Degradation of ascorbic acid in ethanolic solutions

Article abstract of DOI:10.1021/jf3032342

Ascorbic acid occurs naturally in many wine-making fruits. The industry also uses ascorbic acid as an antioxidant and color stabilizer in the making of alcoholic beverages including white wine, wine cooler, alcopop, and fruit liqueur. However, the degradation of ascorbic acid itself may cause browning and the deterioration of color quality. This study was aimed to monitor the degradation of ascorbic acid, the formation of degradation products, and the browning in storage of ascorbic acid containing 0-40% (v/v) ethanolic solutions buffered at pH 3.2 as models of alcoholic beverages. The results show that ascorbic acid degradation in the ethanolic solutions during storage follows first-order reaction, that the degradation and browning rates increase with the increase of ethanol concentration, that the activation energy for the degradation of ascorbic acid is in the range 10.35-23.10 (kcal/mol), that 3-hydroxy-2-pyrone is an indicator and a major product of ascorbic acid degradation, and that aerobic degradation pathway dominants over anaerobic pathway in ascorbic acid degradation in ethanolic solutions.

Full text of DOI:10.1021/jf3032342

Article
pubs.acs.org/JAFC
Degradation of Ascorbic Acid in Ethanolic Solutions
Hsin-Yun Hsu, Yi-Chin Tsai, Chi-Chang Fu, and James Swi-Bea Wu*
Graduate Institute of Food Science and Technology, National Taiwan University, Taipei 10672, Taiwan
ABSTRACT: Ascorbic acid occurs naturally in many wine-making fruits. The industry also uses ascorbic acid as an antioxidant
and color stabilizer in the making of alcoholic beverages including white wine, wine cooler, alcopop, and fruit liqueur. However,
the degradation of ascorbic acid itself may cause browning and the deterioration of color quality. This study was aimed to
monitor the degradation of ascorbic acid, the formation of degradation products, and the browning in storage of ascorbic acid
containing 0−40% (v/v) ethanolic solutions buffered at pH 3.2 as models of alcoholic beverages. The results show that ascorbic
acid degradation in the ethanolic solutions during storage follows first-order reaction, that the degradation and browning rates
increase with the increase of ethanol concentration, that the activation energy for the degradation of ascorbic acid is in the range
10.35−23.10 (kcal/mol), that 3-hydroxy-2-pyrone is an indicator and a major product of ascorbic acid degradation, and that
aerobic degradation pathway dominants over anaerobic pathway in ascorbic acid degradation in ethanolic solutions.
KEYWORDS: ascorbic acid, degradation, kinetics, ethanol, wine, browning
difference in chemical-physical characteristics between juice and
alcoholic beverages.9 Ethanol concentration has been recog-
nized as a major factor that affects Maillard reaction mechanism
and browning rate, chlorophyll stability, and the browning rate
in ascorbic acid-catechin solutions.23−26
INTRODUCTION
■
Ascorbic acid is among the major natural nutritional
components that widely exist in fruits. In aqueous systems,
ascorbic acid is very unstable and easily degraded in aerobic or
anaerobic pathway.1 The major factors that affect the stability of
ascorbic acid in the processing and storage of fruit juice include
oxygen, temperature, pH value, water activity, light, ionic
strength, and sugar species.2−13
Chuang et al. constructed ethanolic solutions containing
ascorbic acid and catechin to model for alcoholic beverages
containing both ascorbic acid and phenolic compounds.26 They
reported that an increase in ethanol concentration in the model
solutions accelerated ascorbic acid degradation and 3OH2P
formation while decelerated browning, and suggested that
3OH2P may not be a good indicator of browning in the
aqueous-ethanolic system containing ascorbic acid and
catechin. They also postulated that the acceleration in browning
is resulted from the reaction between catechin and L-xylosone,
the intermediate product of aerobic ascorbic acid degradation.
In alcoholic beverages, ascorbic acid and phenolic com-
pounds are usually present together. In such an environment,
ascorbic acid may degrade following either its own pathway or a
pathway involving its reaction with phenolic compounds.
Presumably, the former may prevail in alcoholic beverages
containing an overwhelmingly higher amount of ascorbic acid
as compared with that of the active phenolic compounds.
Knowledge pertaining to the degradation of ascorbic acid in
aqueous-ethanolic system in the absence of phenolic com-
pounds would be helpful in composing the complete scheme of
browning in alcoholic beverages. The present study was
therefore initiated.
In the presence of oxygen, ascorbic acid loses electrons and
hydrogen ions to form the unstable intermediate compound
dehydroascorbic acid as the first step of degradation.
Dehydroascorbic acid is quickly hydrolyzed to 2,3-diketo-L-
gulonic acid, which is then decarboxylated into L-xylosone. L-
Xylosone is further converted into other carbon compounds as
the degradation products of ascorbic acid.13 These degradation
compounds may interact with food ingredients to cause
browning, the alteration of flavor, and the reduction of product
quality.14 Shinoda et al. monitored ascorbic acid degradation
and browning in a model solution of orange juice.15·16 They
concluded that furfural, 2-furoic acid, 5-hydroxymaltol, 3-
hydroxy-2-pyrrone (3OH2P) and hydroxymethylfurfural
(HMF) are the major degradation products; that 3OH2P and
furfural are browning contributors; and that 3OH2P can be an
indicator of ascorbic acid degradation in the aqueous system.
Numerous reports are relevant to the degradation of ascorbic
acid in aqueous food while much fewer literatures have dealt
with it in aqueous-ethanolic systems. Fruit liqueur, wine cooler,
and alcopop are among the alcoholic beverages that may
contain a noticeable amount of ascorbic acid originated from
fruit or fruit juice as a raw material or ingredient in the
manufacturing process.17 Ascorbic acid may also be used in
white wine industry as an antioxidant for the prevention of
browning.18−20
MATERIALS AND METHODS
Chemicals. Ascorbic acid and potassium dihydrogen phosphate
were purchased from Sigma Chemical Co. (St. Louis, MO). Furfural
and 2-furoic acid were from Chem Service, Inc. (West Chester, PA),
■
Alcoholic beverages vary in their ethanol contents, or
ethanol/water ratios. Different ethanol/water ratios constitute
different chemical environments.21,22 Jeney-Nagymate and
Fodor found better retention of ascorbic acid in orange juice
as compared with those in wine, and attributed it to the
Received: July 26, 2012
Revised: September 20, 2012
Accepted: September 20, 2012
Published: September 20, 2012
© 2012 American Chemical Society
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and 3-hydroxy-2-pyrone (3OH2P) from Tyger Scientific, Inc. (Ewing,
NJ). Glacial acetic acid was from J. T. Baker Co. (Phillipsburg, NJ).
Acetonitrile (gradient grade), methanol (gradient grade), and ortho-
phosphoric acid (85%) were from Merck Co. (Darmstadt, Germany).
Preparation of Model Solutions. Aqueous and ethanolic (5, 10,
20, 30, 40%, v/v) model solutions containing ascorbic acid (40 mg/
100 mL) were prepared. Wine cooler is the major item to be modeled
after in the present study. The ascorbic acid content in commercial
fruit juices is usually no higher than 43 mg/100 mL.27 Wine cooler is
commonly formulated with neutral spirit, fruit juice and water to reach
an ethanol concentration between 3% and 5%. The ascorbic acid
concentration in wine cooler is usually no higher than 40 mg/100 mL.
It is therefore chosen as the concentration of ascorbic acid in the
model solutions.
The pH of the solution was adjusted to 3.2, which is a common pH
value of white wine, with 1.0 and 0.1 N hydrochloric acids by using a
pH-Stat (PHM 290, Radiometer Analytical SAS, Lyon, France). Each
4.0 mL aliquot of the solution was filled into a 4.5 mL brown glass vial.
The vial was sealed with Teflon gasket and capped. The model
solutions were incubated at 25, 35, or 45 °C up to 35 days, and
sampled periodically during storage for analysis.
Statistical Analysis. Each experiment was performed in triplicate.
The results were expressed in mean
standard error. One-way
analysis of variance (ANOVA) was used to analyze the variance among
samples. Duncan’s new multiple range test, at p < 0.05, was used to
determine the significance of differences among sample means.
RESULTS
■
Kinetics of Ascorbic Acid Degradation. 1. Reaction
Order. Ascorbic acid retention in the pH 3.2 ethanolic solutions
at 0−40% (v/v) ethanol concentrations and 25−45 °C storage
temperatures is shown in Figure 1. The rate of ascorbic acid
degradation was found to increase with the increase in ethanol
concentration, incubating temperature, and storage time. At 45
°C storage, ascorbic acid is completely degraded in 8 days.
The semilogarithmic plot of the ascorbic acid retention (C/
C0) versus time reveals that the degradation of ascorbic acid in
0−40% ethanolic solutions follows the first-order reaction
kinetics (Figure 2).
2. Rate Constant and Half-Life. Table 1 shows the rate
constants of ascorbic acid degradation in the ethanolic model
solutions incubated at various temperatures. The rate of
degradation increases with the increase in the storage
temperature and the ethanol concentration. An especially big
increase occurred when the ethanol concentration was
increased from 30 to 40% (v/v).
Assessment of Browning. The absorbance at 420 nm of each
sample was monitored as the browning index, using a UV−visible
spectrophotometer (Model Helios Alpha, Spectronic Unicam, Cam-
bridge, U.K.).
HPLC Analysis. The samples were then subjected to HPLC
analysis referring to Shinoda et al. and Nour et al. with
modifications.15,28 Ascorbic acid and three major degradation products
(furfural, 2-furoic acid, and 3OH2P) were analyzed using a 250 mm ×
4.6 mm inside diameter, 5 μm particle size, Hypersil Gold C18
reversed-phase high pressure liquid chromatography (HPLC) column
(Thermo Scientific, Waltham, MA). The HPLC analysis was run with
a Prominence liquid chromatography system (Shimadzu, Kyoto,
Japan) comprising a vacuum degasser, LC-20AT pump, SIL-20A
autosampler and SPD-M20A diode array detector (DAD). The mobile
phase for the determination of ascorbic acid was 50 mM potassium
dihydrogen phosphate solution, of which the pH value had been
adjusted to 2.8 with phosphoric acid. For the detection of degradation
products, 50 mM acetic acid/acetonitrile (98:2 v/v) solution was
chosen the mobile phase instead. The mobile phase was vacuum-
degassed, and then set at 0.7 mL/min flow rate for all the
chromatographic separations. The detection wavelength was set at
the maximum optical absorbance, 244 nm for ascorbic acid, 254 nm
for 2-furoic acid, 283 nm for furfural, or 300 nm for 3OH2P. Standard
curves were established with analytical grade ascorbic acid, furfural, 2-
furoic acid and 3OH2P. The injection volume was 20 μL for each
sample. The HPLC analysis was supplemented with internal
calibration by using the standard compounds. The degradation ratio
of ascorbic acid was calculated employing the following equation,
Half-life, calculated as −ln 0.5k−1, is defined as the time
needed for 50% reduction in the concentration of substrate. A
longer half-life corresponds with a slower degradation rate. The
half-life of ascorbic acid in model solutions in storage at a fixed
temperature is shortened when the ethanol concentration is
increased (Table 1).
3. Activation Energy. The slope of the semilogarithmic plot
of the rate constant versus the inverse of temperature depicts
the activation energy (Figure 3). Ascorbic acid has the lowest
value of activation energy at 10.35 kcal mol−1 in 40% (v/v)
ethanol among all the tested ethanol concentrations (Table 1).
Degradation Products of Ascorbic Acid. HPLC DAD
analysis found 2-furoic acid and 3OH2P in the ethanolic
solution incubated at 45 °C (Figure 4). Furfural, which is
another commonly recognized major degradation product of
ascorbic acid in fruit juice, was not detected.
Both the contents of 2-furoic acid and 3OH2P increased with
the increase in storage time (Figure 4). At the end of storage
test, the contents of 2-furoic acid in all the model solutions at
different ethanol concentrations were less than 10 ppm while
those of 3OH2P ranged between 30 and 60 ppm. Restated, the
compound 3OH2P is much more abundant than 2-furoic acid
as an end product of ascorbic acid degradation in the ethanolic
solutions. An increase in ethanol concentration accelerated the
formation of both 3OH2P and 2-furoic acid. The formation of
3OH2P and 2-furoic acid in various concentrations of ethanol
was found to follow zero-order kinetics with a determination
coefficient of correlation in between 0.75−0.90 and 0.94−0.98,
respectively.
Degradation ratio (%) = [(A − B)/A]*100
where A is the initial concentration of ascorbic acid, and B is the
detected concentration of ascorbic acid.
Evaluation of Ascorbic Acid Degradation Kinetic Parame-
ters. The changes in ascorbic acid content in the model solutions in
storage were fitted into the first-order kinetics equation,
ln(C/C0) = −kt
for evaluating the rate constant k, in day−1, for ascorbic acid retention.
C and C0 are the ascorbic acid contents at time t and time zero,
respectively. C/C0 expresses the % retention of ascorbic acid.
The temperature dependence of ascorbic acid retention was
determined by Arrhenius equation:
Browning in Ethanolic Model Solutions. Figure 5 shows
the progress of browning in ethanolic solutions of ascorbic acid
at 45 °C for 35 days as an accelerated storage test. All the
samples browned readily. The browning rate increased with the
increase in ethanol concentration. The model solution at 40%
(v/v) ethanol concentration browned more seriously than all
other samples at lower ethanol concentrations.
k = k0 e−E /RT
a
where Ea is the activation energy (kcal mol−1), R is the universal gas
constant (1.987 cal mol−1 K−1), T is the absolute temperature (K), and
k0 is the pre-exponential factor.
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Figure 1. Retention of ascorbic acid in the model solutions (40 mg/
100 mL, pH 3.2) during storage at various temperatures.
Figure 2. First-order kinetics plots of ascorbic acid degradation in the
model solutions (40 mg/100 mL, pH 3.2) during storage at various
temperatures.
DISCUSSION
■
Reaction Kinetics. Researchers found different chemical
kinetics of ascorbic acid degradation during storage in different
aqueous systems. Among them, zero-order reaction kinetics in
an intermediate moisture model food system, pseudo-first-order
in dehydrated guava fruit, first-order in orange juice and citrus
juice concentrates, and second-order in lemon juice all have
been reported.5,6,10,29,30 However, no literature in regard to the
degradation kinetics of ascorbic acid in ethanolic systems is
available yet. Normal white wine contains approximately 12%
(v/v) ethanol with a pH value between 3.0 and 3.5.31 Wine
cooler and alcopop usually contain ethanol in the ranges of 4−
6% (v/v) and 5−12% (v/v), respectively.17,32 Ascorbic acid
may also be present in fruit liqueurs, which normally contain
ethanol no higher than 40% (v/v). The present study evaluated
ascorbic acid degradation in 0−40% (v/v) ethanolic model
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Table 1. Effect of Ethanol Concentration on the Rate
Constant (k), Half-Life (t1/2), and Activation Energy (Ea) of
Ascorbic Acid Degradation in Model Solutions (40 mg/100
mL, pH 3.2)
ethanol
concentration (v/v)
temperature
rate constant
(day−1
t1/2
Ea
(°C)
)
(day) (kcal/mol)
45
35
25
45
35
25
45
35
25
45
35
25
45
35
25
45
35
25
0.261 0.007
0.251 0.005
0.072 0.005
0.455 0.005
0.307 0.012
0.086 0.002
0.783 0.010
0.418 0.002
0.089 0.001
1.117 0.010
0.431 0.001
0.096 0.002
1.125 0.043
0.875 0.004
0.105 0.001
1.583 0.001
1.737 0.050
0.535 0.002
2.66
2.76
9.69
1.52
2.26
8.08
0.89
1.66
7.76
0.62
1.61
7.19
0.62
0.79
6.58
0.44
0.40
1.30
12.29
15.79
20.52
23.10
22.46
10.35
0%
5%
10%
20%
30%
40%
Figure 4. Formation of 2-furoic acid and 3-hydroxy-2-pyrone in the
model solutions of ascorbic acid (40 mg/100 mL, pH 3.2) during
storage at 45 °C.
Figure 3. Arrhenius plot for ascorbic acid degradation in the model
solutions (40 mg/100 mL, pH 3.2).
solutions buffered at pH 3.2, simulating alcoholic beverages
with fruit as a raw material, and found that the degradation
follows first order reaction kinetics with temperature-depend-
ency (Figure 2).
Effect of Ethanol Concentration. Chuang et al. reported
that a higher ethanol concentration corresponds to a higher
degradation rate of ascorbic acid in ascorbic acid−catechin
model solutions.26 They postulated that a higher ethanol
concentration provides a lower water activity environment and
promotes the dehydration of ascorbic acid degradation
intermediate compound L-xylosone, therefore, exhibits an
increased rate of ascorbic acid degradation. In the present
study, the rate of ascorbic acid degradation in ethanolic
solutions at a fixed temperature was also found to increase with
the increase in ethanol concentration (Table 1). Our findings
are supportive to the above-mentioned postulation. Ethanol
concentration can be an important parameter for ascorbic acid
retention in alcoholic beverages.
Figure 5. Browning in the model solutions of ascorbic acid (40 mg/
100 mL, pH 3.2) during storage at 45 °C.
Temperature Effect. Results from the present study show
that temperature is another important factor for the stability of
ascorbic acid. A higher storage temperature resulted in a higher
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degradation rate of ascorbic acid in the ethanolic model
solutions (Table 1). Burdurlu et al. reported that storage
temperature affected degradation rate of ascorbic acid in orange
juice concentrate with a pH value at 3.2. In their samples, the
degradation rate of ascorbic acid at 45 °C was approximately 9
times that at room temperature.6 In the present study, the
degradation rate of ascorbic acid in 40% (v/v) ethanolic model
solution at 45 °C was approximately 3 times that at room
temperature. The effect of temperature on the degradation rate
of ascorbic acid is obvious in ethanolic solutions, although not
as strong as that in orange juice concentrate. Ascorbic acid in all
the ethanolic model solutions degrades over 99% in 45 °C
storage for 8 days (Figure 2). On the basis of the rate constants,
we predict that a similar degree of ascorbic acid degradation in
25 and 35 °C storages would occur in approximately 54 days
and 15 days, respectively.
Villota and Hawkes concluded that the activation energy of
ascorbic acid degradation ranges from 5 to 40 kcal/mol in
aqueous systems.33 The values of activation energy of ascorbic
acid degradation are 30 and 25.16 kcal/mol in orange juice and
orange juice concentrate, respectively.6,29 The present study
found that the values of activation energy for ascorbic acid
degradation in 0−40% (v/v) ethanol concentrations are
between 10.35 and 23.10 kcal/mol (Table 1), within the
range for reported values in aqueous systems. The model
solution at 40% (v/v) ethanol concentration is characterized
with the lowest value of activation energy, 10.35 kcal/mol. A
change in storage temperature exerts a heavier impact on the
degradation rate of ascorbic acid at this ethanol concentration
as compared with other concentrations.
form brown-colored xanthylium cation pigments.34 Chuang et
al. reported that an increased ethanol concentration in ascorbic
acid−catechin model solutions accelerated ascorbic acid
degradation and 3OH2P formation but decelerated browning.26
They postulated that an increase in ethanol concentration in
their model solutions promotes more L-xylosone to dehydrate
and form 3OH2P, while it leaves less to react with catechin and
form xanthylium cation pigments. Assuming the color intensity
of 3OH2P is lower than that of xanthylium cation pigments,
deceleration in browning of the solution would occur. The
present study found that an increase in ethanol concentration
in the ascorbic acid model solutions promoted the accumu-
lation of 3OH2P and accelerated browning (Figure 5). We
postulate that the ethanolic model solutions in the present
study contained no catechin to react with L-xylosone to form
xanthylium cation pigments; therefore, any accelerated
formation of 3OH2P in the system would simply be reflected
with more browning.
On the basis of the above-described findings, we propose that
3OH2P can be an indicator of browning in ethanolic model
solutions containing ascorbic acid only. It may also be true in
alcoholic beverages containing an overwhelmingly higher
amount of ascorbic acid as compared with the amount of
browning-active phenolic compounds.
Further studies, including the purification and identification
of the browning products, to elucidate the detailed mechanism
of the ascorbic acid browning reaction in ethanolic solutions are
recommended. Ascorbic acid and phenolic compounds are
usually present together in alcoholic beverages. Ascorbic acid
may degrade in its own pathways or following a pathway that
involves the reaction with phenolic compounds. Experiments in
the real systems to evaluate the relative importance of each
pathway are also recommended.
Degradation Pathway and Browning. Yuan and Chen
reported that 3OH2P, 2-furoic acid and furfural are the major
degradation products of ascorbic acid in acid aqueous
solution.14 Shinoda et al. found 3OH2P and 2-furoic acid as
the two major degradation products of ascorbic acid in orange
juice model solution.15,16 Chuang et al. found 3OH2P and 2-
furoic acid in ascorbic acid−catechin ethanolic model
solutions.26 These two degradation products were found in
the present study (Figure 4).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jsbwu@ntu.edu.tw.
Funding
There are aerobic and anaerobic pathways for the non-
enzymatic degradation of ascorbic acid. The end-products
3OH2P and 2-furoic acid are formed in the aerobic
degradation, whereas furfural is formed in the anaerobic
degradation.14 In the present study, 3OH2P and 2-furoic acid
were found in all the ethanolic model solutions while furfural
was never detected, indicating the predominance of aerobic
pathway over anaerobic pathway in the degradation of ascorbic
acid in ethanolic solutions.
Between the two major aerobic degradation products of
ascorbic acid, 3OH2P and 2-furoic acid, only the former causes
browning.15,16 The present study found 3OH2P a major
degradation product of ascorbic acid in the ethanolic model
solutions (Figure 4). An increase in ethanol concentration in
the model solutions accelerates 3OH2P formation and
browning (Figure 5). The browning index correlates well
with the concentration of 3OH2P (coefficient of determination,
R2 = 0.93) in linear regression. The roles of 3OH2P in ascorbic
acid degradation and browning appear similar between aqueous
and aqueous-ethanolic systems.
Financial support (NSC-98-2313-B-002-046-MY3) from the
National Science Council, Taiwan.
Notes
The authors declare no competing financial interest.
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