The effect of MgCl2 on the kinetics of the Maillard reaction in both aqueous and dehydrated systems

Article abstract of DOI:10.1016/j.foodchem.2009.04.084

The modification of Maillard reaction kinetics induced by MgCl₂ was evaluated in both liquid and dehydrated model systems with special emphasis on the interactions of the salt with water and/or the sugars. In liquid trehalose systems, browning is accelerated by the presence of MgCl₂ due to the increased sugar hydrolysis and to the reduction of water mobility caused by the salt (shown by the decrease in ¹H NMR relaxation times T₂), counteracting the inhibitory effect of water on the reaction. In water restricted trehalose systems, MgCl₂ inhibited the Maillard reaction. The salt-sugar interactions, manifested by the delayed sugar crystallization, decreased the reaction rate by affecting the reactivity of reducing sugars. Molecular and supramolecular effects in the presence of MgCl₂ have been observed in the present work, and must be taken into account considering high technological interest in finding strategies to either inhibit or enhance the Maillard reaction depending on the application.

Full text of DOI:10.1016/j.foodchem.2009.04.084

Food Chemistry 118 (2010) 103–108
Contents lists available at ScienceDirect
Food Chemistry
journal homepage: www.elsevier.com/locate/foodchem
The effect of MgCl2 on the kinetics of the Maillard reaction in both aqueous
and dehydrated systems
1
1
,2
*
Silvia B. Matiacevich , Patricio R. Santagapita , María del Pilar Buera
Departamentos de Industrias y de Química Orgánica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, 1428, Buenos Aires, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
The modification of Maillard reaction kinetics induced by MgCl2 was evaluated in both liquid and dehy-
drated model systems with special emphasis on the interactions of the salt with water and/or the sugars.
In liquid trehalose systems, browning is accelerated by the presence of MgCl2 due to the increased sugar
hydrolysis and to the reduction of water mobility caused by the salt (shown by the decrease in 1H NMR
relaxation times T2), counteracting the inhibitory effect of water on the reaction. In water restricted tre-
halose systems, MgCl2 inhibited the Maillard reaction. The salt–sugar interactions, manifested by the
delayed sugar crystallization, decreased the reaction rate by affecting the reactivity of reducing sugars.
Molecular and supramolecular effects in the presence of MgCl2 have been observed in the present work,
and must be taken into account considering high technological interest in finding strategies to either
inhibit or enhance the Maillard reaction depending on the application.
Received 10 February 2009
Received in revised form 12 March 2009
Accepted 23 April 2009
Keywords:
Maillard reaction
MgCl2
Transversal relaxation time
Browning
Ó 2009 Elsevier Ltd. All rights reserved.
Fluorescence
Browning inhibition
1. Introduction
the Maillard reaction in fructose-asparagine model systems; how-
ever, ionic associations between the charged groups of asparagine
The rate of the Maillard reaction and the nature of its products
are governed by the immediate chemical environment of the reac-
tants, defined by the chemical composition of the system (water
content, pH, presence and type of buffer salts, temperature and
exposure to light) (Baisier & Labuza, 1992; Bell, 1997; Cerruti, Res-
nik, Seldes, & Ferro Fontán, 1985; Petriella, Chirife, Resnik, & Loz-
ano, 1988). The development of fluorescent products from the
Maillard reaction (excitation 340–370 nm/emission 420–475 nm)
was described during the storage of different types of food prod-
ucts and model systems. The application of fluorescence measure-
ment is considered a potential tool for addressing key problems of
food deterioration as it is an early marker or index of the biomolec-
ular damage (Matiacevich, Santagapita, & Buera, 2005). The behav-
iour of model systems in the presence of divalent cations (Mg2+ or
Ca2+) was previously studied. Salts may affect important properties
of sugar systems, including fluorescence and browning develop-
ment from the Maillard reaction, the kinetics of sugar crystalliza-
and Ca2+ were not observed. Sugar–cation (Morel-Desrosier, Lher-
met, & Morel, 1991) and brown pigment-cation complexes (O’Brien
& Morrissey, 1997) have been shown to form in solution. The
development of strategies for inhibiting the Maillard reaction
employing additives other than sulphites (Lester, 1995) has a high
technological interest. Alternatively, in other applications it may be
beneficial to enhance the Maillard reaction. However, there has
been little work carried out on the role of food-compatible salts
in the kinetics of the Maillard reaction. The effect of cations can
be analysed through their impact on the modification of solid–
water interactions, and water availability, which govern and mod-
ulate the kinetics of the Maillard reaction (Acevedo, Schebor, &
Buera, 2006).
Nuclear magnetic resonance (NMR) is a very useful technique
for investigating the behaviour of water mobility in foods
(Schmidt, 2007). The differences in relaxation times of protons
from different environments have been exploited in NMR studies
to measure the relative amounts of water with different degrees
of interaction with solids and consequently, with different mobility
(Acevedo et al., 2006; Farroni, Matiacevich, Guerrero, Alzamora, &
Buera, 2008). The spin–spin or transversal relaxation time con-
stants of protons (T2) are related to the molecular mobility of pro-
tons in the system. The relaxation processes are affected by the
chemical exchange taking place in sites of different mobility. Pro-
tons of water molecules which interact tightly with solids, and
therefore have reduced mobility (although they are still exchange-
able), show small T2 values, whereas protons which are readily
tion and sugar hydrolysis (Matiacevich
&
Buera, 2006;
Santagapita & Buera, 2006; Santagapita & Buera, 2008; Schebor,
Burin, Buera, & Chirife, 1999). Gökmen & Sßenyuva (2007) showed
that acrylamide formation is prevented by divalent cations during
* Corresponding author. Tel./fax: +54 11 4576 3366.
E-mail address: pilar@di.fcen.uba.ar (M. del P. Buera).
1
Research fellow of Consejo Nacional de Investigaciones Científicas y Técnicas de la
República Argentina (CONICET).
2
Member of CONICET.
0308-8146/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.foodchem.2009.04.084

104
S.B. Matiacevich et al. / Food Chemistry 118 (2010) 103–108
mobile have relatively long T2 values (Kuo, Gunasekaran, Johnson,
& Chen, 2001).
method (Wiener Lab. S.A.I.C., Rosario, Argentina) described previ-
ously (Bergmeyer & Grassi, 1983).
The purpose of the present work was to analyse the modifica-
tion of Maillard reaction kinetics induced by the presence of MgCl2
in both liquid and dehydrated model systems, with special empha-
sis on the interactions of the salt with water and/or the sugars.
2.4. Fluorescence quantum yield
Sugar systems solutions (70% w/v) were diluted in order to ob-
tain absorbance below 0.05 to avoid inner filter effects (Lakowicz,
1999). Four dilutions (in duplicate) were prepared for each time of
incubation. Fluorescence quantum yields (UF) were determined
using quinine sulphate in 0.1 N sulphuric acid as a reference fluo-
rophore of known quantum yield (UF = 0.546) (Lakowicz, 1999),
using the classical Eq. (2.1):
2. Materials and methods
2.1. Preparation of model systems
2.1.1. Liquid systems
ISk ARk n2S
Liquid systems were prepared from 5% to 70% w/v of
D
(+)glu-
-trehalose (Hayashibara
L-glycine (Merck) with and
UFS
¼
UFR
ꢂ
ꢂ
ꢂ
ð2:1Þ
IkR
nR2
AkS
cose (Merck, Darmstadt, Germany) or
a-a
Co, Ltd., Okayama, Japan) and 0.5%
where USk and URk are the fluorescence quantum yield of the sample
(S) and the reference (R), respectively; ISk and IkR are the integrated
areas of their fluorescence spectra; ASk and AkR are their absorbances
at the excitation wavelength k, and nS and nR are the refractive in-
dex of each solution. In Eq. (2.1), it is assumed that the sample
and reference are excited at the same wavelength.
without MgCl2 (Mallinckrodt Chemical Works, St. Louis, USA), in
a 5:1 sugar:salt molar ratio; controls without MgCl2 and glycine
were also prepared. Aliquots (2.5 ml) of each model system were
placed in 5 ml vials. The systems were prepared in 0.1 M phos-
phate buffer, pH 5 (Merck, Darmstadt, Germany). All reactants
were analytical grade. The vials were hermetically sealed and
stored at 70 1 °C in a forced air convection oven.
2.5. Nuclear magnetic resonance (NMR) measurements
2.1.2. Solid systems
Solid systems consisted of freeze-dried solutions containing
20% w/v trehalose and 1% w/v L-glycine. The systems were pre-
Transversal or spin–spin relaxation times (T2) were measured
by time resolved proton nuclear magnetic resonance (1H NMR) in
a Bruker Minispec mq20 (Bruker Biospin Gmbh, Rheinstetten, Ger-
many) with a 0.47 T magnetic field operating at a resonance fre-
quency of 20 MHz. Proton populations of different mobility were
measured using two spin-echo sequences: (a) Hahn (Hahn,
1950), for protons from solids or from water strongly interacting
with the solid matrix, and (b) Carr–Purcell–Meiboom–Gill (CPMG)
(Carr & Purcell, 1954; Meiboom & Gill, 1958), for more mobile pro-
tons. All samples were previously equilibrated at 25.00 0.01 °C in
a thermal bath (Haake, model Phoenix II C35P, Thermo Electron
Corporation Gmbh, Karlsruhe, Germany).
pared in 0.1 M phosphate buffer at pH 5. The salt MgCl2 was added
in a 5:1 sugar:salt molar ratio; controls without MgCl2 and without
glycine were also prepared. Aliquots (2.5 ml) of each model were
placed in 5 ml vials, frozen at ꢀ26 °C for 24 h and immersed in li-
quid nitrogen immediately before freeze-drying. Freeze-drying
was performed in a Heto Holten A/S, cooling trap model CT 110
freeze-dryer (Heto Lab Equipment, Denmark) operating at a con-
denser plate temperature of ꢀ111 °C and a chamber pressure of
4 ꢁ 10ꢀ4 mbar. After freeze-drying, the systems were transferred
to vacuum desiccators and exposed to different relative vapour
pressure conditions (RVP): 22%, 43%, 52%, 84% and 97% (Greenspan,
1977) at 25 1 °C for 2 weeks. After humidification, the vials were
hermetically sealed and stored at 70 1 °C in a forced air convec-
tion oven.
2.5.1. Measurements using a Hahn sequence
Hahn spin-echo consists of a (90°–s–180°) sequence. This meth-
od allows for analysis of the less mobile protons interacting with
the solid matrix; the more mobile protons could not be analysed
using this sequence due to diffusion problems during measure-
ment at long times, caused by inhomogeneities in the magnetic
field. Trehalose dehydrated systems humidified between 22% and
97% RVP were analysed using this sequence. The following settings
were used: scans = 4, recycle delay = 1.5 s, gain = 75 dB, number of
2.2. Determination of browning and fluorescence development
The progress of the Maillard reaction was followed by a brown-
ing index defined as absorbance units at 445 nm (UV–vis 1203
spectrophotometer, Shimadzu, Kyoto, Japan) multiplied by the
dilution factor, and by fluorescence intensity with excitation at
340 nm/emission 492 nm (model USB 2000 spectrofluorometer,
Ocean Optics Inc., FL, USA). The samples employed for the determi-
nation of fluorescence were diluted in order to avoid inner filter ef-
fect, with absorbance values lower than 0.1 at excitation
wavelength (340 nm). Since trehalose is a non-reducing sugar, its
participation in Maillard reaction can only occur after hydrolysis.
Browning and fluorescence development as a function of reac-
tion time followed a quadratic dependence, which indicates a half
order reaction, which results from the combination of two steps of
zero and first order (Matiacevich & Buera, 2006).
points = 20, time for decay curve display = 1 s and interpulse (s)
range of 0.001–0.5 ms. The interpulse range was selected in order
to record the complete relaxation of the signal. No phase cycling
was used. A polyvinylpyrrolidone (PVP of 58 000 Da) system equil-
ibrated at 43% RVP was used for the automatic update of the equip-
ment which tunes the pulse duration, detection angles, gain and
magnetic field homogeneity.
2.5.2. Measurements using a CPMG sequence
Liquid systems were analysed using a CPMG sequence, which
h
i
consists of 90ꢃx—
s
— 180ꢃy—
s
—echo—
s
sequence, with the follow-
n
ing setting:
s
= 0.5, scans = 4, number points = 256, dummy
The confidence intervals for browning and fluorescence values
were 3% and 5% for 95% certainty, respectively.
shots = 15, gain = 68 dB; phase cycling was used. The automatic
update of the equipment was performed employing a 40% w/v tre-
halose system without thermal treatment.
2.3. Disaccharide hydrolysis
For both sequence measurements, an exponential function (as
stated below in Eq. (2.2)) was found to fit the experimental data
adequately, from which transverse relaxation time constant (T2)
was obtained.
Sugar hydrolysis was analysed by measuring the amount of glu-
cose released during the incubation time by means of an enzymatic

S.B. Matiacevich et al. / Food Chemistry 118 (2010) 103–108
105
2Þ
A ¼ Aðꢀt=T
ð2:2Þ
colour
changes:
uncoloured ? yellow ? golden ? cinna-
mon ? reddish brown. The type and sugar concentration and the
presence of MgCl2 were critical for the rate of pigment and fluores-
cent compounds formation. In the model systems without amino
acid (control samples), no increase in the absorbance or the fluo-
rescence was detected, indicating that there was no contribution
to colour or fluorescence due to caramelization in the experimental
time frame. The decreasing order for browning development was
found according to sugar reactivity/stability: glucose > trehalose
(Burton, Mc Weeny, Pandhi, & Biltcliffe, 1962; O’Brien & Morrissey,
1997) in all analysed sugar concentrations, as shown in Fig. 1a and
c for the 70% w/v samples. In liquid glucose systems, inhibitory ef-
fects of MgCl2 on browning and development of fluorescent prod-
where A is the signal amplitude that is proportional to the amount
of protons in the sample relaxing after the pulse sequence. This
mono-exponential model was used for the liquid samples and for
most of the dehydrated systems, obtaining the contributions of pro-
tons in the most representative fraction. A bi-exponential model fit-
ted some dehydrated systems with high water content
(corresponding to samples at 97% and 84% RVP containing MgCl2),
but it is important to note that, for these samples, the signal ampli-
tude of the detected second proton fraction represented a very
small contribution to the total proton population, and the mono-
exponential approach was selected as the best model for compara-
tive purposes.
ucts, observed for the 70% w/v samples in Fig. 1a and
b
(respectively), were also noticed in the whole composition range
studied (5–70% w/v), extending our previous results for 5% and
40% w/v samples (Matiacevich & Buera, 2006; Santagapita & Buera,
2006). However, as shown in Fig. 1c and d for the 70% w/v systems,
an accelerating effect of MgCl2 was observed for both browning
and development of fluorescence in liquid systems containing tre-
halose, in the whole composition range studied. Fig. 2 shows the
kinetic constants for browning development in trehalose systems,
calculated through a quadratic model, plotted as a function of dif-
ferent water contents. Although trehalose is very stable to hydro-
lysis (Santagapita & Buera, 2006; Schebor et al., 1999), it was
hydrolysed during heat treatment at 70 °C and the hydrolysis rate
was accelerated by the presence of MgCl2 (Santagapita & Buera,
2006), as confirmed by glucose analysis (data not shown). For
example, for the range between 5% and 40% w/v trehalose system,
the rate of trehalose hydrolysis was lower than 1% in systems with-
out salt and increased to 2.3–2.5% in the presence of MgCl2. Pig-
ment development through the Maillard increased accordingly,
as observed in Fig. 1c and d.
2.6. Determination of water content
The total water content of the trehalose solid rehumidified sys-
tems was determined gravimetrically by difference in weight be-
fore and after drying in a vacuum oven for 48 h at 96 2 °C.
These drying conditions were selected in previous studies with
samples of similar composition (Mazzobre, Longinotti, Corti, &
Buera, 2001) and they were adequate to determine water content
in the studied systems with a confidence interval of 6% for a 95%
certainty. Duplicate analyses were performed and the average of
the results is reported on a dry basis (db).
2.7. Thermal transitions measurements
Melting events were determined by dynamic differential scan-
ning calorimetry (DSC), in the temperature range from 20 to
120 °C, by means of a Mettler Toledo model 822 equipment (Met-
tler Toledo AG, Greifensee, Switzerland) at a heating rate of 10 °C/
min. Melting temperature values were taken as peak temperatures.
All measurements were made in duplicate with 10–25 mg sample
3.1.2. Solid systems
mass using hermetically sealed aluminium pans of 40 ll inner vol-
In the freeze-dried trehalose samples humidified at relative va-
pour pressures (RVPs) in the range 22–97%, the development of
fluorescence was parallel to the development of browning, and
the hydrolysis rate increased in the presence of salt as was seen
in the analysed liquid systems (also in agreement with Santagapita
& Buera, 2006). In the samples containing MgCl2 at 10% (db) water
content, only 1.1% trehalose was hydrolysed after 24 h at 70 °C. Un-
der the same conditions, disaccharide hydrolysis was 0.13% for tre-
halose samples without salt. However, contrary to the results
observed in trehalose liquid samples, in solid trehalose samples
the presence of MgCl2 delayed browning development, as shown
in Fig. 2. It is clearly shown that MgCl2 inhibited the Maillard reac-
tion at low water contents in solid trehalose systems (Fig. 2, left
side of the dotted line), but in liquid systems, the presence of the
salt increased the reaction rate (Fig. 2, right side of the dotted line,
and Fig. 1c). Thus, the effect of MgCl2 on browning development in
trehalose samples was dependent on the state of the systems.
ume (Mettler Toledo AG). An empty pan was used as a reference
and average values are reported. The instrument was calibrated
using standard compounds (indium and zinc) of defined melting
point and heat of melting (DHm). All thermograms were analysed
using STARe Software v. 6.1 (Mettler Toledo AG). The samples were
initially amorphous as determined by the absence of endothermal
peaks in the DSC scans. The amount of trehalose dihydrate formed
(degree of crystallization, /) during the storage time was calcu-
lated from the ratio of the area of the endothermic melting peak
of the sample and the calorimetric enthalpy of the melting of pure
trehalose dihydrate measured under the same conditions in a dy-
namic DSC run, as Eq. (2.3).
D
Hm
/ ¼
ꢄ 100
ð2:3Þ
D
HmT
where
HmT is the heat of melting of pure trehalose (139 J/g) measured
D
Hm is the heat of melting of trehalose in the sample and
D
under the same conditions (Santagapita & Buera, 2006). The relative
error for / (calculated with a 95% confidence interval) was about
10% of the / value.
3.2. Fluorescence characteristics and quantum yield study
In order to analyse the effects associated with sugar type and
their interactions with MgCl2 on the Maillard reaction kinetics in
liquid systems shown in Fig. 1, fluorescence characteristics of the
samples were studied. In agreement with previous studies (Mati-
acevich et al., 2005), the maximum wavelength for emission was
450 nm in all systems, and the fluorescence development at zero
time of incubation was negligible. Fig. 3 shows the fluorescence
emission spectra of trehalose or glucose systems with and without
MgCl2, after 0 and 120 h of storage at 70 °C. As seen in this figure,
the spectral characteristics were independent of the studied
3. Results and discussion
3.1. Maillard reaction development: fluorescence and absorbance data
3.1.1. Liquid systems
Browning and fluorescence development of the liquid systems
increased with increasing storage time at 70 °C, for the whole com-
position range studied (5–70% w/v), as shown in Fig. 1 (a–d). The
70% w/v sugar systems, for example, showed corresponding visual

106
S.B. Matiacevich et al. / Food Chemistry 118 (2010) 103–108
Fig. 1. Browning and fluorescence development for 70% w/v glucose (a, b), or trehalose (c, d) systems with MgCl2 (open symbols) and without MgCl2 (closed symbols) as a
function of storage time at 70 °C. Excitation/emission 340/450 nm. T, trehalose; G, glucose; M, MgCl2. Bars on symbols represent the 95% confidence interval.
crease of the fluorophores quantum yield. The same conclusion
could be obtained for trehalose systems (Fig. 1d), considering that
trehalose must be hydrolysed to two glucose units in order to par-
ticipate of the Maillard reaction.
Therefore, in agreement with previous studies, the presence of
MgCl2 affected the kinetics of the reaction without affecting the
chromatic and fluorescent spectral characteristics of samples (Mat-
iacevich & Buera, 2006; Santagapita, Matiacevich, & Buera, in
press) or the fluorescence quantum yield properties of the
fluorophores.
3.3. Proton mobility studies by NMR
Fig. 2. Kinetic constants for the Maillard reaction (obtained by a quadratic model)
In order to evaluate the possible contribution of water–salt
interactions in the results observed in Figs. 1 and 2, proton mobil-
ity in each liquid system was studied by 1H NMR transversal relax-
ation times (T2). In liquid systems, T2 values were calculated after
the spin-echo CPMG sequence by a mono-exponential decay model
and the results are shown in Fig. 5. With increasing sugar concen-
tration, T2 values diminished for all sugar systems, indicating a re-
duced mobility of water protons. As also observed in Fig. 5, T2
values were not affected by MgCl2 in systems containing glucose
(i.e., water–glucose interaction was unmodified by salt), whilst in
trehalose systems they were between 6% and 14% lower in the
presence of salt, reflecting a reduced mobility. The T2 values ob-
tained indicated that the MgCl2 interaction affects water mobility
in the trehalose samples, but not in glucose systems. Since an
inhibitory effect of water has been observed on the Maillard reac-
tion (Acevedo et al., 2006; Labuza, 1994), the accelerating effect of
MgCl2 on the reaction rate observed in the disaccharide samples in
Figs. 1 and 2 can be associated with water–salt interactions.
According to Miller and de Pablo (2000), the local environment
as a function of water content for trehalose systems. Solid lines correspond to sugar
systems without salt and dashed lines to MgCl2 -containing systems. T, trehalose;
M, MgCl2. Bars on symbols represent the 95% confidence interval.
system, indicating that neither the type of sugar nor the presence
of salt affected the type of fluorophore obtained in each system.
The fluorescence quantum yield, which is defined as the ratio
between the number of photons involved in the emission and the
total number of excited photons, may provide information on
molecular structural properties of the fluorophores and their inter-
actions. The fluorescence quantum yield was calculated for the
samples as a function of heating time at 70 °C, in order to further
analyse the effect of MgCl2 on the Maillard reaction (shown in Figs.
1 and 2). As shown in Fig. 4, the fluorescence quantum yield was
constant as a function of heating time in glucose systems after a
slight initial increase. It is important to note that the presence of
MgCl2 did not affect the quantum yield. Thus, the increase of fluo-
rescence observed in Fig. 1b is due to the increasing concentration
of fluorophores with increased incubation time, and not to an in-

S.B. Matiacevich et al. / Food Chemistry 118 (2010) 103–108
107
Fig. 3. Fluorescence emission spectra of 70% w/v sugar systems heated at 70 °C for 120 h (-.-.-.-.-. glucose, and —— trehalose systems; grey curves correspond to MgCl2
containing systems). The curve labelled as t = 0 h, G or T corresponds to the fluorescence for zero time of incubation for all sugar systems. T, trehalose; G, glucose; M, MgCl2.
Table 1
Degree of trehalose crystallization (/) obtained by differential scanning calorimetry
for systems of different water contents (w.c., in % of dry basis) with or without MgCl2.
W.C. 2 (% db)
/ (%)*
w.o. MgCl2
with MgCl2
11
24
27
32
38
86
88
80
6
61
54
35
*
D
HmT
Hm
/ ¼
ꢄ 100; where
D
Ym is the melting enthalpy of trehalose at a given time
D
and
DYmT is the melting enthalpy of pure trehalose (Santagapita & Buera, 2006). The
relative error for / (calculated for a 95% confidence interval) was about 10% of the /
value.
Fig. 4. Fluorescence quantum yield as a function of time of incubation for glucose
systems with (open symbols) and without (closed symbols) MgCl2. Excitation/
emission 340/450 nm. G, glucose; M, MgCl2. Bars on symbols represent the 95%
confidence interval.
systems the reaction is accelerated by MgCl2, whilst it is delayed in
the glucose systems in the presence of salt. Thus, it is not an effect
of the type of reactants involved in the reaction, but of the kind of
water-matrix interactions: whilst in the glucose systems the
water–glucose interactions were not affected by the presence of
MgCl2 (as shown by 1H NMR relaxation times), in the trehalose sys-
tems a decreased relaxation time indicated lower water mobility in
the presence of salt. A probable interaction between Mg2+ and
phosphate buffer, which may cause the inhibition of Maillard reac-
tion, has been reported (Akagawa, Miura, & Suyama, 2002) and
must also be taken into account.
In other set of experiments, in the trehalose solid systems (with
and without MgCl2), the spin–spin 1H NMR relaxation times were
obtained by the spin-echo Hahn sequence (indicated as T2H) in
samples at different RVP. The T2H values in samples of similar
water content (between 10% and 20% db) were 25
2 ls either in
Fig. 5. 1H NMR spin–spin relaxation times (T2) obtained by a mono-exponential
decay model, using a CPMG sequence as a function of sugar concentration for liquid
systems. T, trehalose; G, glucose; M, MgCl2. Bars on symbols represent the 95%
confidence interval.
the presence of MgCl2 or without the salt, showing no differences
between them. The T2H values obtained correspond to matrix and
water protons displaying strong interactions with the matrix, and
no changes in their mobility could be detected by the presence of
MgCl2. Therefore, the water–salt interactions manifested by the
relaxation times from 1H NMR in the trehalose solid systems could
not explain the inhibitory effect of salts on the browning kinetics.
Of the dehydrated systems, only the trehalose–MgCl2 systems at
RVP 84% and 97% and the trehalose systems at 97% showed a sec-
ond set of T2H with values higher than 1 ms, which correspond to
water molecules of weak interactions with the solid matrix. It is
to be noted that the effect of MgCl2 in these solid systems (in which
mobile water molecules were detected) was not as important on
the browning kinetic constants of these systems as at lower water
contents (Fig. 4).
of the ions contained in a trehalose system has more water mole-
cules compared to those uniformly distributed in water, which can
explain the lower T2 values obtained in the disaccharide-salt con-
taining systems.
In the case of glucose liquid systems (Fig. 1a and b) the retard-
ing effect of MgCl2 can be attributed to sugar–salt interactions. The
formation of sugar-cation complexes, which may affect the sugar
availability for the reaction, has been previously reported and de-
pends on the tautomeric forms and spatial position of the hydroxyl
groups of the sugar (Angyal, 1973). It is to be noted that in treha-
lose systems, the reactant participating in the Maillard reaction is
glucose, as in the pure glucose systems. However, in the trehalose
Another effect of the salt interactions in restricted water envi-
ronments could be manifested by the modification of the sugar

108
S.B. Matiacevich et al. / Food Chemistry 118 (2010) 103–108
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crystallization kinetics by MgCl2. Table 1 shows that the degree of
trehalose crystallization (/) in solid trehalose systems containing
MgCl2 was lower than in samples without salt. The delay of sugar
crystallization by the presence of salts in supercooled systems has
been previously reported (Longinotti, Mazzobre, Buera, & Corti,
2002; Santagapita & Buera, 2008) and may be explained by dy-
namic water–salt–sugar interactions, which take place at a molec-
ular level and are related to the charge/mass ratio of the cation
present. An interaction/complexation between the magnesium cat-
ion and the Maillard reaction products has also been reported
(O’Brien and Morrissey, 1997) and is not discarded as another fac-
tor influencing molecular mobility.
Labuza, T. P. (1994). Interpreting the complexity of the kinetics of the Maillard
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In conclusion, the Maillard reaction kinetics can be affected by
the presence of salts to a different degree, depending on the type
of sugar present (either acting as reactant or as part of the solid
matrix) and the state of the system. Molecular and supramolecular
effects of the presence of MgCl2 have been observed in this work.
Due to the inhibitory effect of water on the Maillard reaction,
modifications in water–solids interactions promoted by salts could
be responsible for changes in the reaction rates. In liquid trehalose
systems (or when mobile water protons were detected), the
browning reaction is accelerated by the presence of MgCl2 due to
the increased sugar hydrolysis and the reduction of water mobility
caused by the salt, counteracting the inhibitory effect of water on
the Maillard reaction. On the other hand, in water restricted
(freeze-dried) trehalose systems, MgCl2 inhibited the Maillard
reaction. In this case, the salt–sugar interactions, manifested by
the delayed sugar crystallization, decreased the reaction rate by
affecting the reactivity of reducing sugars.
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This work was supported by Agencia Nacional de Promoción
Científica y Tecnológica (PICT 20545), CONICET (PIP 5977) and Uni-
versidad de Buenos Aires (X024).
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