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4140 Chem. Soc. Rev., 2012, 41, 4140–4149 This journal is c The Royal Society of Chemistry 2012
Cite this: Chem. Soc. Rev., 2012, 41, 4140–4149
Flavour chemistry of methylglyoxal and glyoxal
Yu Wangaand Chi-Tang Ho*
b
Received 30th January 2012
DOI: 10.1039/c2cs35025d
Methylglyoxal (MGO) and glyoxal (GO), known as reactive carbonyl species, can be generated
endogenously and exogenously (human body and food system). They are attracting increased
attention because of their relationship with diabetes and flavour generation. In this review, their
characteristics relating to flavour chemistry are discussed. MGO and GO can be detected in food
systems by GC and HPLC after derivatization. MGO and GO formed in the Maillard reaction
play important roles as precursors of aroma and colour compounds, especially in Strecker
degradation, a major flavour generation reaction. When combined with amino acids they undergo
Schiff base formation, decarboxylation and a-aminoketone condensation leading to heterocyclic
aroma compounds such as pyrazines, pyrroles and pyridines. They attack amine groups in amino
acids, peptides and proteins to form advanced glycation end products (AGEs) and cause carbonyl
stress followed by oxidative stress and tissue damage. Therefore, many studies about scavengers
of MGO and GO are seen. The influence of these scavengers on flavour generation is also
discussed.
1. Introduction
Reactive carbonyl species (RCS) such as glyoxal (GO) and
methylglyoxal (MGO), that can be generated endogenously
and exogenously, are attracting increased attention because of
their relationship with diabetes and flavour generation. In vivo
MGO is primarily formed during glycolysis in cells and
generated from the metabolism of ketone body degradation
of threonine, and by the fragmentation of triosephosphates.1
In vitro, particularly in the Maillard reaction, RCS can be
generated from Schiff’s base and Amadori compounds.
A carbonyl group of RCS can attack an amine group in
amino acids, peptides or proteins to form advanced glycation
end products (AGEs) and cause carbonyl stress, followed by
oxidative stress and tissue damage.2 Increasing evidence in
both clinical and pre-clinical studies shows that MGO is
associated with hyperglycemia in both Type I and Type II
diabetes and diabetes-related complications such as nephropathy,
Alzheimer’s disease and cataracts.3 Therefore, RCS are inducing
factors of diabetes or its complications. However, RCS that
are formed in Maillard reaction play important roles as
precursors of aroma and colour compounds, especially in Strecker
degradation, a major flavour generation reaction. MGO and
GO, in combination with amino acids, undergo Schiff’s base
formation, decarboxylation and a-aminoketone condensation,
leading to heterocyclic aroma compounds such as pyrazines,
pyrroles and pyridines. In other words, MGO and GO, that
contribute to flavour generation in food, can induce diabetes
and diabetes complications. How to evaluate their paradoxical
roles becomes very interesting and challenging. Therefore, in
this tutorial review, flavour chemistry of MGO and GO is
discussed to provide an overview of controlling their formation
in food.
2. Chemical properties of methylglyoxal and
glyoxal
MGO, a yellow hygroscopic liquid, is known as 2-oxopropanal,
pyruvaldehyde, or 2-ketopropionaldehyde. It is present in three
rapidly equilibrium forms in aqueous solution; monohydrate is the
highest (71%) followed by dihydrate (28%), and the anhy-
drated form is only about 1%.4 Depending on temperature
and water content, MGO can change from a less reactive
noncarbonyl form to more reactive carbonyl and dicarbonyl
forms.5 GO, which is also a yellow coloured liquid, is the
smallest dialdehyde.
3. Analytical methods for quantification of MGO
and GO
A derivatization process is needed, prior to chromatographic
analysis, to quantify MGO or GO. Several derivatization agents
listed in Table 1 and their adduct products shown in Fig. 1,
including diamino derivatives of benzene and naphthalene, react
withMGO to form quinoxalines. Quinoxalines have been analysed
a Chair of Food Chemistry and Molecular Sensory Science,Lise-Meitner-Straße 34, Technische Universitat Munchen,D-85354 Freising, Germany
bDepartment of Food Science, Rutgers University, New Brunswick,NJ 08901, USA. E-mail: [email protected]
Chem Soc Rev Dynamic Article Links
www.rsc.org/csr TUTORIAL REVIEW
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This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 4140–4149 4141
and quantified with high-performance liquid chromatography
(HPLC) and can be monitored by anUV detector at 300–360 nm,
by a fluorescent detector at 300–360 nm with excitation
wavelengths and at 380–450 nm, with emission wavelengths,
or by a mass detector (MS). Other agents, such as 6-hydroxy-
2,4,5-triaminopyrimidine that forms a pteridine derivative,
and a cysteamine forming 2-acetylthiazolidine, have also been
analysed by HPLC. A reverse phase HPLC column is often
applied. In the GC method, oxime derivatives of MGO with
O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA) can
be detected by a flame ionization detector, a MS/SIM detector,
an electron-capture detector, or a flame photometric detector.
1,2-Diaminobenzene derivatives of MGO can be analysed
using a flame ionization detector, a MS/SIM, or a specific
nitrogen/phosphorus detector. For biological samples, additional
procedures might be needed to separate MGO from proteins.
More than 90% of MGO was demonstrated to be bound to
proteins, and perchloric acid was needed as a deproteinization
agent. Another benefit of the use of perchloric acid is to
keep the samples in low pH, which prevents degradation of
dihydroxyacetone phosphate and glyceraldehyde 3-phosphate
to MGO.
4. Generation of MGO and GO in foods
MGO can be exogenously generated from sugar autoxidation,
Maillard reaction, as well as degradation of lipid and microbial
fermentation. In sugar autoxidation, MGO is formed from
fragmentation of sugar by retro-aldol condensation in which
oxygen plays an important role. This process mainly occurs
in food containing a lot of carbohydrates, especially mono-
saccharides, from which the amount of MGO is higher than
that from disaccharides.6 And the amount of MGO produced
from glucose is higher than that from fructose. Honey with a
high content of glucose and fructose formsMGO through sugar
degradation during the heating processes in manufacturing and
storage. The concentrations of GO and MGO in honey are in
the range of 0.3–1.3 mg kg�1 and 0.8–33 mg kg�1, respectively.7
The most frequently used sweetener in foods or beverages is
high fructose corn syrup which contains 90, 55 or 42%
fructose. Levels of GO and MGO in commercial beverages
which contain high amount of HFCS were 15.8–104.6 and
23.5–139.5 (mg per 100 ml), respectively.8 Coffee, as one of the
popular drinks, was also studied for its MGO and GO levels.
Four types of black coffee (espresso, bold, mild, and a
decaffeinated mild roast) were tested. Espresso was shown to
contain the highest level of MGO at 230.9 mM, followed by
bold coffee.9 The roasting process influences the content of
MGO and GO in coffee beans because of the occurrence of
Maillard reaction. When green beans were roasted at 210 1C for
20 min, the glyoxal content increased in the first 6 min, peaked at
13.07 � 0.39 mg per 100 g, then slowly decreased to 1.93 �0.05 mg per 100 g at 20 min. Methylglyoxal showed similar
behavior with its peak concentration (21.19 � 0.42 mg per 100 g)
at 10 min and subsequently declined.10 Accumulation of MGO
in lipids is caused by lipid degradation during processing and
storage. The amount of MGO formed in fish oils (tuna, cod
liver and salmon oils) heated at 60 1C for 7 days ranged from
2.03–0.13 to 2.89–0.11 mg kg�1, whereas among vegetable oils
(soybean, olive and corn oil) under these conditions, only olive
oil yielded MGO (0.61–0.03 mg kg�1).11 During fermentation,
microorganisms releaseMGO into food products most commonly
into alcoholic drinks and dairy products. Levels of MGO in
brandy, vinegar and wine were 1.9, 35 and 10 ppm, respectively.12
5. Relationship between MGO and GO formation
and the Maillard reaction
Maillard reaction plays an important role in MGO and GO
formation in vitro and in vivo. In fact, scavenging RCS may
terminate Maillard reaction, and suppressing Maillard reaction
Table 1 Some examples of derivatization methods for methylglyoxal (MGO) analysis
Derivatization reagent Derivatives products Detector
HPLC 6-Hydroxy-2,4,5-triaminopyrimidine Pteridine UVCysteamine 2-Acetylthiazolidine UVMeso-stilbenediamine 2,3-Diphenyl-5-methyl-2,3-dihydropyrazine UV
GC 1,2-Diaminobenzene Quinoxaline MS/SIM, NPDO-(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine-hydrochloride (PFBHA)
Oxime MS/SIM, ECD, NPD, FPD
a Source: Ref. 1. Note: ECD, electron-capture detector; NPD, nitrogen phosphorus detector; FPD, flame photometric detector.
Fig. 1 Derivatization agents and products for methylglyoxal (MGO).
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4142 Chem. Soc. Rev., 2012, 41, 4140–4149 This journal is c The Royal Society of Chemistry 2012
may reduce the level of RCS. Maillard reaction is the major
route for the generation of flavour and colour. Therefore, it is
important to study the relationship between flavour generation
and RCS formation.
5.1. Chemistry of Maillard reaction
Maillard reaction, which can be generally defined as the chemical
interaction involving carbohydrates and amino compounds, is
responsible for the generation of roasted, toasted and caramel-like
aromas, as well as for the development of brown colour in foods.
Maillard reaction also has both nutritional and toxicological effects
on processed food.13–15 Many of the antinutritional aspects of the
Maillard reaction, such as effects on the availability of essential
amino acids, on enzyme activity, as well as on the absorption/
utilization of minerals, have been extensively studied.13,14
Maillard reaction occurs in three stages.13 The initial stage
(Fig. 2) is the condensation of the carbonyl group of a reducing
sugar with an amino compound to form the Schiff base which
then cyclizes to the N-substituted aldosylamine. These Schiff
bases can rearrange to form RCS such as 1-deoxyglycosone or
3-deoxyglycosone through amino-deoxyaldose or ketose by
Amadori or Heyns rearrangements. The Amadori rearrangement
product is not stable, so the intermediate stages (Fig. 2) include
enolization, deamination, dehydration, cyclization, retroaldoliza-
tion, isomerization and fragmentation to carbonyl compounds
(small molecular RCS), furan derivatives and other intermediates.
The final stage is the reaction of these RCS and furan derivatives
to form flavour and colour compounds. In the intermediate
stage, an amine group is released from the reaction, this means
amino acids play important roles in catalysing sugar activation
and fragmentation. On the other hand, sugar can be degraded into
carbonyl compounds at high temperature through enolization,
dehydration and fragmentation.
5.2. Strecker degradation
Generally, flavour compounds can be categorised into two groups:
cyclization/condensation products and fragmentation products.
Strecker degradation, which is considered a significant source of
flavour compounds, is associated with the intermediate stage of
Maillard reaction. If Maillard reaction can be seen as the
degradation of sugar catalysed by amino compounds, from
another point of view, Strecker degradation can be taken as the
degradation of amino acids initiated by RCS.
In Strecker degradation, dicarbonyl compounds such as
MGO and GO react with amino acids to produce carbon dioxide
and aldehydes with one less carbon atom and a-aminoketones
that are key precursors of heterocyclic flavour compounds such
as pyrazines, oxazoles and thiazoles (Fig. 3).16
5.3. Proposed mechanism of methylglyoxal formation
RCS, including glyoxal, methylglyoxal and 3-deoxyglucosone,
have been demonstrated to be formed in early glycation
in vitro from the degradation of glucose and Schiff base
adduct. The reactions were influenced by the concentration
of phosphate buffer and availability of trace metal ions.17
Glucosone can be formed frommonosaccharide autoxidation.18
Glyoxal can be generated in the degradation of glucose by
retro-aldol condensation, which is activated by deprotonation
of the 2- or 3-hydroxy groups. Hydrogen peroxide, which is
formed in autoxidation of glycoaldehyde to glyoxal, and
glucose to glucosone, can also stimulate glyoxal formation
by hydroxyl radical-mediated acetal proton abstraction from
glucopyranose and a-elimination reactions.18 Deprotonation
of carbon-2 of glucose and re-distribution of the electron density
between carbon-1 and carbon-2 or carbon-2 and carbon-3 in
glucose lead to dehydration, forming the 1,2-enol or 2,3-enol and
thereby 1-deoxyglucosone (1-DG) or 3-deoxyglucosone (3-DG),
respectively.19 Methylglyoxal may be formed by fragmentation
of 3-DG (Fig. 4). The formation of glyoxal, methylglyoxal and
3-DG from glucose is dependent on phosphate buffer and
availability of trace metal ions.20 This may be due to phosphate
dianion HPO42� and metal ions catalysing the deprotonation of
glucose and the autoxidation of glycoaldehyde and hydroxyl
radical formation implicated in glyoxal formation.18 Moreover,
trace metal ion phosphate complexes may be related to the
activation of glucose for 3-DG formation.20
The formation of fructosamine residues is the major pathway
of early glycation by glucose, but not the only one. Originally,
RCS were considered to be formed from fructosamine only.
However, some studies show that 3-DG, MGO and GO were
generated throughout the whole reaction and changes in theirFig. 2 Initiate and intermediate stages of the Maillard reaction.13
Fig. 3 Strecker degradation.
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concentrations did not follow that of fructosamine.21 In other
words, RCS may be formed from other sources, such as from
glucose degradation and from the Schiff base (Fig. 5). The
formation mechanism of RCS is similar to glucose degradation
except for the presence of aldimine that could be hydrolyzed to
MGO, GO and 3-DG. The presence of the aldimine group
accelerates the formation of RCS.20 The formation of 3-DG
and fructosamine is parallel reaction pathways in which the
deprotonation of carbon-2 of the Schiff’s base is a critical point.
Furthermore, in the Namiki pathway, glyoxal can be formed
directly from the Schiff’s base through erythritol, while methyl-
glyoxal is from 3-DG though retroaldolisation.21 Formation of
a superoxide radical, which is a key in GO generation, can be
detected in the early stage of glycation.22 MGO and 3-DG
generation, however, does not involve oxidation. The formation
of glyoxal, methylglyoxal and 3-DG in glucose–amine reaction is
also dependent on phosphate buffer concentration and availability
of trace metal ions.20
6. Importance of MGO and GO as intermediates
in flavour generation
Flavour generated from Maillard reaction can follow three
paths, (i) from carbohydrates only, (ii) from a combination of
carbohydrates and amino acids, and (iii) from amino acids
only, among all of which MGO and GO play important roles as
intermediates. Carbohydrates can transform into glycosones at
the beginning of Maillard reaction, and then these glycosones
either cyclize into flavour compounds or break into a-dicarbonylssuch as MGO and GO, then follow a recombination of these
intermediates. One of these is 2,5-dimethyl-4-hydroxy-3(2H)-
furanone (DMHF), which is generally considered the product
of cyclization of intact glycosones and/or the recombination of
MGO and GO. Aromas formed from carbohydrates and
amino acids can be divided into amino acid-specific and amino
acid-non-specific pathways. For the amino acid-non-specific
pathway, a-dicarbonyl (MGO or GO) reacts with most types
of amino acids forming a-aminoketone via Strecker degradation
which leads to the formation of alkylpyrazines, oxazoles and
oxazolines. In the amino acid-specific pathway, amino acids
such as cysteine and proline, a-aminoketone or a-dicarbonylare involved in the generation of thiazoles, thiazolines, pyrrolines,
and pyridines. Although some flavour compounds (e.g., methional,
phenylacetaldehyde) are generated from amino acids only,
a-dicarbonyl compounds are still involved in their formation
pathways, particularly through Strecker degradation. In addition,
peptides can also react with a-dicarbonyl compounds to generate
some peptide-specific aromas (e.g., pyrazinones).
6.1. Formation of 2,5-dimethyl-4-hydroxy-3(2H)-furanone
(DMHF) from carbohydrate or MGO
2,5-Dimethyl-4-hydroxy-3(2H)-furanone (DMHF, known as
furaneol), with an intense caramel-like aroma, was originally
discovered as a key flavour component of strawberry in 1965.23
DMHF has been found to be an important odour-active
compound in various natural and processed foods, such as
pineapple, raspberry, tomato and grape, as well as in roasted
coffee, bread crust, roasted almond and soy sauce.24 In fruits,
DMHF is usually formed by biosynthesis. Because of its
widespread occurrence, DMHF became a major reactant in
generating other flavour compounds. At low pH, DMHF has
been shown to react with cysteine or hydrogen sulfide in
generating meat-like aroma compounds.25 Some roast aroma
compounds such as alkylpyrazines can also be generated
through the decomposition of DMHF with phenylalanine.26
The formation pathways of DMHF have been studied in model
experiments of thermal degradation of 6-deoxysugars, hexoses and
pentoses in the presence or absence of amino acids.27–29 Generally,
DMHF can be formed through 2,3-enolization of 6-deoxysugars,
hexoses and pentoses leading to 1-deoxyosones as intermediates.30
Compared to hexoses and pentoses, 6-deoxysugars, such as
rhamnose, are more effective in forming DMHF through
2,3-dioxo-4,5-dihydroxyhexane that is not easily formed from
hexoses and cannot be generated from pentoses.27 Schieberle
in 1992,28 and later Hofmann and Schieberle in 200131 showed
that DMHF can be formed from hexose via acetylformoin
(2,4-dihydroxy-2,5-dimethyl-3(H)-furanone) reduction, which
may proceed either by disproportionation reaction or a
Strecker reaction with amino acids. Only one study has shown
the formation of DMHF from pentoses. Blank and Fay
indicated that elongation of pentoses by Strecker aldehyde
of glycine was an alternative pathway of DMHF formation in
which acetylformoin was also proposed as an intermediate.29
Fig. 4 Oxidative formation of methylglyoxal (MGO) from glucose.20
Fig. 5 Formation of methylglyoxal (MGO) and glyoxal (GO) in
Maillard reaction.21
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Hexose or pentose can be cleaved into MGO and 1-hydroxy-
2-propanone to generate DMHF. This reaction has been
proved to be a major pathway when glucose is reacted with
proline in an aqueous solution.32 MGO, depending on the pH,
differently affected DMHF generation in the presence or absence
of amino acids. The DMHF level increased as pH increased when
cysteine reacted withMGO, whereas the trend was reversed in the
presence of glycine.24 When MGO was heated alone at 120 1C,
the formation of DMHF was observed, and the MGO level was
significantly increased as the pH of the reaction increased. MGO
may transform into 1-hydroxy-2-propanone and pyruvic acid
through the Cannizzaro reaction (Fig. 6), and subsequently lead
to DMHF by reacting MGO with 1-hydroxy-2-propanone.32
DMHF formation from MGO was pH-dependent because the
Cannizzaro reaction is a base preferential reaction.
6.2. Formation of aroma compounds from carbohydrates and
amino acids
6.2.1. Formation of alkylpyrazines via Strecker degradation
(non-specific amino acids). Alkylpyrazines, which are nitrogen-
containing heterocyclic compounds, occur in a wide range of
raw and processed foods with potent and characteristic aroma.
The most plausible formation mechanism of pyrazines is the
condensation of two a-aminoketones. Dicarbonyls from sugar
degradation at the early stage of Maillard reaction react with
amino acids via Strecker degradation to form a-aminoketones
leading to pyrazine formation through oxidation of dihydro-
pyrazines. If the alkyl groups in a-aminoketones are different,
the isomers of pyrazines are observed33 (Fig. 7).
6.2.2. Formation of oxazoles and oxazolines via Strecker
degradation (non-specific amino acids). Oxazoles and oxazolines
are two closely related heterocyclic flavour compounds containing
nitrogen and oxygen atoms. Oxazoles and oxazolines occur in
various processed foods, such as roasted and ground coffee,
roasted cocoa, heated soy sauce, baked potatoes and roasted
peanuts.34,35
Formation pathways of oxazoles and oxazolines were first
proposed between a dicarbonyl compound and an amino acid
undergoing decarboxylation (Fig. 8).
6.2.3. Formation of 2-acetyltetrahydropyridine and 2-acetyl-
pyrroline via Strecker degradation (specific amino acids). Flavour
generation reactions occur most often at high temperatures
especially Strecker degradation in which carbonyl groups react
with amine groups through nucleophilic attack. However, some
flavour compounds such as popcorn like flavour 2-acetyltetra-
hydropyridine and roasted aroma 2-acetylpyrroline can be
formed from MGO in the presence of a specific amino acid
proline, but without forming a-aminoketones. For example,
2-acetyltetrahydropyridine, which is a popcorn like flavour in
rice and bread crust, can be formed from Strecker degradation
of proline andMGO through decarboxylation and dehydration36
(Fig. 9). The degradation product from the reaction between
proline and MGO can continually react with MGO to form
2-acetylpyrroline (Fig. 10). On the other hand, 2-acetyltetra-
hydropyridine can be biosynthesized through lysine and MGO.37
The gene(lat) encoding the L-lysine e-aminotransferase (LAT)
in Streptomyces clavuligerus was cloned and expressed in
Escherichia coli. Lysine was found to be transformed to
1-piperideine-6-carboxylic acid. And 2-acetyltetrahydropyridine
was characterized from the reaction mixture of 1-piperideine-6-
carboxylic acid and MGO (Fig. 11).
Fig. 6 Reaction pathway leading methylglyoxal to 2,5-dimethyl-4-
hydroxy-3(2H)-furanone via Cannizzaro reaction.
Fig. 7 Proposed pathways of alkylpyrazines formation.33
Fig. 8 Proposed pathway of oxazoles and oxazolines formation.
Fig. 9 Hypothetical mechanism for the formation of 2-acetyltetra-
hydropyridine.36
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6.2.4. Formation of thiazoles and thiazolines via Strecker
degradation (cysteine specific). Other aroma compounds
formed in the presence of cysteine via Strecker degradation are
thiazoles and thiazolines. They are closely related to oxazoles
and oxazolines, except a sulfur atom replaces the oxygen atom in
position 1 and a nitrogen atom occupies position 3. The first
thiazole and thiazoline isolated from foods were 4-methyl-5-
vinylthiazole and 2-acetyl-2-thiazoline, respectively.38,39 Later,
various thiazoles and thiazolines were identified in many food
systems, especially in thermally treated foods such as baked
potatoes40 and roasted peanuts.35 Generally, 2-alkylthiazoles
possess green, vegetable-like aroma. Increasing substitutions in
positions 4 and 5 adds more nutty, roasted and sometimes meaty
notes. Because they contain sulfur thiazoles and thiazolines that
are formed mostly in the presence of cysteine. The most accepted
formation mechanism of these is that a-dicarbonyls, such as
2,3-butanedione, react with hydrogen sulfide, ammonia and
acetaldehyde to form either 3-hydroxy-3-mercapto-2-butanone
or 3-mercapto-2-butanone, leading to 2,4,5-trimethythiazole and
2,4,5-trimethyl-3-thiazoline through condensation, cyclization
and condensation with ethylideneamine41 (Fig. 12). MGO and
GO should undergo similar reaction.
6.3. Formation of aroma compounds from amino acids via
Strecker degradation
Aromas, such as 3-methylbutanal (malty) or phenylacetaldehyde
(honey-like), that are derived from leucine and phenylalanine,
respectively, in the presence of a-dicarbonyl, generally are
Strecker aldehydes formed in Strecker degradation. Besides
aldehydes, some odour-active acids were also identified in
the presence of RCS. In Strecker degradation, for example,
phenylalanine reacts with MGO or GO, preferentially leading
to phenylacetic acid generation42 (Fig. 13), whereas reacting
with 3-DG favors the formation of phenylacetaldehyde.43
6.4. Formation of pyrazinones from peptides and alpha-
dicarbonyls
Besides different amino acids, peptides are also important
precursors of flavour generation with RCS. The major volatile
compounds from Maillard reaction of glycine, diglycine and
triglycine with glucose are alkylpyrazines. Triglycine was not
stable and degraded to cyclic diglycine and glycine, whereas
diglycine had a higher stability than triglycine toward hydrolytic
cleavage of the peptide bond.44 Pyrazinone is a peptide-specific
reaction product, and could generate from diglycine, triglycine
and tetraglycine with glucose.45 These compounds are formed
from the decarboxylation of 2-(30-alkyl-20-oxopyrazin-10-yl)
alkanoic acid at high temperatures, such as 180 1C (Fig. 14),
or the condensation of a-dicarbonyl with isoasparagine.46
Model systems of Gly-Leu and Leu-Gly with MGO were also
studied. It was found that a peptide sequence did not affect the
type of aromas, but only changed the amount of pyrazinones
slightly.47
7. Trapping of MGO and GO by phenolic
compounds
MGO and GO are extremely reactive and readily modify
lysine, arginine, and cysteine residues on proteins in vivo to
form advanced glycation end products (AGEs) that are linked
to hyperglycemia and diabetes complications. As the first step
of AGEs formation, proteins in the tissues are modified by
reducing sugars through the reaction between a free amino
Fig. 10 Hypothetical mechanism for the formation of 2-acetyl-
pyrroline.36
Fig. 11 Biosynthesis of 2-acetyltetrahydropyridine.37
Fig. 12 Formation pathways of thiazoles and thiazolines.41
Fig. 13 Formation of phenylacetaldehyde and phenylacetic acid
from methylglyoxal (MGO) and phenylalanine.42
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group of proteins and a carbonyl group of the sugars, leading to
the formation of fructosamines via a Schiff base by Amadori
rearrangement. Then, both Schiff base and Amadori product
undergo a further series of reactions through dicarbonyl inter-
mediates (e.g., GO and MGO), to form AGEs. The amount of
AGEs is dependent on the concentration of dicarbonyl inter-
mediates, reactivity and concentration of amino acid residues,
the half-life of protein, and the activity of glyoxalase system.
Accumulation of MGO and GO in cells may cause carbonyl
stress that in its first step induces the formation of hydrogen
peroxide. The hydrogen peroxide may then increase oxidative
stress and, therefore, tissue damage. Moreover, MGO can activate
NF-kB and induce the associated gene that is responsible for
inflammation and proliferation.2 Therefore, studies on the
prevention of accumulation of dicarbonyl compounds become
very urgent. Many synthetic chemicals have been reported to
inhibit formation of AGEs. More recently, researchers have
documented the scavenging effect of MGO and GO by natural
phenolic compounds, such as flavanols, chalcones, stilbenes,
isoflavones and phenolic acids.
An inhibition of protein glycation on different stages was
studied by using different dietary flavonoids. Luteolin, rutin,
epigallocathin-3-gallate (EGCG), and quercetin demonstrated
significant inhibitory effects on MGOmediated AGEs formation
by 82.2, 77.7, 69.1 and 65.3%, respectively, while catechin,
epicatechin (EC), epicatechin gallate (ECG), epigallocatechin
(EGC), kaempferol, and naringenin showed a lower inhibitory
effect (13–54%).48
Among all flavonoids, molecules with catechin-like structure
show the strongest effect on direct trapping of MGO. In tea
polyphenol adduct investigation, the mole ratio of MGO to each
specific polyphenol was set at three, and the % reduction of
MGO was compared with that of the control sample at 0 1C on
an ice/salt bath for 1 h.49 After 1 h at 37 1C incubation, MGO
was very stable, only decreasing by 5.8%. All tea polyphenolic
compounds scavenge MGO.Most tea catechins decreased MGO
by about 33%, which indicates that one catechin molecule reacts
with one MGO molecule, since the initial mole ratio of MGO to
a tea polyphenolic compound was 3 to 1. Theaflavins, the
main black tea components, were more reactive toward MGO
than other polyphenols tested. Theaflavins showed high levels
of MGO reduction with respect to the control sample, which
suggested that theaflavins would be excellent candidates in the
treatment of MGO scavenging in future in vivo studies. The
decreased amounts of MGO in TF1 (theaflavin), TF2 (theaflavin-
3- and -30-gallate), and TF3 (theaflavin-3,30-digallate) were
63.1, 60.1, and 66.7%, respectively. All tested theaflavins
decreasedMGO by about 66%, which implies that one theaflavin
molecule can trap two MGO molecules. In addition, the efficacy
of EGCG trapping to GO was much lower than that of MGO,
because GO is much easier to polymerise in aqueous solution.
Trapping efficacy could be slowed down by the transformation
from polymers to free GO. The major adduct between EGCG
andMGO has been identified (Fig. 15). It is concluded that the
reaction between EGCG and MGO dominantly occurs at the
C6- and C8-position in the A ring of EGCG. Slightly alkaline
pH facilitates the addition of MGO at these two positions
to form mono- and di-MGO adducts. Isomers with R- and
S-configuration could also be seen at each position.50
Chalcones, represented by phloretin and phloridzin, have
been studied for their trapping efficacy of MGO and GO.51
GO still showed a lower attachment than MGO because of
easier polymerization. Phloretin had a much higher trapping
rate than did phloridzin suggesting that glycosylation of a
hydroxyl group at position 2 significantly slows down the
formation of adducts. A similar phenomenon was observed in
EGCG. Positions 3 and 5 of the A ring in phloretin and
phloridzin were active sites to bind with MGO to form mono-
and di-MGO adducts (Fig. 16). Stilbenes were compared for
their efficacy of trapping MGO.52 Among 2,3,5,40-tetrahydroxy-
stilbene-2-O-b-D-glucoside (THSG), pterostilbene and resveratrol,
THSG was the most effective trapping agent. Positions 4 and 6 of
the A ring in stilbenes were the major active sites for trapping
MGO (Fig. 17). Genistein, one of the isoflavones, was reported to
trap MGO under neutral and alkaline conditions in vitro.53
Mono- and di-MGO adducts were identified. Mono-MGO
adducted at position 8 on the A ring of genistein, and di-MGO
conjugated at both positions 6 and 8 on the A ring of genistein
(Fig. 18). Based on all the results obtained here, it is concluded
Fig. 14 Mechanism of the formation of alkylpyrazinone from
dipeptide and methylglyoxal (MGO).45
Fig. 15 Adducts of methylglyoxal (MGO) and epigallocathin-3-
gallate (EGCG).
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that phenolic compounds having the same A ring structure
(EGCG, phloretin, chalcone, stilbene and genistein) can efficiently
trap MGO or GO to form mono- or di-MGO adducts even with
different C rings. The mechanism is that the slightly alkaline pH
can increase the nucleophilicity of the unsubstituted carbons at
the A ring and facilitate the addition of MGO at these two
positions to form mono- or di-MGO adducts. This observation
also applies to polymers of flavonoids such as proanthocyanidins.
Peng et al. (2008) demonstrated that proanthocyanidin
B2 could effectively scavenge MGO, in a similar manner to
aminoguanidine.54 Compounds consisting of a single benzene
ring with the addition of at least one hydroxyl group were
studied forMGO trapping. Compounds with one hydroxyl group
on the benzene ring cannot react with MGO. Benzenetriols
showed relatively higher trapping capability. The decreasing rate
of MGO trapping for pyrogallol, 1,2,4-trihydroxybenzene, and
1,3,5-trihydroxybenzene were 55%, 49% and 64%, respectively.
Steric hindrance and carbon electron charges on the benzene ring
were reported as the influential factors. A carbon electron charge
of �0.24 was the minimum value for high reactivity using a
computational chemistry calculation.55 The carbon electron
charges of epicatechin are shown in Fig. 19. The carbon
electron charge on position 6 showed the lowest number of
�0.26, followed by the one on position 8 (�0.25). These resultsexplain why positions 6 and 8 were the active sites for flavanols.
8. Effect of polyphenolic compounds on Maillard
flavour generations
Some polyphenolic compounds can efficiently trapMGO and GO
usually generated from Maillard reaction. Therefore, scavenging
RCS may influence Maillard reaction, while suppressing Maillard
reaction may reduce the level of RCS. Moreover, Maillard
reaction is the major route for the generation of flavour and
colour. Thus, it is important to study the effect of polyphenolic
compounds on flavour generation by trapping RCS.
Epicatechin was first studied for its influence on flavour
generation in an aqueous model with glucose and glycine.56
Addition of epicatechin (EC) in the reaction decreased the
generation of 2,3-butanedione, acetol, pyrazine, methylpyrazine,
2,3,5-trimethylpyrazine and cyclotene, because these are volatile
compounds formed from C2/C3 or C3/C3 sugar fragments, and
epicatechin can directly react with C2, C3 and C4 sugar fragments.
Through NMR analysis, glyoxal, hydroxyacetone, and erythrose
were identified as the sugar fragments. Methylglyoxal could
also bind with EC, but EC-MGO underwent conformation/
constitutional exchange.57 One of the isomers consisted of a
covalent binding between C1 of MGO and either the C6 or C8
position of the EC A ring, in accordance with a previous
study.50 Besides EC, other flavan-3-ols, such as ECG and EGCG,
showed the same phenomenon.58 Generation of phenolic-C2, C3,
C4 and C6 fragment adducts statistically fit the reduction of
glyoxal, glycolaldehyde,MGO, hydroxyacetone, diacetyl, acetoin,
and 3-deoxyglucosone. Under a roast condition, addition of
EC can significantly reduce the generation of hydroxyacetone,
2-methylpyrazine, 2,3,5-trimethylpyrazine, furfural, 2-acetylfuran,
Fig. 16 Adducts of methylglyoxal (MGO) and phloridzin.51
Fig. 17 Adducts of methylglyoxal (MGO) and stilbenes.52
Fig. 18 Adducts of methylglyoxal (MGO) and genistein.53
Fig. 19 Carbon electron charges on epicatechin.55
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4148 Chem. Soc. Rev., 2012, 41, 4140–4149 This journal is c The Royal Society of Chemistry 2012
5-methylfurfural, 2(5H)-furanone, 2-acetylpyrrole and furfuryl
alcohol. However, it only reacted with C5 and C6 sugar
fragments. EC can quench 3-deoxy-2-hexosulose.59 Phloretin
and phloridzin were also studied for their effect on flavour
generation in a model reaction with lysine and glucose.60 The
amount of lysine could significantly affect the generation of
flavour compounds. Furfural, 1,2,3,4-tetrahydroquinoline
and eight pyrazines were identified in the reactant ratio of
1 : 3 (glucose : lysine), while other additional compounds,
such as 2,3-dihydro-1-methyl-1H-indole, N-ethyl-2,3-xylidine
and 3-ethyl-2,5-dimethylpyrazine, were only found in lower
amounts of lysine. Both phloretin and phloridzin inhibited
the formation of volatile compounds, but phloretin showed
a higher activity. Moreover, phloretin and phloridzin were
reported to show lower inhibitory effect with a higher content
of lysine, suggesting the competition of these phenolic compounds
and amino groups for RCS.
9. Conclusion
MGO and GO, which are intermediates in the Maillard
reaction, are involved in flavour and colour generation for
foods and drinks. Phenolic compounds having the same A ring
structure (EGCG, phloretin, chalcone, stilbene and genistein)
can efficiently trap MGO or GO to form mono- or di-MGO/
GO adducts even with different C rings. Because of the
trapping efficacy of phenolic compounds, adding them into
Maillard reaction would change the formation of MGO and
GO, and change the flavour properties. Therefore, RCS trapping
agents provide an alternative way to influence flavour generation
and affect the quality of food.
Notes and references
1 I. Nemet, L. Varga-Defterdarovic and Z. Turk, Mol. Nutr. FoodRes., 2006, 50, 1105–1117.
2 L. Wu and B. H. Juurlink, Hypertension, 2002, 39, 809–814.3 H. Juergens, W. Haass, T. R. Castaneda, A. Schuermann,C. Koebnick, F. Dombrowski, B. Otto, A. R. Nawrocki,P. E. Scherer, J. Spranger, M. Ristow, H. Joost, P. J. Havel andM. H. Tschoep, Obes. Res., 2005, 13, 1146–1156.
4 D. J. Creighton, M. Migliorini, T. Pourmotabbed andM. K. Guha, Biochemistry, 1988, 27, 7376–7384.
5 I. Nemet, D. Vikic-Topic and L. Varga-Defterdarovic, Bioorg.Chem., 2004, 32, 560–570.
6 A. Hollnagel and L. W. Kroh, Z. Lebensm.-Unters. Forsch., 1998,207, 50–54.
7 E. Marceau and V. A. Yaylayan, J. Agric. Food Chem., 2009, 57,10837–10844.
8 C. Y. Lo, S. Li, Y. Wang, D. Tan, M. Pan, S. Sang and C. T. Ho,Food Chem., 2008, 107, 1099–1105.
9 J. Wang and T. Chang, J. Food Sci., 2010, 75, 167–171.10 M. Daglia, A. Papetti, C. Aceti, B. Sordelli, V. Spini and
G. Gazzani, J. Agric. Food Chem., 2007, 55, 8877–8882.11 K. Fujioka and T. Shibamoto, Lipids, 2004, 39, 481–486.12 J. A. Rodrigues, A. A. Barros and P. G. Rodrigues, J. Agric. Food
Chem., 1999, 47, 3219–3222.13 J. E. Hodge, J. Agric. Food Chem., 1953, 1, 928–943.14 T. Hayashi and T. J. Shibamoto, J. Agric. Food Chem., 1985, 33,
1090–1093.15 D. S. Mottram, B. L. Wedzicha and A. T. Dodson, Nature, 2002,
419, 448–449.16 G. P. Rizzi, J. Agric. Food Chem., 1972, 20, 1081–1085.17 P. J. Thornalley, A. Langborg and S. Harjit, Biochem. J., 1999,
344, 109–116.
18 P. J. Thornalley, S. P. Wolff, J. Crabbe and A. Stern, Biochim.Biophys. Acta, 1984, 797, 276–287.
19 P. J. Thornalley, Environ. Health Perspect., 1985, 64, 297–307.20 H. Nursten, The Maillard Reaction: Chemistry, Biochemistry and
Implication, The Royal Society of Chemistry, Cambridge, UK,2005.
21 T. Hayashi and M. Namiki, Agric. Biol. Chem., 1980, 44,2575–2580.
22 C. J. Mullarkey, D. Edelstein and M. Brownlee, Biochem. Biophys.Res. Commun., 1990, 173, 932–939.
23 B. Willhalm,M. Stoll and A. F. Thomas,Chem. Ind. (London, U. K.),1965, 38, 1629–1630.
24 Y. Wang and C. T. Ho, J. Agric. Food Chem., 2008, 56,7405–7409.
25 Y. Zheng, S. Brown, W. O. Leding, C. Mussinan and C. T. Ho,J. Agric. Food Chem., 1997, 45, 894–897.
26 K. K. Jutta and B. Werner, Z. Lebensm.-Unters. Forsch., 1990, 190,14–16.
27 T. Hofmann and P. Schieberle, J. Agric. Food Chem., 1997, 45,898–906.
28 P. Schieberle, Flavor Precursors: Thermal and Enzymatic Conversions,ed. R. Teranishi, G. R. Takeoka and M. Guntert, ACS SymposiumSeries 490, Washington, DC, 1992, pp. 164–175.
29 I. Blank and L. B. Fay, J. Agric. Food Chem., 1996, 44, 531–536.30 J. E. Hodge, F. D. Mills and B. E. Fisher, Cereal Sci. Today, 1972,
17, 34–40.31 T. Hofmann and P. Schieberle, Flavour 2000, Perception Release
Evaluation Formation Acceptance Nutrition/Health, Proceedings ofthe 6th Wartburg Aroma Symposium. Eisenach, ed. M. Rothe,Eisenach, Germany, 2001, pp. 311–322.
32 P. Schieberle, Ann. N. Y. Acad. Sci., 2005, 1043, 236–248.33 G. Vernin, The chemistry of heterocyclic flavoring and aroma
compounds, Ellis Horwood, Chichester, UK, 1982.34 O. G. Vitzthum, P. Werkhoff and P. Hubert, J. Food Sci., 1975, 40,
911–916.35 C. T. Ho, Q. Z. Jin, M. H. Lee and S. S. Chang, J. Agric. Food
Chem., 1983, 31, 1384–1386.36 P. Schieberle, J. Agric. Food Chem., 1995, 43, 2442–2448.37 M. L. Wu, J. H. Chen, C. T. Ho and T. C. Huang, J. Agric. Food
Chem., 2007, 55, 1767–1772.38 M. Stoll, P. Dietrich, E. Sundte and M. Winter, Helv. Chim. Acta,
1967, 50, 2065.39 C. H. Tonsbeek, H. Copier and A. J. Placken, J. Agric. Food
Chem., 1971, 19, 1014–1016.40 E. C. Coleman, C. T. Ho and S. S. Chang, J. Agric. Food Chem.,
1981, 29, 42–48.41 H. J. Takken, L. M. van der Linde, P. J. de Valois, H. M. van Dort
and M. Boelens, Phenolic, Sulfur, and Nitrogen Compounds inFood Favors, ed. G. Charalambous and I. Katz, ACS SymposiumSeries 26, American Chemical Society, Washington, DC, 1976,pp. 114–121.
42 T. Hofmann and P. Schieberle, J. Agric. Food Chem., 2000, 48,4301–4305.
43 T. Hofmann, P. Munch and P. Schieberle, J. Agric. Food Chem.,2000, 48, 434–440.
44 C. Y. Lu, Z. Hao, R. Payne and C. T. Ho, J. Agric. Food Chem.,2005, 53, 6443–6447.
45 Y. C. Oh, C. K. Shu and C. T. Ho, J. Agric. Food Chem., 1991, 39,1553–1554.
46 C. K. Shu and B. M. Lawrence, J. Agric. Food Chem., 1995, 43,779–781.
47 Y. C. Oh, C. K. Shu and C. T. Ho, J. Agric. Food Chem., 1992, 40,118–121.
48 C. H. Wu and G. C. Yen, J. Agric. Food Chem., 2005, 53,3167–3173.
49 C. Y. Lo, S. Li, D. Tan, S. Sang, M. H. Pan and C. T. Ho,Mol. Nutr. Food Res., 2006, 50, 1118–1128.
50 S. Sang, X. Shao, N. Bai, C. Y. Lo, C. S. Yang and C. T. Ho,Chem. Res. Toxicol., 2007, 20, 1862–1870.
51 X. Shao, N. Bai, K. He, C. T. Ho, C. S. Yang and S. Sang,Chem. Res. Toxicol., 2008, 10, 2042–2050.
52 L. Lv, X. Shao, L. Wang, D. Huang, C. T. Ho and S. Sang,J. Agric. Food Chem., 2010, 58, 2239–2245.
53 L. Lv, X. Shao, H. Chen, C. T. Ho and S. Sang, Chem. Res.Toxicol., 2011, 24, 579–586.
Dow
nloa
ded
by F
AC
DE
QU
IMIC
A o
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Sep
tem
ber
2012
Publ
ishe
d on
16
Apr
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pubs
.rsc
.org
| do
i:10.
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/C2C
S350
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View Online
This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 4140–4149 4149
54 X. Peng, J. Ma, J. Chao, Z. Sun, R. C. Chang, I. Tse,E. T. Li, F. Chen and M. Wang, J. Agric. Food Chem., 2010, 58,6692–6696.
55 C. Y. Lo, W. T. Hsiao and X. Y. Chen, J. Food Sci., 2011, 76,H90–H96.
56 V. M. Totlani and D. G. Peterson, J. Agric. Food Chem., 2005, 53,4130–4135.
57 V. M. Totlani and D. G. Peterson, J. Agric. Food Chem., 2006, 54,7311–7318.
58 Y.Noda andD.G. Peterson, J. Agric. Food Chem., 2007, 55, 3686–3691.59 V. M. Totlani and D. G. Peterson, J. Agric. Food Chem., 2007, 55,
414–420.60 J. Ma, X. Peng, K. M. Ng, C. M. Che and M. Wang, Food Funct.,
2012, 3, 178–186.
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