2253, Feb., 2012. Available from:
<http://www.ncbi.nlm.nih.gov/pubmed/22367285>. Accessed: Nov. 19, 2013. doi:
10.1007/s00216-012-5776-2.
http://www.ncbi.nlm.nih.gov/pubmed/22367...
).
The combination H2O2 / NH4OAc, where the addition of
NH4OAc prevents resorption of the extracted metals, has been adopted in
many schemes for extracting metals associated with organic matter and sulfides (KRISHNAMURTI et al., 1995LI, X. et al. Chemical forms of Pb, Zn and Cu in the sediment profiles
of the Pearl River Estuary. Marine Pollution Bulletin, Netherlands, v.42, n.3,
p.215-223, Mar., 2001. Available from:
<http://www.ncbi.nlm.nih.gov/pubmed/11381876>. Accessed: Nov. 19, 2013. doi:
10.1016/S0025-326X(00)00145-4.
http://www.ncbi.nlm.nih.gov/pubmed/11381...
).
Residual fraction
The extraction of this metal fraction occurs by digestion with strong acids such as HF,
HClO4, HCl and HNO3. The amount of metals associated with this
fraction is also reported by some authors as the difference between the total
concentration and the sum of all fractions of metals extracted during the previous steps
(CHLOPECKA et al., 1996DAS, K.B. et al. Metal speciation in solid matrices. Talanta,
Netherlands, v.42, n.8, p.1007-1030, Feb., 1995. Available from:
<http://www.ncbi.nlm.nih.gov/pubmed/18966324>. Accessed: Nov. 19, 2013. doi:
10.1016/0039-9140(95)01557-R.
http://www.ncbi.nlm.nih.gov/pubmed/18966...
).
Several reagents used in sequential extraction procedures have advantages and
disadvantages and there is not a reactant or ideal protocol for general use in the case
of food. Therefore, the selection process must be related to a defined objective, taking
into account the nature of the sample (RUZIK,
2012SUTHERLAND, R.A. et al. Operationally defined metal fractions in road
deposited sediment, Honolulu, Hawaii. Journal of Environmental Quality, Madison,
v.29, n.5, p.1431-1439, Oct., 2000. Available from:
<https://dl.sciencesocieties.org/publications/jeq/abstracts/29/5/JEQ0290051431?access=0&view=pdf>.
Accessed: Nov. 21, 2013. doi: 10.2134/jeq2000.00472425002900050009x.
https://dl.sciencesocieties.org/publicat...
). Note that, after the choice of the sequential extraction procedure,
only soluble species can be extracted, which reproduce the conditions of bioavailability
of each element (BOUYSSIERE et al., 2003BOUYSSIERE, B. et al. (Ed.). Sample preparation tyechniques for
elemental speciation studies. In: Handbook of elemental speciation: techniques and
methodology. John Wilwy & Sons, Ltd. Chichester, England, p. 95-118, 2003.
Available from:
<http://onlinelibrary.wiley.com/doi/10.1002/0470868384.fmatter/pdf> .Accessed:
Nov. 19, 2013. doi: 10.1002/0470868384.ch4.
http://onlinelibrary.wiley.com/doi/10.10...
).
Sequential extraction in food matrices
The verification of the concentrations of trace toxic and those considered essential
nutrients in food elements is performed in many countries. In the last two decades has
been shown that food safety and nutritional quality of foods also depend on the chemical
form in which an element is present therein, since the bioavailability, activity, and
toxicity of beneficial minerals are a function of metal concentration, its oxidation
state and the chemical form in which it is found in the sample (EBDON et al., 2001FERREIRA, P.R.G.; SANTANA, G.P. Tratamento químico seletivo: uma
especiação operacional. Scientia Amazonia, Manaus, v.1, n.3, p.40-48, Nov., 2012.
Available from:
<http://www.scientia.ufam.edu.br/attachments/article/16/v.1%20n.3,%2040-48,%202012.pdf>.
Accessed: Nov. 21, 2013. doi: 2238.1910.
http://www.scientia.ufam.edu.br/attachme...
).
The number of studies on chemical speciation in food has increased significantly and
reached a rate of ninety articles per year, but only a few elements are widely studied
(such as arsenic, selenium and mercury), while others do not attract the attention of
researchers. Elements such as Co, I, Mn, Fe, Zn, Cu and Mo have received little
attention so far because of their low concentrations in foods, complex instability and
chemical behavior of the species, making it a challenge for the chemistry (RUZIK, 2012SUTHERLAND, R.A. et al. Operationally defined metal fractions in road
deposited sediment, Honolulu, Hawaii. Journal of Environmental Quality, Madison,
v.29, n.5, p.1431-1439, Oct., 2000. Available from:
<https://dl.sciencesocieties.org/publications/jeq/abstracts/29/5/JEQ0290051431?access=0&view=pdf>.
Accessed: Nov. 21, 2013. doi: 10.2134/jeq2000.00472425002900050009x.
https://dl.sciencesocieties.org/publicat...
).
Analysis by sequential extraction in food should provide some care, since each has a
different foodstuff and chemical matrix requires a special control interference. The
complex of minerals that are formed with nutrients in foods, especially those formed
with labile metal matrix lose their stability with the change of state of oxidation,
hydrolysis, methylation and the action of microorganisms (RAYMAN, 2004RAYMAN, M.P. The use of high-selenium yeast to raise selenium status:
how does it measure up? British Journal of Nutrition, United States, v.92, n.4,
p.557-574, Jun., 2004. Available from:
<http://www.ncbi.nlm.nih.gov/pubmed/15522125>. Accessed: Nov. 21, 2013. Doi:
10.1079/BJN20041251
http://www.ncbi.nlm.nih.gov/pubmed/15522...
; SCHRAUZER,
2006SCHRAUZER, G.N. Selenium yeast: composition, quality, analysis, and
safety. Pure and Applied Chemistry, United States, v.78, p.105-109, Oct., 2006.
Available from:
<http://pac.iupac.org/publications/pac/pdf/2006/pdf/7801x0105.pdf>. Accessed:
Nov. 21, 2013. doi: 10.1351/pac200678010105.
http://pac.iupac.org/publications/pac/pd...
).
The sequential extraction should be performed to separate the matrix elements without
loss and without altering the original forms, however, extraction from solid samples
that do not generate losses or contamination and no changes to the species are extremely
difficult. It must have adequate recovery of these samples (attack with strong acid) and
preservation of the species (QUEVAUVILLER et al.,
1996 QUEVAUVILLER, P. et al. Quality control of results of speciation
analysis. In: CAROLI, S. Element speciation in bioinorganic chemistry. New York:
Wiley, 1996. p.195-222. Available from:
<http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471576417.html>. Accessed:
Nov., 19, 2013. doi: 978-0-471-57641-9.
http://www.wiley.com/WileyCDA/WileyTitle...
).
The choice of technique for extraction should have some considerations: the chemical
properties of the substances to be analyzed, their chemical forms, the matrix and the
application technique (RUZIK, 2012SUTHERLAND, R.A. et al. Operationally defined metal fractions in road
deposited sediment, Honolulu, Hawaii. Journal of Environmental Quality, Madison,
v.29, n.5, p.1431-1439, Oct., 2000. Available from:
<https://dl.sciencesocieties.org/publications/jeq/abstracts/29/5/JEQ0290051431?access=0&view=pdf>.
Accessed: Nov. 21, 2013. doi: 10.2134/jeq2000.00472425002900050009x.
https://dl.sciencesocieties.org/publicat...
). Depending on
the matrix of the sample to be analyzed, and the elements of interest, extracting,
dissolving, leaching or solid-liquid extraction may be performed for the isolation of
species, since the procedure preserves it (BOUYSSIERE et
al., 2003BOUYSSIERE, B. et al. (Ed.). Sample preparation tyechniques for
elemental speciation studies. In: Handbook of elemental speciation: techniques and
methodology. John Wilwy & Sons, Ltd. Chichester, England, p. 95-118, 2003.
Available from:
<http://onlinelibrary.wiley.com/doi/10.1002/0470868384.fmatter/pdf> .Accessed:
Nov. 19, 2013. doi: 10.1002/0470868384.ch4.
http://onlinelibrary.wiley.com/doi/10.10...
).
ANDRADE et al. (2005ANDRADE, E.C.B. et al. Determinação dos teores de zinco em diferentes
extratos de hortaliças dos tipos A e B. Ciência e Tecnologia de Alimentos, Campinas,
v.25, n.2, p.591-596, Apr./Jun., 2005. Available from:
<http://www.scielo.br/pdf/cta/v25n2/25022>. Accessed: Nov. 19, 2013. doi:
10.1590/S0101-20612005000200014.
http://www.scielo.br/pdf/cta/v25n2/25022...
) analyzed samples of
vegetables types A and B for total zinc content by atomic absorption spectrometry in
flame (FAAS), and the method of sequential extraction was applied with the following
extractors: 1M calcium chloride, 0.1M acetic acid / 5% ammonium acetate (pH 5.0), 0.5M
acetic acid, 0.5 hydrochloric acid. At least six different chemical species for zinc
were found in these samples.
SILVA et al. (2010SILVA, C.S. et al. Avaliação do uso da casca do fruto e das folhas de
Caesalpinia ferrea Martius como suplemento nutricional de Fe, Mn e Zn. Ciência e
Tecnologia de Alimentos, Campinas, v.30, n.3, p.751-754, Jul./Sept., 2010. Available
from: <http://www.scielo.br/pdf/cta/v30n3/v30n3a28.pdf>. Accessed: Nov. 21,
2013. doi: 10.1590/S0101-20612010000300028.
http://www.scielo.br/pdf/cta/v30n3/v30n3...
) evaluated the total content
and bioavailability of iron, manganese and zinc in extracts of the peels and leaves of
Caesalpinia ferrea Martius also using FAAS. The extractors used in the
sequential extraction were: 0.05M NaOH, 0.05M Tris-HCl (pH = 8), 1% (w / v) sodium
dodecyl sulfate (SDS), 0.05M HCl and hot water (60°C). The peel of the fruit and the
leaves showed high levels of iron and manganese. Iron showed better bioavailability in
the peel and leaves of the fruit. Manganese and zinc were more potentially bioavailable
in the leaves.
GONÇALVES & TAKASE (2005)GONÇALVES, E.C.B.A.; TAKASE, I. Determinação dos teores de cobre em
diferentes extratos de hortaliças do tipo A e B. Ciênc. Tecnol. Aliment. vol.24 no.2
Campinas Apr./June 2004. Available from:
<http://www.scielo.br/scielo.php?pid=S0101-20612004000200020&script=sci_arttext>.
Accessed: Nov. 19, 2013. doi: 10.1590/S0101-2061200400020002.
http://www.scielo.br/scielo.php?pid=S010...
evaluated the
sequential extraction of copper and zinc in vegetables type C, using FAAS. The
extractors used were: 1M MgCl2, pH = 7.0; 0.5M acetic acid / 5% ammonium
acetate (pH 5.0); 1M acetic acid; 0.1M sodium hidroxyde and 0.5M HCl. It was observed
that copper was presented in at least three different chemical species and the zinc was
extracted with efficacy in the majority of samples studied with four extractors.
There are few studies related to chemical speciation in food and it is of interested to
expand these analyzes to this area, since the speciation of metals is already recognized
in biological and environmental analysis (DAS et al.,
1995EBDON, L. et al. Trace element speciation for environment, food and
health. Quevauvilier: Royal Society of Chemistry, 360 p. 2001. Available from:
<http://pubs.rsc.org/en/Content/eBook/978-0-85404-459-7#!divbookcontent>.
Accessed: Nov. 19, 2013. doi: 10.1039/9781847552204.
http://pubs.rsc.org/en/Content/eBook/978...
). In this way, information about chemical speciation in food is extremely
important for studies of human nutrition and health (KHOUZAM et al., 2012KRISHNAMURTI, G.S.R. et al. A new soil test method for the determination
of plant-available cadmium in soils. Communications in Soil Science & Plant
Analysis, United Kigdom, v.16, n.17-18, p.2857-2867, Nov., 1995. Available from:
<http://www.tandfonline.com/doi/abs/10.1080/00103629509369493#.Uowwo8SThqU>.
Accessed: Nov. 19, 2013. doi: 10.1080/00103629509369493.
http://www.tandfonline.com/doi/abs/10.10...
).
Considering that chemical speciation allows to separate compounds by their different chemical characteristics, this technique is a useful tool for assessing the bioavailability of minerals in foods. The chemical interactions that minerals have in a food are likely to identification by this technique. The exchangeable fraction by extracting the weakly adsorbed in the matrix metal, released by ionic exchange, most likely identifies the metals as salts. The bound to carbonate fraction represents the fraction of the chemical compounds in the matrix interaction that involves covalent forces as well as changes in pH, promotes the extraction of metals strongly bound to the matrix, probably intermolecular interactions. The oxidic fraction causes degradation of organic matter, possibly featuring the metals with intramolecular interaction with the macronutrients. These chemical influence the specificity, single or facilitated absorption occurring during the diffusion process, affecting the bioavailability of the metal.
CONCLUSION:
The chemical speciation in some micronutrients and trace elements in food is important to human health, but it is a challenge to researchers and also to the chemistry. Low concentrations of these elements in food, complex chemical behavior and instability of the species are the reasons why there are not many studies in this area. As the food matrix and sequential extraction procedures in foods are very varied, it is possible to observe the diversity of chemical interactions between species.
It is known that there is no general protocol for sequential extraction in foods and therefore it should be always take into consideration the nature of the sample. Mechanisms of sequential extraction in foods should be stimulated to provide a better understanding of metal speciation into the matrix as well as mobility and bioavailability.
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Publication Dates
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Publication in this collection
June 2015
History
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Received
04 Dec 2013 -
Accepted
14 Oct 2014