Role of aldehydic derivatives in the condensation of phenolic compounds with emphasis on the sensorial properties of fruit-derived foods

Article abstract of DOI:10.1021/jf025503y

The reactions between (epi)catechin, mavidin 3-O-glucoside, and some aldehydes were investigated by LC/DAD and LC/ESI-MS analysis. The obtained results showed that the acetaldehyde-mediated condensation occurred more generally and glyoxylic acid, furfural

Full text of DOI:10.1021/jf025503y

J. Agric. Food Chem. 2002, 50, 5571−5585 5571
Role of Aldehydic Derivatives in the Condensation of Phenolic
Compounds with Emphasis on the Sensorial Properties of
Fruit-Derived Foods
‡
NOUR-EDDINE ES-SAFI,*,† VEÄ RONIQUE CHEYNIER,‡ AND MICHEL MOUTOUNET
UMR Sciences Pour l’Œnologie, INRA, 2 place Viala, 34060 Montpellier, France, and
EÄ cole Normale Supe´rieure, Laboratoire de Chimie Organique et d’Etudes physico-chimiques,
B.P. 5118, Takaddoum Rabat, Morocco
The reactions between (epi)catechin, mavidin 3-O-glucoside, and some aldehydes were investigated
by LC/DAD and LC/ESI-MS analysis. The obtained results showed that the acetaldehyde-mediated
condensation occurred more generally and glyoxylic acid, furfural, and 5-(hydroxymethyl)furfural (HMF)
react in the same way in the first stages of the reactions. In terms of reactivity, reactions were faster
with acetaldehyde than with glyoxylic acid, furfural, or HMF, where the reactions were slower. In the
case of acetaldehyde, the obtained purple derivatives were more predominant and stable than the
colorless adducts and no xanthylium salt was detected. Interactions involving glyoxylic acid yield
purple adducts, which were obtained in small amount compared to the colorless ones. The latter
were shown to proceed to more polymerized and yellowish derivatives. Finally, in the case of furfural
and HMF, purple compounds involving flavanol and anthocyanin units were detected, and colorless
compounds were shown to be predominant and to yield yellowish xanthylium salts.
KEYWORDS: (+)-Catechin; (-)-epicatechin; malvidin 3-O-glucoside; phenolic compounds; aldehyde;
acetaldehyde; glyoxylic acid; furfural; 5-(hydroxymethyl)furfural; HMF; LC/DAD; LC/MS; browning;
darkening
INTRODUCTION
continue throughout processing and aging. They lead to a great
diversity of products, thus adding to the complexity of food
phenolic composition. The new compounds formed often show
specific organoleptic properties, distinct from those of their
precursors. Therefore, a better understanding of their structures
and properties and the mechanisms generating them appears to
be essential to predict and control food quality.
Phenolic compounds have attracted considerable interest
because of their ubiquitous occurrence within the plant kingdom
and numerous important properties. In particular, they are major
fruit-derived food constituents, responsible for some of their
organoleptic features. They are also receiving increasing atten-
tion as natural antioxidants and potential health-promoting
agents. Their eventual health effects are attracting considerable
interest in the international scientific community.
Polyphenols show a great diversity of structures from rather
simple molecules (monomers and oligomers) to polymers. These
phenolic compounds are highly unstable and are rapidly
transformed into various reaction products when plant cells are
damaged and also during food processing. Indeed, phenolic
compounds are highly reactive species due to the acidic character
of their hydroxyl groups and the nucleophilic properties of the
phenolic rings that undergo various types of reactions in the
course of food processing. Among these reactions, oxidation
processes and addition reactions, leading to various adducts and
eventually tannin-like polymeric compounds, are particularly
important (1-13). These reactions start as soon as plant cells
are damaged or broken (i.e., at crushing or pressing) and
Major progress achieved in food technology over the past
decade concerns, on the one hand, the structural determination
of phenolic compounds and, on the other hand, the elucidation
of reaction mechanisms modifying polyphenols in the course
of food processing and storage. Among the reactions involved
in the change of color or astringency during aging or storage
of fruit-derived foods, the interaction referred to as antho-
cyanin-tannin condensation has been studied by several authors.
Within this reaction, two mechanisms have been postulated:
direct nucleophilic addition of flavanols onto the electrophilic
anthocyanin, generating orange xanthylium salts, and aldehyde-
mediated condensation, yielding purple pigments. The interac-
tion of phenolic compounds with aldehydic reactants plays a
major role during the storage of fruits and fruit-derived foods
(3-12, 14, 15).
Fruit-derived foods are complex mixtures able to undergo
many different reactions. The full characterization of these
reactions has not been achieved because of difficulties involved
in extracting and separating the newly formed compounds
* Author to whom correspondence should be addressed (fax 212 377
750047; e-mail b.essafi@iav.ac.ma, n.es_safi@caramail.com).
† EÄ cole Normale Supe´rieure.
‡ INRA.
10.1021/jf025503y CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/30/2002

5572 J. Agric. Food Chem., Vol. 50, No. 20, 2002
Es-Safi et al.
aldehydes [acetaldehyde, glyoxylic acid, furfuraldehyde, and
5-(hydroxymethyl)furfuraldehyde] as well as the structures of
the newly formed compounds. A comparative study on the
reactivity of these aldehydes on phenolic compounds is pre-
sented. Finally, the implication of these reactions and these new
products on the sensorial properties of fruit-derived foods is
also discussed.
MATERIALS AND METHODS
Reagents. Deionized water was purified with Milli-Q water system
(Millipore, Bedford, MA) prior to use. Acetonitrile was purchased from
BDH (Poole, U.K.). Methanol, formic acid, and acetic acid were
obtained from Prolabo (Fontenay S/Bois, France). (+)-Catechin and
(-)-epicatechin were purchased from Sigma (St. Louis, MO). Malvidin
3-O-glucoside was purchased from Extrasynthe`se (Genay, France).
Acetaldehyde was obtained from Merck (Darmstadt, Germany). Gly-
oxylic acid was obtained from Aldrich (Paris, France). Furfural and
5-(hydroxymethyl)furfural (HMF) were obtained from Interchim (Mont-
culon, France) and Lancaster Synthesis (Strasbourg, France), respec-
tively.
Reactions. An acidic solution was prepared with 17 µL of acetic
acid and 50 µL of ethanol in 433 µL of water, giving a pH value of
2.2. Various pH values ranging from 2.2 to 4.0 were obtained by the
addition of 1 M sodium hydroxide to the medium described above and
were adjusted using a 93313 Bioblock pHmeter (Illkirch, France). (+)-
Catechin or (-)-epicatechin (20 mM) was prepared in triplicate in each
obtained medium (0.5 mL). The aldehydic derivatives [acetaldehyde,
glyoxylic acid, furfuraldehyde, and 5-(hydroxymethyl)furfuraldehyde]
were added to give a final concentration of 20 mM.
Figure 1. Evolution of (+)-catechin with time and pH when incubated
with acetaldehyde in a model solution system.
directly from food products. Model solutions mimicking food
products constitute a simplified medium for their exploration,
allowing the detection of the newly formed compounds, their
isolation, and their structure elucidation. Whereas the color of
a new fruit-derived product is due to its initial chemical
composition, the subsequent color changes during aging involve
generally condensation of phenolic compounds. Direct conden-
sation between anthocyanins and flavanols is very slow.
However, rapid polymerization between anthocyanin and cat-
echins or tannins occurs in the presence of aldehyde derivatives
with increased color intensity and stability, but further reactions
with polymerized flavanols lead to instability, precipitation, and
decreased color (4, 16-21).
Solutions containing malvidin 3-O-glucoside or cyanidin 3-O-
glucoside and (+)-catechin or (-)-epicatechin were prepared in the
same way. The prepared solutions were incubated in triplicate at 20
°C and in absence of light, and reactions were periodically monitored
by liquid chromatography (LC) coupled with a diode array detector
The present paper summarizes and gathers our recent findings
about the reactions between phenolic compounds and some
Figure 2. General structures of the dimeric colorless (IA, IIA, IIIA, and IVA) and colored (IB, IIB, IIIB, and IVB) derivatives obtained by interaction
involving flavanols, anthocyanins, and acetaldehyde (R ) CH ), glyoxylic acid (R ) COOH), furfural (R ) H), and HMF (R ) CH OH).
3
1
1
2

Interactions between Phenolic Compounds and Aldehydic Derivatives
J. Agric. Food Chem., Vol. 50, No. 20, 2002 5573
furfural and HMF depending on the reaction rate, which was found to
be influenced by the used aldehyde. Quantification of residual flavanols
and of all colorless compounds was achieved on the basis of peak areas
at 280 nm, using (+)-catechin or (-)-epicatechin as standard. Quan-
tification of residual anthocyanin and of all purple compounds was
achieved on the basis of peak areas at 520 nm, using malvidin 3-O-
glucoside as standard. Identification of the formed compounds was
achieved on the basis of their UV-visible and MS spectroscopy and
by comparison with standards.
Analytical HPLC/DAD Analyses. HPLC/DAD analyses were
performed by means of a Waters 2690 separation module system
including a solvent and a sample management system, a Waters 996
photodiode array detector, and Millenium 32 chromatography manager
software. UV-visible spectra were recorded from 250 to 600 nm. The
column was a reversed-phase Lichrospher 100-RP18 (5 µm packing,
250 × 4 mm i.d.) protected with a guard column of the same material.
Elution conditions were as follows: 1 mL/min flow rate; temperature,
30 °C; solvent A, water/formic acid (98:2, v/v); solvent B, acetonitrile/
water/formic acid (80:18:2, v/v); elution from 5 to 30% B in 40 min,
from 30 to 40% B in 10 min, and from 40 to 100% B in 5 min, followed
by washing and re-equilibrating the column.
MS Apparatus and LC/ESI-MS Analyses. MS measurements were
performed on a Sciex API I Plus simple quadruple mass spectrometer
equipped with an electrospray ionization source. The mass spectrometer
was operated in positive or negative ion mode. Ion spray voltage/orifice
voltages were selected at -4 kV/-70 V and +5 kV/+60 V,
respectively. For direct injection, the solution was introduced into the
electrospray source at a constant flow rate of 5 µL/min with a medical
syringe infusion pump in combination with a 100 µL syringe.
HPLC separations were carried out on a narrowbore reversed-phase
column with an ABI 140 B solvent delivery system (Applied Biosys-
tems, Weiterstadt, Germany). The column was connected with the ion
spray interface via a fused-silica capillary (length ) 100 cm, 100 µm
i.d.). The reaction mixture was injected with a rotary valve (Rheodyne
model 8125) fitted with a 20 µL sample loop. The separation was
achieved on a Lichrospher 100-RP18 column (5 µm packing, 250 × 4
mm i.d., Merck), with a flow rate of 280 µL/min. The elution was
done with solvents A and B used in HPLC/DAD analysis and the
conditions adapted as follows: isocratic 10% B in 4 min, linear gradient
from 10 to 15% B in 11 min, from 15 to 50% B in 25 min, and from
50 to 100% B in 5 min, followed by washing and reconditioning of
the column. The absorbance at 280 nm was monitored by an ABI 785A
programmable absorbance detector and by a Waters 990 DAD linked
to 990 system manager software.
Absorption Spectra. Spectrophotometric measurements and UV-
visible spectra were recorded with a 111 GBC spectrophotometer fitted
with a quartz cell and equipped with GBC Scan Master software.
RESULTS AND DISCUSSION
Reactions Involving Acetaldehyde. Among the aldehydic
derivatives, acetaldehyde has been found in many foods and
beverages including fruits, vegetables, bread, dairy products,
meats, and alcoholic or nonalcoholic beverages (22-24). It is
widely used in such artificial fruit flavors as apple, apricot,
banana, and peach and is commonly found in fruits such as
citrus fruits because it is an intermediate product in the
respiration of higher plants. It is also formed by oxidation of
ethanol or by decarboxylation of pyruvic acid during fermenta-
tion of grapes (25).
The reactions of (+)-catechin and (-)-epicatechin in the
presence of acetaldehyde were studied in model solution
systems. When incubated separately with acetaldehyde and at
pH values varying from 2.2 to 4.0, (+)-catechin or (-)-
epicatechin was transformed to various compounds initially
absent in the mixtures; reactions were faster with (-)-epicatechin
than with (+)-catechin. HPLC-DAD analysis showed that the
formed compounds were characterized by UV-visible spectra
Figure 3. HPLC chromatograms of a mixture of (+)-catechin, malvidin
3-O-glucoside, and acetaldehyde recorded at 280 (A), 440 (B), and 520
nm (C) showing the formation of more polymerized colored derivatives.
MED1 and MED2 represent the two formed colored dimers. The spectra
drawn in panel D show the bathochromic shift observed in the visible
region for more polymerized compounds.
(DAD) and with an electrospray mass spectrometry (ESI-MS) detector
during a period varying from 6 days for acetaldehyde to 3 months for

5574 J. Agric. Food Chem., Vol. 50, No. 20, 2002
Es-Safi et al.
Figure 4. General mechanism of the aldehyde-induced polymerization of flavanols and anthocyanins with formation of colorless (A) and colored com-
pounds (B).
similar to that of their precursor with absorption maxima at 280
nm showing that the original flavanic skeleton was retained.
Besides, reaction kinetics slowed when the pH increased, as
can be observed in Figure 1, in agreement with the literature
data (26-28).
by our group (18, 19). The ethanol adduct then loses a water
molecule to give a new carbocation intermediate, which is in
turn attacked by another flavanol unit to yield an ethyl-linked
dimer adduct as shown in Figure 2 (IA). The presence of
intermediate ethanol adducts of dimer and trimer species
confirmed that polymerization continued following the same
mechanism.
ESI-MS analysis of solutions containing (+)-catechin or (-)-
epicatechin and acetaldehyde conducted in the negative ion
mode allowed detection of many peaks corresponding to various
oligomers with several mass peaks corresponding to the doubly
charged ions of larger molecular weight polymers as previously
reported by us (18). These compounds consisted of flavanol
units linked with ethyl bridges as shown in Figure 2 (IA).
Detection of the ethanol-flavanol adduct (Cat.-CH2OH-CH3)
by HPLC coupled to ion spray mass spectrometry (ESI-MS) as
the negative ion at m/z 333 amu allowed us to demonstrate that
the reaction starts with protonation of acetaldehyde in acidic
medium followed by nucleophilic addition of the flavanol (C-6
or C-8 of the A-ring) on the resulting carbocation, as previously
postulated by Timberlake and Bridle (4) and confirmed later
In mixtures containing both flavanols and acetaldehyde, new
compounds besides the homogeneous bridged derivatives were
detected. These compounds were concluded to be heterogeneous
oligomers consisting of (+)-catechin and (-)-epicatechin linked
with an ethyl bridge. In this case, (-)-epicatechin decreased
more rapidly than (+)-catechin, showing that the reaction was
faster with the former. This was also observed in solutions
containing the two flavanols and the (+)-catechin-ethanol
intermediate. Under these conditions, the homogeneous (+)-
catechin bridged dimers and heterogeneous dimers were ob-
tained by the action of the intermediate on (+)-catechin and
(-)-epicatechin, respectively. In addition, the homogeneous

Interactions between Phenolic Compounds and Aldehydic Derivatives
J. Agric. Food Chem., Vol. 50, No. 20, 2002 5575
Figure 5. General structures of the trimeric (V and VI) and tetrameric (VII and VIII) colored derivatives detected in mixtures containing (−)-epicatechin,
malvidin 3-O-glucoside, and acetaldehyde.
(-)-epicatechin ethyl-bridged dimers were also detected,
showing that ethyl linkages underwent depolymerization and
recombination reactions.
HPLC-ESI-MS of the mixture conducted in the positive ion
mode showed the presence of both colorless ethyl-linked
flavanol oligomers and colored flavanol-anthocyanin adducts,
meaning that competition took place between both species in
the addition process. Thus, both colorless (IA) and colored (IB)
dimers were detected as their molecular ion in the positive ion
mode at m/z 607 and 809 amu, respectively. From a mechanistic
point of view, the reaction proceeds in the same way in the
first stage, giving the monomer ethanol intermediate as shown
in Figure 4 (R ) CH3). This intermediate then loses a water
molecule to give a new carbocation, which is in turn attacked
either by another flavanol unit to yield an ethyl-linked dimer
or by an anthocyanin to give a flavanol-ethyl-anthocyanin
adduct. The fact that only the flavanol unit could be attacked
by the first ethanal carbocation was supported by the absence
of a malvidin 3-O-glucoside intermediate. Thus, no signals was
detected at m/z 537 amu corresponding to a methyl carbinol
In a mixture containing malvidin 3-O-glucoside, (-)-epicat-
echin, and acetaldehyde, the two reactions compete and the
formation of adducts containing only homogeneous ethyl-
bridged flavanols or involving (-)-epicatechin and the antho-
cyanin was observed. Monitoring the evolution of the reaction
indicated that the homogeneous bridged derivatives were formed
first and then transformed to colored derivatives, which seemed
to be more stable (Figure 3A). The latter were characterized
by absorption maxima bathochromically shifted (around 540
nm) compared to that of malvidin 3-O-glucoside (525 nm). This
bathochromic shift was more accentuated for more polymerized
products as shown in Figure 3D, where absorption maxima
attained 555 nm for compounds eluted in the end of the
chromatogram.

5576 J. Agric. Food Chem., Vol. 50, No. 20, 2002
Es-Safi et al.
Figure 6. Reaction pathways showing the formation of the trimeric (V and VI) and tetrameric (V and VI) colored adducts from colorless (IA′) and colored
(IB′) dimers.
group fixed on an anthocyanin unit, and the only detected
monomer intermediate was that of epicatechin with a molecular
ion observed at m/z 335 amu. In this reaction, both the flavanol
and the anthocyanin act as nucleophiles, whereas the protonated
form of the aldehyde is the electrophilic species. The resulting
ethyl-linked pigments are relatively stable, probably protected

Interactions between Phenolic Compounds and Aldehydic Derivatives
J. Agric. Food Chem., Vol. 50, No. 20, 2002 5577
Figure 8. HPLC chromatograms of a mixture of (+)-catechin and furfural
recorded at 280 nm (A). Compounds F1−4 are the four formed colorless
adducts. The UV−visible spectra of the four major derivatives are also
presented (B).
and one anthocyanin (m/z 809 amu), two flavanols and one
anthocyanin (m/z 1125 amu), one flavanol and two anthocyanins
(m/z 664 amu), three flavanols and one anthocyanins (m/z 1441
amu), and two flavanols and two anthocyanins (m/z 822 amu)
were detected. The structures corresponding to the trimeric (V
and VI) and tetrameric (VII and VIII) colored adducts are
shown in Figure 5. It seems from the detected oligomeric
derivatives that the reaction stops when both chain ends are
occupied by an anthocyanin moiety. Thus, no adducts with more
than two anthocyanin moieties were detected. In other words,
this means that the anthocyanin played the role of ending chain,
whereas the flavanol allowed the formation of more polymerized
colorless and colored adducts as indicated above.
Figure 7. HPLC chromatogram of a mixture of (+)-catechin and glyoxylic
acid recorded at 280 nm (A) showing residual flavanol and new formed
compounds (2a−d are the four colorless dimers). Evolution of (+)-catechin
with time and pH (B) and evolution of the four dimeric derivatives with
time during reaction are also presented (C).
The formation of the trimeric (V and VI) and tetrameric (VII
and VIII) colored derivatives detected in a mixture containing
(-)-epicatechin, malvidin 3-O-glucoside, and acetaldehyde
could be envisaged either from the colorless (IA′) or the colored
(IB′) dimers as shown in Figure 6. Whereas the colorless dimer
(IA′) could yield a colored trimer (V) by linking an anthocyanin
moiety through an ethyl bridge, both colored trimers (V and
VI) could be obtained from the colored dimer (IB′) by
involvement of the flavanol or the anthocyanin moieties. As
the linkage between the ethyl bridge and the anthocyanin or
the flavanol moieties is sensitive to acidic conditions, where
they were shown to be cleaved (19), each colored trimer could
be converted to the other by a depolymerization and recombina-
tion process. Each conversion is accompanied by a loss of a
flavanol unit and a reaction of the obtained intermediate with
an anthocyanin moiety and vice versa. As may be expected,
the formation of compounds involving more anthocyanins units
is prevalent because the linkage between (-)-epicatechin and
the bridge is more sensitive to acid conditions than that between
from the hydration reaction by self-association phenomenon,
leading to non-covalent dimers associated by a sandwich-type
stacking (29).
The acetaldehyde-induced condensation proceeds more easily
without the anthocyanins. In fact, pigments were shown to react
through a single position (only the C-8 top), whereas flavanols
were able to react twice, through the C-8 and C-6 positions. In
other words, this means that anthocyanins are terminal units
ending off the polycondensation reaction, whereas flavanols are
propagation units maintaining the polymerization process. Under
acidic conditions, the ethyl bond connecting two flavanol units
is more labile than those connecting an anthocyanin to a
flavanol. This was supported by thiolysis, by which colored
dimers were shown to be more resistant than the colorless ones
(19).
When the reaction was allowed to proceed, more polymerized
adducts were detected by LC/ESI-MS analysis conducted in the
positive ion mode. Thus, signals corresponding to one flavanol

5578 J. Agric. Food Chem., Vol. 50, No. 20, 2002
Es-Safi et al.
prevalence of that involving two anthocyanin moieties. The
polymerization process could obviously continue from the
tetramer involving only one anthocyanin unit, giving pentamers
and so on. The formation of more polymerized derivatives was
recently shown in a model solution system containing (+)-
catechin, malvidin 3-O-glucoside, and acetaldehyde, where a
pentamer containing four flavanol units and an anthocyanin
moiety was detected by LC/ESI-MS analysis (30). Each obtained
oligomer or polymer could involve at most two anthocyanin
moieties, whereas more flavanol units could be involved. This
explains the fact that the flavanol peak area decreased more
rapidly compared to that of the anthocyanin as observed in the
chromatographic profile recorded at 280 nm at the end of the
reaction (Figure 3A). This fact showed that during food storage
and aging, acetaldehyde could strongly contribute to the
formation of oligomeric colored pigments by bridging flavanol
and anthocyanin units and then participate in the stability of
the food color.
In addition to monomer flavanols, the acetaldehyde-induced
condensation was also shown to occur between procyanidins
and anthocyanins, where similar bathochromic shifts were
observed on the UV-visible spectra of the mixtures (31, 32).
Some of the bridged oligomeric colorless and colored com-
pounds formed from condensation between (+)-catechin, mal-
vidin 3-malvidin-3-O-glucoside, and acetaldehyde were detected
in grape-derived beverages (33, 34). Nevertheless, other antho-
cyanins are expected to behave similarly. This suggests that a
great diversity of products can be generated during food
processing, maturation, and storage. Their levels depend obvi-
ously on the nature and relative amount of flavanols and
anthocyanins present.
In food-processing technology, practices that increases ac-
etaldehyde formation, such as aeration, may promote polym-
erization of anthocyanins and catechins or tannins, thereby
improving color intensity and stability. Indeed, addition of
exogenous acetaldehyde, or that produced by a coupled autoxi-
dation of ethanol and phenolic compounds (35), provokes
copolymerization of anthocyanins and catechins or tannins and
then enhances color intensity. From a sensory point of view,
this polymerization is sometimes desirable, because it reduces
the tannins level and, hence, alleviates astringency and bitterness
(7, 36). However, it can cause turbidity and deposits, presumably
due to precipitation of larger molecular anthocyanin-tannin
complexes (4, 7, 19) or formation and precipitation of catechin-
acetaldehyde colloidal polymers (18, 20, 21). Thus, acetaldehyde
is known to have an impact on haze formation in beer through
reaction with catechins and preformed complex phenolics (37).
An appreciable presence of acetaldehyde in the free state (>5
mg/L) can destabilize a red wine by inducing a phenolic haze
and eventual deposition of condensed pigments, which occurs
very rapidly at higher levels of acetaldehyde (38).
Figure 9. HPLC chromatograms of a mixture of (+)-catechin and furfural
recorded with an electrospray mass detector (A). The mass spectrum of
the infused reaction (B) and the extracted ion current recorded at m/z
657 amu and corresponding to the colorless dimers are also pre-
sented (C).
the ethyl group and malvidin 3-O-glucoside (19). This was
demonstrated on the chromatographic profile of a more evolved
solution summarized in Figure 3C, which clearly showed that
the detected peaks absorbed in the visible region, meaning that
they contained the flavylium units.
As indicated above, the anthocyanin moiety plays the role
of ending chain in the polymerization process, which stops when
both chain ends were occupied by an anthocyanin unit. The
trimer (VI), which presents two malvidin 3-O-glucoside units
as chain ends, could not conduct directly to tetramers because
only one summit of the anthocyanin moiety could be involved
in the polymerization process, whereas the trimer (V) containing
two flavanols units could. Thus, two tetramer derivatives (VII
and VIII) could be obtained by linking an anthocyanin or a
flavanol moiety through an ethyl bridge. The depolymerization
and recombination processes observed above between the two
trimers could also occur between the formed tetramers with a
Reactions Involving Glyoxylic Acid. Another example
involving the interaction between phenolic compounds and
aldehydic derivatives is the evolution of (+)-catechin in iron-
catalyzed medium containing tartaric acid, which is transformed
by oxidation to glyoxylic acid (CHO-COOH) (39, 40). This
prompted us to study the role that glyoxylic acid could play as
an aldehydic compound in the polymerization of phenolic
compounds. It may be noted that this compound is a carboxylic
acid found in green fruits, seedlings, and young leaves and is
the starting point for the glyoxylate cycle.
The reaction between (+)-catechin and glyoxylic acid was
conducted in hydroalcoholic medium, and bridged derivatives
analogous to those obtained with acetaldehyde were obtained.

Interactions between Phenolic Compounds and Aldehydic Derivatives
J. Agric. Food Chem., Vol. 50, No. 20, 2002 5579
Figure 10. Mass spectra of the monomer (A) and dimer (B) intermediates and the corresponding extracted ion current chromatograms (A′, B′) detected
in a mixture of (+)-catechin and furfural.
LC/MS analysis of the incubated solution, conducted in the
negative ion mode, showed an m/z signal located at 635 amu
for the four major compounds 2a-d shown in the chromato-
graphic profile drawn in Figure 7A. This indicated that they
consisted of two (+)-catechin units bridged by a carboxymethine
group (Figure 2, IIA) as previously reported (40, 41). These
experiments showed that the formation of these dimers proceeds
via a mechanism similar to acetaldehyde-induced polymeriza-
tion, in which acetaldehyde is replaced by glyoxylic acid,
indicating that glyoxylic acid reacts in the same way in the very
first steps. However, various compounds with absorptions
between 440 and 460 nm were also detected in the case of
glyoxylic acid and were not observed with acetaldehyde.
To evaluate the effect of pH changes on the composition of
the incubated solutions, the condensation of (+)-catechin with
glyoxylic acid was monitored by HPLC in the pH range from
2.2 to 4.0. Quantification of all products was achieved on the
basis of peak areas at 280 nm, using (+)-catechin as standard.
The concentration of residual (+)-catechin is presented in
Figure 7B. (+)-Catechin concentration showed a gradual
decrease at all pH values, owing to the higher availability of
glyoxylic acid carbocation at lower pH values. This phenomenon
was also observed in the condensation of (+)-catechin with
acetaldehyde in the presence or absence of malvidin 3-O-
glucoside (19, 27, 28) and is in agreement with the postulated
mechanism (4, 18).
The four compounds accumulated gradually at the beginning
of the reaction and then decreased, probably as more polym-
erized products were formed. The figure also shows that among
the four formed dimers, compound 2a (8-8 isomer) was the one
formed at higher concentration, followed by compounds 2b and
2c (8-6 and 6-8 isomers), which were formed at similar amount,
and finally compound 2d (6-6 isomer), which was formed at
lower amount. All of these dimers were shown to evolve easily
to yellowish xanthylium salt derivatives with absorption maxima
located between 440 and 460 nm (42-44). In addition to these
yellowish compounds other derivatives with absorption maxima
around 300 nm were also detected. These compounds were
shown to be mono- and biformylated catechin derivatives (45).
When malvidin 3-O-glucoside was incubated in the presence
of (+)-catechin and glyoxylic acid, both colorless and colored
compounds, initially absent in the mixture, were obtained,
meaning that the anthocyanin and the flavanol competed in the
interaction. LC/DAD and ESI-MS analyses of the sample
conducted in the positive ion mode enabled the detection of
colorless along with the colored dimers, which were detected
as their negative ion at m/z 839 amu. However, it seems that
the process involving only flavanols and leading to colorless
bridged compounds was largely predominant compared to the
reaction involving the anthocyanin. Thus, the areas of the two
peaks corresponding to the colored dimers consisted of (+)-
catechin and malvidin 3-O-glucoside linked through a car-
boxymethine bridge (Figure 2, IIB) and were small compared
to those corresponding to the colorless ones (Figure 2, IIA).
Variations in relative amounts of the four dimers formed
during incubation of the mixture are reported in Figure 7C.

5580 J. Agric. Food Chem., Vol. 50, No. 20, 2002
Es-Safi et al.
Figure 11. Mass spectrum (A) and the corresponding extracted ion current
chromatogram (B) recorded at m/z 859 amu showing the two colored
dimers MF1 and MF2 detected in a mixture of (+)-catechin, malvidin 3-O-
glucoside, and furfural.
In addition to these dimers, which present an absorption
maxima around 545 nm as their analogous catechin-malvidin
ethyl-linked derivatives, other compounds showing similar UV-
visible spectra with a maximum at 500 nm were also observed.
These compounds result probably from a cycloaddition reaction
involving malvidin 3-O-glucoside. Similar malvidin-derived
pigments were detected in the case of acetaldehyde, and analo-
gous adducts involving malvidin 3-O-glucoside, procyanidin B3,
and acetaldehyde were also detected in a model solution system
(46). As the reaction was allowed to proceed, successive
condensations led to numerous oligomers and polymers, which
were eluted as a large unresolved bump in the end of the
chromatogram.
Figure 12. HPLC chromatograms of a mixture of (+)-catechin, malvidin
3-O-glucoside, and furfural recorded at 280 (A), 440 (B), 500 (C), and
520 nm (D) showing the formation of more polymerized colorless and
colored compounds.
These results indicated that polyphenols can react with
acetaldehyde and glyoxylic acid to form various oligomeric
colorless and colored bridged adducts. This demonstrates the
effect of these aldehydic derivatives on the polymerization
reactions and their influence on color change and astringency
and, by the way, their participation in browning reactions
observed in food-processing technology.
Reactions Involving Furfural. In addition to polyphenols,
other compounds such as carbohydrates can participate in the
alterations observed during heat processing or storage of
foodstuffs rich in reducing sugars. These compounds were also
shown to give rise to components such as furfural and its
derivatives, which were reported to be responsible for undesir-
able flavor in juices (47).
Figure 13. Comparison of (+)-catechin evolution when incubated with
furfural or HMF at pH 2.2.
The formation of furfural compounds and the increase of their
concentration during storage of fruit-derived juices are well
documented and are considered as indications of quality
deterioration and a degree of damage to the product caused by
excessive heat during processing or subsequent storage (48, 49).
HMF and furfural are the main furfural compounds used to
evaluate nonenzymatic browning in foods and have been used
as indicators of the Maillard reaction in numerous foodstuffs
such as juices, honey citrus products, infant milk, and breakfast
cereals (49, 54).

Interactions between Phenolic Compounds and Aldehydic Derivatives
J. Agric. Food Chem., Vol. 50, No. 20, 2002 5581
(+)-catechin units are linked by a furfuryl bridge as shown in
Figure 2 (IIIA).
In addition to these dimers, other oligomeric bridged com-
pounds were also detected. Thus, the formation of intermediate
furfuryl adducts of monomer (m/z 385 amu) and dimer deriva-
tives (m/z 753 amu) was observed as shown in the extracted
mass chromatographic profiles recorded, respectively, at m/z
385 and 753 amu (Figure 10). The detection of these intermedi-
ate adducts demonstrated the role of furfural in the polymeri-
zation process and suggested the formation of more polymerized
compounds. This was confirmed by the appearance of a bump
in the end of the chromatographic profile and showed that the
reaction evolved to more polymerized compounds, which finally
precipitate as a black solid in the assay vial. In addition, other
compounds presenting UV-visible spectra similar to those of
xanthylium salts with absorption maximum located around 440
nm were also observed either through LC/DAD or through LC/
ESI-MS analysis.
When malvidin 3-O-glucoside was added to the mixture,
oligomeric colorless and colored pigments involving both (+)-
catechin and anthocyanin moieties were detected, showing that
the two polyphenols competed in the condensation process as
was observed in the case of acetaldehyde and glyoxylic acid.
Among the obtained colored pigment adducts, two colored
compounds were observed. Their UV-visible spectra were
similar to that of malvidin 3-malvidin-3-O-glucoside but showed
an additional shoulder around 450 nm, and the wavelengths of
their maximum absorbance in the visible range (545 nm) were
significantly higher than that of malvidin 3-O-glucoside.
Figure 14. Mass spectrum (A) and corresponding extracted ion current
chromatograms (B) recorded at m/z 889 amu showing the two colored
dimers HM1 and HM2 detected in a mixture of (+)-catechin, malvidin
3-O-glucoside, and HMF.
Upon LC-MS analysis conducted in the negative ion mode,
a molecular ion at m/z 859 amu was observed for both pigments
(Figure 11A). This is consistent with one flavylium moiety
linked to one (+)-catechin moiety through a furfuryl bridge
(Figure 2, IIIB), in agreement with the previously postulated
mechanism concerning acetaldehyde (4, 18, 19). An extracted
ion current chromatogram recorded at m/z 859 amu (Figure
11B) showed the presence of the two colored adducts MF1 and
MF2, confirming thus the results obtained through LC/DAD
analysis.
When (+)-catechin was incubated with furfural, the mixture
underwent a progressive darkening, suggesting that a reaction
occurs and a possible contribution of such interactions to fruit-
derived foods darkening observed during storage (49). Monitor-
ing the evolution of the mixture by HPLC/DAD analysis showed
the appearance of new peaks in addition to those of the reactants
(Figure 8A). Among these compounds four major compounds
with absorption maxima at 280 nm were observed. The UV-
visible spectra of the major formed compounds recorded
between 250 and 500 nm were similar to that of (+)-catechin
with a maximum absorbance near 280 nm (Figure 8B),
suggesting thus that the original flavanol structure was retained
(55).
As may be expected, the evolution of the reaction allowed
successive condensations yielding numerous oligomers and
polymers. These were eluted as a large unresolved bump in the
end of the chromatogram obtained through LC/DAD analysis.
Analysis of the sample enabled the detection of colorless
oligomers along with the colored adducts, as shown in the
chromatographic profiles recorded at 280, 440, 500, and 520
nm (Figure 12). As shown, the chromatogram recorded at 280
nm (Figure 12A) showed residual (+)-catechin and furfural in
addition to a bump consisting probably of more polymerized
bridged derivatives. The chromatographic profile clearly shows
that the polymeric fraction of the analyzed solution predomi-
nantly consisted of colorless adducts detected at 280 nm as
opposed to yellowish or purple ones detected at 440 (Figure
12A) or 520 nm (Figure 12B), respectively. This fact showed
that the bridged oligomers obtained are not rapidly transformed,
yielding purple colored compounds. This was not observed in
the case of acetladehyde, where the peaks recorded at 280 and
520 nm showed equivalent intensities as shown in Figure 3A,C
in the case of (-)-epicatechin and malvidin 3-O-glucoside.
The total ion current chromatogram recorded when the
reaction was monitored by LC-MS indicated in particular the
presence of the four adducts F1-4 (Figure 9A). The mass
spectrum obtained in the negative ion mode when the reaction
was infused through a syringe allowed the detection of both
oligomers and intermediate ions (Figure 9B). Among the
detected compounds catechin ion (m/z 289 amu), monomer
intermediate ion (m/z 385 amu), dimer ion (m/z 657 amu), and
dimer intermediate ion (m/z 753 amu) can be observed in Figure
9B. In addition to each m/z value there were several peaks,
suggesting that they could be regio- and/or stereoisomers. Thus,
four dimeric adducts along with intermediate adducts in each
solution were detected by LC/ESI-MS analysis. This was
confirmed on the extracted ion current (XIC) chromatogram
profile recorded at m/z 657 amu (Figure 9C), where the presence
of four dimeric adducts can be observed. Mass spectrometric
analysis revealed thus that compounds F1-4 all had a molecular
weight of 658 and corresponded to a structure in which two
The fact that, in the case of furfural, the disappearance of
malvidin 3-O-glucoside was not accompanied by an equivalent
formation of bridged colored compounds showed that the
anthocyanin should probably suffer a degradation reaction in

5582 J. Agric. Food Chem., Vol. 50, No. 20, 2002
Es-Safi et al.
Figure 15. Phenolic reactions with aldehydes leading to colorless and colored compounds. Absorption maxima of the formed compounds are also
indicated.
addition to the condensation process involving (+)-catechin and
furfural. This fact also showed that the contribution of furfural
in color stabilization through anthocyanin condensation is
negligible compared to its participation in flavanol polymeri-
zation. Its more pronounced influence in the condensation of
(+)-catechin suggests that furfural could play a role in taste
transformation such as astringency decrease during storage or
aging of foods.
presence was also confirmed by LC/ESI-MS analysis with
detection of compounds showing mass spectra with [M+ - 2H]-
ion signals at m/z 667 amu and was also confirmed on the ion-
extracted mass chromatogram recorded at m/z 667 amu. The
detection of their corresponding xanthene derivatives at m/z 669
amu supports the proposed structures.
In the presence of HMF, malvidin 3-O-glucoside and (+)-
catechin underwent a polymerization reaction as revealed either
through LC/DAD or through LC/ESI-MS analysis, and detection
Reactions Involving HMF. When (+)-catechin was incu-
bated in the presence of HMF, reaction between the two
reactants occurred with formation of analogous oligomeric
compounds, which were detected upon LC/DAD analysis.
Comparison of the results obtained between the two furfural
derivatives showed that reaction was faster with furfural that
with HMF as shown in Figure 13, which clearly showed that,
with time, (+)-catechin decreased more rapidly with furfural
than with HMF. This may be due to the difference in reactivity
of both molecules as previously indicated (56, 57).
of colorless (λmax ) 280 nm) and colored compounds (λmax
)
545 nm) initially absent in the mixture was observed. The UV-
visible spectra of these colored compounds were similar to that
of malvidin 3-O-glucoside with a bathochromic shift of their
visible absorption maxima, which were located around 545 nm.
LC/ESI-MS analysis showed the presence of four dimeric
colorless (Figure 2, IVA) and two colored dimers (Figure 2,
IVB) adducts as their molecular ions located at m/z 687 and
889 amu, respectively. An extracted ion chromatogram recorded
at m/z 889 amu showed the presence of two dimeric adducts
HM1 and HM2 consisting of (+)-catechin and malvidin 3-O-
glucoside linked through an HMF bridge as shown on the
obtained extracted mass chromatogram (Figure 14).
Mass spectrometric analysis conducted in the negative ion
mode revealed the presence of four compounds with ion signals
at m/z 687 amu and corresponding to colorless dimers (Figure
2, IVA). Intermediate adducts of monomer (m/z 415 amu) and
dimer (m/z 783 amu) were also observed, and the formation of
more polymerized compounds was also observed in the case of
HMF by the appearance of a brown precipitate in the vial. Other
compounds with absorption maxima around 440 nm, corre-
sponding probably to xanthylium salts, were also observed. Their
CONCLUSION
Fruit-derived foods are generally complex mixtures able to
undergo, during storage, many different reactions that lead to

Interactions between Phenolic Compounds and Aldehydic Derivatives
J. Agric. Food Chem., Vol. 50, No. 20, 2002 5583
the formation of brown polymers. Whereas the color of a new
fruit-derived product is due to its high chemical content, the
subsequent color changes during aging involve generally
condensation of phenolic compounds. Direct condensation
between anthocyanins and flavanols is very slow. However,
rapid polymerization between anthocyanin and catechins or
tannins occurs in the presence of aldehyde derivatives, with
increased color intensity and stability, but further reaction with
polymerized catechin and tannins leads to instability, precipita-
tion, and decreased color. A number of competing reactions
involving polyphenols and aldehydes were shown to occur
during food processing, maturation, storage, and aging. The
results presented in this paper and summarized in Figure 15
showed the major role played by aldehydic compounds in the
condensation of flavanols giving various colorless and colored
polyphenolic compounds. The formation of such compounds
in a model solution system indicates their probable contribution
to the color transformation of fruit-derived foods.
which have different pH values and complex polyphenolic
composition and thus showed different evolutions and trans-
formations. It seemed that the formation of the products
described in this paper proceeded very quickly compared to
those probably formed in fruit-derived foods, which may be
relatively slow during the conservation and aging. Nevertheless,
it is expected that such experiments may at least enable
elucidation of the simpler polymerization products such as those
slowly formed during maturation and storage. The role of these
interactions in grape-derived foods color evolution may be
increased by the involvement of other polyphenolic compounds.
In particular, proanthocyanidins, which are the main grape
flavanol constituents, could also be involved in such reactions,
giving xanthylium salt derivatives. This may increase the
importance of such reactions in the color evolution of grape-
derived foods.
Although the reactions described above have been shown to
modify the properties of polyphenols, they are not sufficient to
explain the variations observed in the organoleptic properties
(taste and color) among the various types of fruit-derived foods.
As well, the relationships existing between structure and taste
(e.g., astringency and bitterness) of proanthocyanidins and
proanthocyanidin-derived molecules remain to be established.
Answers to these questions will require the use of different
complementary approaches, including chemical, sensory, and
statistical analysis.
The acetaldehyde-mediated condensation was found to occur
more generally, and glyoxylic acid, furfural, and HMF react in
the same way in the first stage of the reactions. Thus, similar
colorless (IA, IIA, IIIA, and IVA) and colored (IB, IIB, IIIB,
and IVB) derivatives were shown to be formed with the four
studied aldehydes. However, some of the dimers formed in the
case of glyoxylic acid, furfural, and HMF evolved to yellow
pigments, corresponding to xanthylium salts, whereas this was
not observed in the case of acetaldehyde.
Comparison of the results obtained with the studied aldehydic
derivatives showed that in terms of reactivity, reactions were
faster with acetaldehyde where purple derivatives were more
stable than the colorless adducts. The obtained results also
showed that in the case of acetaldehyde no compound with a
UV-visible spectrum similar to that of a xanthylium salt was
detected either through LC/DAD or through LC/ESI-MS
analysis. Interactions involving glyoxylic acid were slower than
those observed in the case of acetaldehyde, as shown by
comparison of Figures 1 and 7C. Colorless adducts were
predominant compared to the purple formed adducts and were
shown to proceed easily to more polymerized compounds and
to give various yellowish derivatives. Purple compounds were
detected with small peak areas compared to those obtained in
the case of acetaldehyde. In the case of furfural and HMF,
reaction was faster with the former but proceeded more slowly
than with acetaldehyde or glyoxylic acid. Purple compounds
involving flavanol and anthocyanin units were also detected,
and colorless compounds were shown to be predominant.
Finally, yellowish compounds with xanthylium salt skeletons
were also detected either by LC/DAD or by LC/ESI-MS
analysis. Their formation was confirmed by detection of their
corresponding xanthene derivatives.
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Received for review February 21, 2002. Revised manuscript received
June 4, 2002. Accepted July 2, 2002. N.E.-S. thanks the Third World
Academy of Sciences (Project 00-193 RG/CHE/AF/AC) for partial
financial support.
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