Article pubs.acs.org/JAFC
Impact of Fluorescent Lighting on Oxidation of Model Wine Solutions Containing Organic Acids and Iron Paris Grant-Preece,* Celia Barril, Leigh M. Schmidtke, and Andrew C. Clark National Wine and Grape Industry Centre, School of Agricultural and Wine Sciences, Charles Sturt University, Locked Bag 588, Wagga Wagga, New South Wales 2678, Australia S Supporting Information *
ABSTRACT: Previous studies have provided evidence that light exposure can increase oxygen consumption in wine and that the photodegradation of iron(III) tartrate could contribute to this process. In the present study, model wine solutions containing iron(III) and various organic acids, either alone or combined, were stored in sealed clear glass wine bottles and exposed to light from fluorescent lamps. Dissolved oxygen was monitored, and afterward the organic acid degradation products were determined and the capacity of the solutions to bind sulfur dioxide, the main wine preservative, was assessed. In the dark controls, little or no dissolved oxygen was consumed and the organic acids were stable. In the irradiated solutions, dissolved oxygen was consumed at a rate that was dependent on the specific organic acid present, and the latter were oxidized to various carbonyl compounds. For the solutions containing tartaric acid, malic acid, and/or citric acid, irradiation increased their sulfur dioxide-binding capacity. KEYWORDS: succinic acid, lactic acid, glyoxylic acid, iron(III) carboxylate, photochemistry, photo-Fenton
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INTRODUCTION Light exposure has been shown to contribute to white wine spoilage, and there is evidence that certain photochemical oxidation reactions, such as the riboflavin-sensitized oxidation of wine components, play an important role.1 After bottling, the total package oxygen can be 1−9 mg/L, and additional oxygen may enter the bottle through the closure over time.2 Light exposure has been shown to increase the uptake of gaseous oxygen by various wines3,4 and to affect the reduction potential of wine,5,6 which is strongly influenced by the dissolved oxygen (DO) concentration.6,7 Molecular oxygen has a triplet ground state, which prevents it from reacting directly with organic compounds in their singlet ground states. Transition metal ions in wine, in their reduced forms, such as iron(II) and copper(I), can reduce oxygen to hydrogen peroxide independently of the presence or absence of light. It has been proposed that iron(II) reduces oxygen to a radical species such as the hydroperoxyl radical (reaction 5, Figure 1) or an iron(III) superoxide radical complex, which is reduced by a second iron(II) ion to hydrogen peroxide (reaction 6, Figure 1), although the intermediate oxygen radical species is yet to be identified in wine and model solutions.8,9 Hydrogen peroxide is then reduced by iron(II) to the hydroxyl radical in the Fenton reaction10 (reaction 7, Figure 1). In an air-saturated model wine solution with tartaric acid, iron(II), and copper(II) stored in a sealed bottle in darkness, DO consumption was initially rapid, but essentially stopped after 10 h.9 After 23.5 h, 2 mg/L oxygen (approximately 25%) had been consumed, and the ratio of iron(II) to iron(III) was about 1:5.9 DO consumption was linked to the presence of tartrate anions, which interact more strongly with iron(III) than with iron(II) and thus promote iron(II) oxidation and at the same time suppress iron(III) reduction.9 Although iron(III) tartrate is relatively stable in model wine solutions stored in darkness, it is degraded upon exposure to © XXXX American Chemical Society
Figure 1. Possible reactions involved in the consumption of oxygen in model wine solutions containing α-hydroxy organic acids and iron.
light, giving rise to tartaric acid oxidation products.11−13 Model wine solutions containing tartaric acid and iron(III) absorb wavelengths below 500 nm, with a maximum absorbance near 340 nm due to iron(III) tartrate.13,14 In acidic aqueous solutions Received: Revised: Accepted: Published: A
October 19, 2016 February 21, 2017 February 25, 2017 February 25, 2017 DOI: 10.1021/acs.jafc.6b04669 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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12% (v/v) aqueous ethanol, pH 3.2 ± 0.1. bNot observed in dark controls, except when indicated. cNot specified. dTrace amount of hydrogen peroxide detected in dark control. eMeasured intensity of photosynthetically active radiation (400−700 nm) reaching the sample was 16200 μmol/m2/s. fCarbonyl compounds identified are shown in Figure 2. gLow amount of glyoxylic acid detected in tartaric acid dark control.
with tartaric acid and iron(III), exposure to wavelengths within the iron(III) tartrate absorption band causes the complex to degrade via ligand-to-metal charge transfer (LMCT), ultimately resulting in the production of iron(II) and an oxidized tartrate radical15−17 (reaction 1, Figure 1). In bottled model wine with tartaric acid and 10 ± 5 μg/L iron, sunlight exposure resulted in the accumulation of hydrogen peroxide and glyoxylic acid, a tartaric acid degradation product, as well as acetaldehyde, an ethanol oxidation product, whereas storage in darkness had no effect12 (Table 1). In the irradiated solution, it was proposed that the photochemical reduction of iron(III) to iron(II) led to the reduction of oxygen to hydrogen peroxide.12 Although some hydrogen peroxide was reduced by iron(II) to the hydroxyl radical, which then oxidized ethanol, ultimately giving rise to acetaldehyde, some hydrogen peroxide was suggested to remain in solution due to the limited amount of iron.12 This sequence of reactions could be referred to as a photo-Fenton process.18 Although this process has been the subject of numerous studies in acidic aqueous systems, including natural aquatic19−21 and wastewater systems,18 only a small number of studies have been conducted in wine22 and wine-like solutions (Table 1). α-Hydroxy carboxylate ligands are oxidized by iron(III) via light-induced LMCT to carboxylate radicals (reaction 1, Figure 1), which undergo oxidative decarboxylation to form an aldehyde or ketone15 (reactions 2−4, Figure 1). Tartaric acid is oxidized to 2-hydroxy-3-oxopropanoic acid, which, in the irradiated model wine, was suggested to degrade into glyoxylic acid and other products.12 Both iron(II) (5 mg/L) and exposure to 300−520 nm radiation from a xenon arc lamp were necessary for the formation of glyoxylic acid in model wine with tartaric acid during short-term storage, and decreasing the initial DO concentration or using a green glass filter instead of a clear glass filter decreased its production, in agreement with the proposed pathways.13 Furthermore, in a similar model wine system containing tartaric acid and iron(II) (5 mg/L) irradiated using a xenon arc lamp (≥300 nm), increasing the sulfur dioxide concentration (0−40 mg/L) decreased glyoxylic acid production, whereas addition of caffeic acid (100 mg/L) had no appreciable effect and pH (2.8−4.0) and temperature (15− 45 °C) combined had some influence.23 Other α-hydroxy organic acids present in wine, namely, malic acid, citric acid, and lactic acid, were degraded to the corresponding carbonyl compounds in oxygen-saturated model wine solutions with iron(II) (5 mg/L) exposed to xenon arc lamp radiation (≥300 nm), but were stable in dark controls24 (Figure 2). The citric acid oxidation product, 1,3-acetonedicarboxylic acid, was partially degraded via β-carbonyl decarboxylation to acetoacetic acid.24 Succinic acid, which does not have an α-hydroxyl group, was oxidized to 3-oxopropanoic acid in the irradiated solution.24 Carbonyl compounds in wine react reversibly with sulfur dioxide, in the form of hydrogen sulfite (bisulfite ion), to generate addition products and therefore can decrease the concentration of free sulfur dioxide, which protects wine from oxidation mainly by reacting irreversibly with hydrogen peroxide.25 The main aims of the present study were to determine whether exposure to light from fluorescent lamps commonly used in factories and retail environments can accelerate DO consumption in model wine solutions containing iron(III) and organic acids stored in clear glass wine bottles; to determine whether the carbonyl compounds previously identified in oxygen-saturated model wine solutions irradiated using a xenon
a
24 organic acid degradation, carbonyl compound productionf,g measured initial dissolved oxygen, 33 ± 2 mg/L iron(II) added, 5 mg/L 60 min quartz cuvettes with Teflon stoppers xenon arc lamp with heatabsorbing filter (≥300 nm)e
3 mL
15 ± 0.5 °C
tartaric acid, malic acid, succinic acid, citric acid, lactic acid
23 glyoxylic acid production decreased by increasing sulfur dioxide concentration (0−40 mg/L), but not appreciably affected by caffeic acid (100 mg/L) addition, glyoxylic acid production affected by pH and temperature combined measured initial dissolved oxygen, 7.4 ± 0.5 mg/L iron(II) added, 5 mg/L tartaric acid 30 min 15, 30, or 45 ± 1 °C quartz cuvettes with Teflon stoppers xenon arc lamp with heatabsorbing filter (≥300 nm)e
3 mL
glyoxylic acid production only upon exposure to 0.99). Ion-Exchange High-Performance Liquid Chromatography with Diode Array Detection (HPLC-DAD). Solutions were analyzed before and after the 4 h storage time using a Waters 600 controller, with a 717 plus autosampler and a 2996 diode array detector, run by Empower Pro software, as described previously for dry and mediumdry wines,28 except that the column temperature was 60 °C. The organic acids and their degradation products were detected at 210 nm, except for 3-oxopropanoic acid and acetaldehyde, which were detected
Figure 2. Carbonyl compound degradation products of the organic acids identified in oxygen-saturated model wine solutions of the individual organic acids and iron(II) exposed to xenon arc lamp radiation (≥300 nm).24
arc lamp (≥300 nm)24 were produced under the above conditions; to investigate the photochemical oxidation of model wine with the organic acids of interest combined as they are in wine; and finally, to determine whether light exposure can increase the capacity of the solutions to bind sulfur dioxide, as this could have implications for wine shelf life.
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MATERIALS AND METHODS
General. Acetaldehyde (≥99.5%), anhydrous citric acid (≥99.5%), ethyl acetoacetate (≥99.0%), L-malic acid (≥99%), and succinic acid (≥99.5%) were obtained from Fluka (Switzerland). Ethyl acetoacetate was hydrolyzed to acetoacetic acid as described previously.26 1,3-Acetonedicarboxylic acid (98.7%), formic acid (≥98%), fumaric acid (neat), glyoxylic acid monohydrate (98%), iron(III) sulfate hydrate (97%), pyruvic acid (98%), sodium L-lactate (∼98%), and L-tartaric acid (99.5%) were obtained from Sigma-Aldrich (USA). Anhydrous sodium metabisulfite (>95%) was obtained from Rowe Scientific (Australia). Tartronic acid (98%) was obtained from ABCR GmbH & Co. (Germany). All glass and plastic items were soaked in 10% (v/v) nitric acid for at least 12 h and then rinsed thoroughly with water filtered through a Millipore Milli-Q water purification system, with a resistivity of 18.2 MΩ·cm. All solutions were prepared using Milli-Q water. Model Wine Solutions. Solutions were prepared in 12% (v/v) aqueous ethanol, and the final pH was adjusted to 3.2 ± 0.1 by adding 1 mol/L sodium hydroxide or 0.5% (v/v) sulfuric acid. Solutions containing each of the individual organic acids (tartaric acid, malic acid, succinic acid, citric acid, and lactic acid) at 18 mmol/L were prepared. Tartaric acid and malic acid can be present at 18 mmol/L in white wines; however, succinic acid, citric acid, and lactic acid are typically present at lower levels,27 and thus additional solutions of the latter were prepared at wine-like concentrations, that is, 6.8, 2.6, and 2.2 mmol/L, respectively. Finally, solutions with all of the organic acids combined at 18 mmol/L or the wine-like concentrations were prepared. Samples (200 mL) of the solutions were added to six 250 mL Schott bottles and treated as described below over 5 days. On each day, two 200 mL samples of six different model wine solutions were aerated by rapid stirring for 1 min using a magnetic stirrer. C
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Figure 3. Absorption spectra of model wine solutions containing iron(III) and the organic acids at 18 mmol/L or the wine-like concentrations before irradiation. Mean (n = 3). split 1:3 (MS:waste), resulting in a flow to the Q-TOF of approximately 0.18 mL/min. The Q-TOF was operated in extended dynamic range (2 GHz) mode and in negative ion mode, with the following settings: range m/z, 70−1100; scan rate, 1 spectrum/s; reference ions for internal mass correction, m/z 112.985587 and 966.000725; drying gas temperature, 325 °C; drying gas flow, 8 L/min; nebulizer pressure, 40 psi; sheath gas temperature, 350 °C; sheath gas flow, 11 L/min; capillary voltage, 4000 V; nozzle voltage, 500 V; fragmentor voltage, 90 V; skimmer 1 voltage, 65 V; and octopole RF peak voltage, 750 V (unless otherwise stated below). Targeted MS/MS experiments in which the molecular ion was isolated and fragmented were performed using the following settings: range, m/z 50−1080; MS scan rate, 1 spectrum/s; MS/MS scan rate, 1 spectrum/s; isolation width, ∼4 amu; and collision energy, 2 V (unless otherwise stated below). Data for acetoacetic acid were obtained using an injection volume of 10 μL, and the corresponding molecular ion was fragmented using a collision energy of 5 V. Data for 1,3-acetonedicarboxylic acid were obtained using an injection volume of 20 μL and with both the drying gas temperature and sheath gas temperature at 250 °C. Free and Total Sulfur Dioxide. A sulfur dioxide stock solution (4.04 g/L) was prepared by dissolving sodium metabisulfite in 12% (v/v) ethanol at pH 3.1 ± 0.1. Aliquots (0.2 mL) of the stock solution were added to 20 mL samples of the solutions previously exposed to light and the dark controls to give a total sulfur dioxide concentration of 40 mg/L (0.62 mmol/L). The samples were equilibrated in darkness at ambient temperature for 3 h, and then free and total sulfur dioxide were measured using a FOSS FIAstar 5000 wine analyzer with a 5027 sampler, run by SoFIA version 2.00 software, as described previously.30
at 230 and 278 nm, respectively. Carbonyl compound degradation products for which reference compounds could be obtained commercially were identified on the basis of their retention time, UV absorption spectrum, and/or high-resolution molecular mass and MS/MS fragmentation pattern (see next section and Table S1). 2,3-Dioxopropanoic acid and 3-oxopropanoic acid were not commercially available and therefore were identified solely on the basis of their UV absorption spectrum and/or molecular mass and fragmentation pattern, which were consistent with data reported in the literature (see Grant-Preece et al.24 and references therein). For each carbonyl compound, the limit of detection (LOD) was calculated using the formula LOD = xb + 3sb, where xb is the mean of the amplitude of the baseline noise between the peak start and end times of six blank solutions and sb is the standard deviation of these values; the limit of quantitation (LOQ) was calculated using the formula LOQ = xb + 10sb.29 Fumaric acid (0−14 μmol/L) and glyoxylic acid (0− 1.9 mmol/L) standard solutions were prepared in 12% (v/v) aqueous ethanol at pH 3.1 ± 0.1 and analyzed to obtain calibration graphs (r2 ≥ 0.98) to quantitate these compounds in the model wine solutions. Ion-Exchange High-Performance Liquid Chromatography with High-Resolution Mass Spectrometry and Diode Array Detection (HPLC-HRMS-DAD). Solutions were analyzed using an Agilent 6530 quadrupole-time of flight (Q-TOF) LC-MS instrument with a dual Agilent Jet Stream Technology Electrospray Ionization Source and an Agilent 1290 Infinity LC system, run by MassHunter Workstation software for 6500 series Q-TOF B.06.01. HPLC was performed as described above, except the mobile phase was 10 mmol/L formic acid in water and the injection volume was 3 μL (unless otherwise stated below). After the diode array detector, the flow was D
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Table 2. Iron(III) Concentrations before and after Irradiation, Dissolved Oxygen (DO) Concentrations before Irradiation and at the Time of the Final Measurement, and Average Rates of DO Consumption over the First 2 h of the 4 h Storage Timea iron(III) (mg/L) solution tartaric acid, 18 mmol/L malic acid, 18 mmol/L succinic acid, 18 mmol/L citric acid, 18 mmol/L lactic acid, 18 mmol/L succinic acid, 6.8 mmol/L citric acid, 2.6 mmol/L lactic acid, 2.2 mmol/L all, 18 mmol/L all, wine-like concns
before irradiated dark irradiated dark irradiated dark irradiated dark irradiated dark irradiated dark irradiated dark irradiated dark irradiated dark irradiated dark
4.45 4.4 4.7 4.7 5 5 5 5 4.8 4.78 4.9 4.9 5 5 4.7 4.7 5.06 5.06 4.8 4.8
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.07 0.2 0.9 0.8 1 1 1 1 0.2 0.08 0.2 0.2 1 1 0.5 0.5 0.08 0.08 0.5 0.4
DO (mg/L) after
1.9 4.2 4.4 4.72 4.7 4.84 5.1 5.1 0.7 4.7 4.6 4.75 1 4.9 0.0 4.9 5.0 5.0 4 4.88
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.7 0.2 0.8 0.08 0.1 0.09 0.4 0.3 0.7 0.2 0.2 0.06 1 0.6 0.1 0.2 0.1 0.1 2 0.09
before (DO0) 7.7 7.9 7.7 7.7 7.74 7.8 7.9 7.8 7.8 7.9 7.7 7.8 7.7 7.9 7.7 7.8 7.8 8.0 7.7 7.7
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.5 0.4 0.4 0.4 0.02 0.4 0.6 0.3 0.4 0.2 0.2 0.1 0.7 0.4 0.7 0.4 0.2 0.6 0.4 0.4
final (DO3.5)b 0.06 7.5 4.2 7.5 7.0 7.5 1.9 7.6 0.3 7.5 6.90 7.51 0.13 7.6 0.05 7.5 1.7 7.5 0.7 7.5
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.04 0.3 0.1 0.3 0.5 0.3 0.6 0.1 0.3 0.3 0.06 0.05 0.07 0.5 0.06 0.4 0.6 0.4 0.5 0.2
ratec (mg/L/h) 3.1 0.2 0.9 0.13 0.3 0.1 1.7 0.10 2.2 0.2 0.23 0.1 2.2 0.16 2.9 0.11 1.60 0.20 1.8 0.1
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.3 0.1 0.2 0.09 0.2 0.2 0.1 0.00 0.2 0.2 0.06 0.1 0.4 0.04 0.4 0.01 0.09 0.08 0.4 0.1
Mean (n = 3). Uncertainties indicate 95% confidence limits. bDO concentrations were measured in the order indicated, and the final measurements for the 18 mmol/L tartaric acid solution and the solution with all organic acids combined at the wine-like concentrations were recorded on average after 3.52 and 3.86 h of irradiation/storage in darkness. cAverage rate of DO consumption = (DO0 − DO2)/t2, where DO0 is the initial concentration, DO2 is the concentration measured between 2.0 and 2.5 h, and t2 is the average time at which the latter measurement was recorded. a
Statistics. All data reported in the following section are the mean of three replicates. All uncertainties indicate 95% confidence limits. Means for different solutions with nonoverlapping 95% confidence intervals are significantly different (p < 0.05).31,32
with maxima at 338 and 346 nm, respectively (Figure 3). The malic acid and 2.6 mmol/L citric acid solutions both exhibited a shoulder in the region above 300 nm, whereas the 18 mmol/L citric acid solution did not show a defined shoulder. The spectra of the 18 and 6.8 mmol/L succinic acid solutions were almost identical; both had shoulders at 333 and 400 nm. The combined organic acid solutions both exhibited a shoulder above 300 nm, which was slightly more pronounced for the wine-like concentration solution. The spectra of the individual organic acid solutions are similar to those reported for acidic aqueous solutions containing the organic acids and iron(III).16,17,34 These studies and others15,35 have shown that exposure to wavelengths above 300 nm induces the photodegradation of iron(III) carboxylate complexes present, ultimately resulting in the production of iron(II) and organic acid oxidation products. Therefore, it was expected that this would occur in the bottled model wine solutions exposed to light emitted by the fluorescent lamps. Dissolved Oxygen Consumption. The initial DO concentrations (DO0) of the solutions were between 7.7 and 8.0 mg/L (Table 2). The final measured DO concentrations, that is, those measured after 3.5 h (DO3.5), of the dark controls were between 7.5 and 7.6 mg/L (Table 2). In all cases, the DO3.5 concentration of the irradiated solution was significantly lower (p < 0.05) than that of the dark control (Table 2). In the irradiated 18 mmol/L solutions, DO consumption was dependent on the specific organic acid. There was no appreciable difference between the DO3.5 concentrations of the irradiated 18 mmol/L tartaric acid and lactic acid solutions; however, there were significant differences (p < 0.05) between the DO3.5 concentrations of the other irradiated 18 mmol/L individual organic acid solutions (Figure 4). In the irradiated 18 mmol/L combined organic acid solution, the DO3.5
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RESULTS AND DISCUSSION Fluorescent Lamp Radiation and Spectral Filtration by Bottle Glass. In the current study, the spectral distribution of light emitted by the fluorescent lamps was not assessed. However, it is known that many commonly used “cool white” fluorescent lamps show mercury emission peaks centered at approximately 313, 365, 405, 436, 546, and 578 nm and phosphor emission peaks with maxima at around 480 and 580 nm.33 The transmission spectrum of the bottle glass (Figure S2) indicated that wavelengths below 300 nm were not transmitted. Transmittance increased to 82% at 360 nm and was between 82 and 89% at wavelengths between 360 and 550 nm, similar to clear glass wine bottles used in previous work.13 Iron(III) Carboxylate Complexes. The UV−visible absorption spectra of the model wine solutions before irradiation are shown in Figure 3. Samples with iron(III) concentrations of 0, 2.5, 5.0, and 7.5 mg/L were also analyzed to obtain iron(III) calibration graphs. Those without added iron(III) essentially showed no absorbance at wavelengths above 300 nm, whereas those with added iron(III) exhibited absorbance values increasing with the iron(III) concentration (Figures S3 and S4); at the specific wavelengths chosen to quantitate iron(III), the absorbance values were directly proportional to the iron(III) concentration. Before irradiation, the iron(III) concentrations of the solutions were between 4.4 and 5.1 mg/L (Table 2). The tartaric acid solution exhibited a peak with a maximum at 338 nm, and the 18 and 2.2 mmol/L lactic acid solutions showed peaks E
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resulted in consumption of approximately 2.5 mg/L oxygen before 24 h had elapsed; however, thereafter, very little additional oxygen was consumed.36 As noted in the Introduction, iron(II) essentially had the same effect in a similar model wine solution without (+)-catechin stored in darkness, and in this case, after 23.5 h, the ratio of iron(II) to iron(III) was about 1:5.9 These observations were linked to the ability of tartrate anions to stabilize iron(III).9 In the current study, it appears that the main pathway for DO consumption in the dark controls would involve reduction of iron(III) to iron(II) and subsequent reaction of iron(II) with oxygen. The measured DO concentrations, as well as the measured iron(III) concentrations described in the next section (Table 2), suggest that in all dark controls, iron(III) was reduced at a negligible rate, most likely due to the stability of the iron(III) carboxylate complexes under these conditions. In the irradiated tartaric acid solution, DO was initially consumed at a relatively constant rate, and the rate slowed as the oxygen concentration fell below 1 mg/L (Figure 4). The measured DO and iron(III) concentrations (Table 2 and Figure 4) provide evidence that exposure to light from the fluorescent lamps induced the degradation of iron(III) tartrate, ultimately resulting in the production of iron(II) and an oxidized tartrate radical16,17 (reaction 1, Figure 1). The oxidized tartrate radical undergoes decarboxylation to form an alkyl radical (reaction 2, Figure 1), which could reduce a second iron(III) ion or oxygen16,17 (reactions 3 and 4, Figure 1). Iron(II) generated by the above reactions could reduce the initial oxygen radical species formed, in this paper represented by the hydroperoxyl radical, to hydrogen peroxide and could also reduce hydrogen peroxide to the hydroxyl radical (reactions 6 and 7, Figure 1). In addition, if the ratio of iron(II) to iron(III) was increased sufficiently, iron(II) could reduce oxygen (reaction 5, Figure 1), in accordance with the findings described above.9,36 Low oxygen and hydrogen peroxide concentrations are expected to limit the oxidation of iron(II) back to iron(III) and, therefore, slow DO consumption, and this is consistent with the decreased rate of DO consumption observed at oxygen concentrations below 1 mg/L. To summarize, it appears that iron(III) tartrate photodegradation (reaction 1, Figure 1) will lead to reactions involving the products, that is, iron(II) and the oxidized tartrate radical, whereby oxygen is reduced to hydrogen peroxide (reactions 4−6, Figure 1) and iron(II) is oxidized back to iron(III) (reactions 5−7, Figure 1). These reactions were among those implicated in the increased rate of DO consumption in bottled acidic aqueous solutions containing iron(III) and individual organic acids, including malic acid and citric acid, exposed to light from tungsten filament lamps,37 and the accumulation of hydrogen peroxide in bottled model wine with tartaric acid and very little iron (10 ± 5 μg/L) exposed to sunlight.12 The results suggest that the reactions described above also occurred to some extent in the irradiated solutions containing the other α-hydroxy organic acids. Like the α-hydroxy organic acids, it is expected that succinic acid will be oxidized by iron(III) via light-induced LMCT, resulting in the production of a succinate radical, which will undergo decarboxylation to form an alkyl radical.24 Whereas the alkyl radicals derived from the α-hydroxy organic acids can be readily oxidized to carbonyl compounds (reactions 3 and 4, Figure 1), it appears that the main pathway for the degradation of the succinic acid-derived alkyl radical will involve combination with oxygen to form an alkylperoxyl radical, which could then degrade via different
Figure 4. Dissolved oxygen (DO) consumption in model wine solutions containing iron(III) and the organic acids, individually or combined, at 18 mmol/L, during irradiation or storage in darkness. DO varied in a similar manner in all dark controls; thus, only the tartaric acid and the combined organic acid dark controls are shown. Mean (n = 3). Error bars indicate 95% confidence limits.
concentration was similar to that of the irradiated 18 mmol/L citric acid solution (Figure 4). In each solution, DO was consumed at a relatively constant rate during the first 2−2.5 h of the storage time (Figure 4). The average rates of DO consumption during this time indicated that for the lightexposed 18 mmol/L solutions DO consumption decreased significantly (p < 0.05) in the order tartaric acid, lactic acid, citric acid, malic acid, and succinic acid, and in the combined organic acid solution, the rate was similar to that in the citric acid solution (Table 2). For the irradiated succinic acid, lactic acid, and combined organic acid solutions, the DO3.5 concentrations of the 18 mmol/L and wine-like concentration solutions were similar, and this was also the case for the average rates of DO consumption over the first 2 h, with the exception of the lactic acid solutions, where the average rate of DO consumption was marginally faster in the 2.2 mmol/L solution (Table 2). In the irradiated 18 and 2.6 mmol/L citric acid solutions, although the average rates of DO consumption over the first 2 h were similar, there was a significant difference (p < 0.05) between the DO3.5 concentrations, with essentially no oxygen remaining in the 2.6 mmol/L solution at this time. The negligible rate of DO consumption in the tartaric acid dark control (Figure 4) is consistent with the previous observation that in air-saturated model wine with tartaric acid, (+)-catechin, copper(II), and iron(III) stored in darkness, on average, 0.34 mg/L oxygen was consumed over a period of 8 days.36 In contrast, addition of iron(II) instead of iron(III) F
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Journal of Agricultural and Food Chemistry pathways to form stable products.24 The reaction of the succinic acid-derived alkyl radical with oxygen in the irradiated succinic acid solutions most likely contributed to the slightly lower DO3.5 concentrations of these solutions compared to that of the dark controls. Evidently, when the bottles were first exposed to light, the efficiency of iron(III) carboxylate photodegradation (reaction 1, Figure 1) would have had a direct effect on DO consumption. The efficiency of iron(III) carboxylate complex photodegradation in acidic aqueous solutions has been previously investigated by measuring the quantum yield of iron(II),16,17,35 that is, the amount of iron(II) produced in moles per mole of photons absorbed by the solution. Many of these studies were conducted in deaerated solutions to prevent oxidation of iron(II), and under these conditions iron(II) could be generated by reactions 1 and 3 (Figure 1). Abrahamson et al.35 reported the quantum yields of iron(II) (366 nm), in deaerated acidic (pH 2.9) aqueous solutions containing iron(III) (16.8 mg/L, 0.30 mmol/L) and either DL-tartaric acid, DL-malic acid, citric acid, or succinic acid (50 mmol/L), to be 0.29, 0.26, 0.17, and 0.13, respectively. For the structurally related organic acids, tartaric acid, malic acid, and succinic acid, it was proposed that the α-hydroxy organic acids gave rise to higher quantum yields than succinic acid because they are more easily oxidized to stable products.35 The irradiation wavelength used by Abrahamson et al.35 (366 nm) coincides with one of the emission bands of typical fluorescent lamps.33 In the present study, when the bottles were first exposed to light, the yields of iron(II) and oxidized carboxylate radicals were dependent on both the quantity of photons absorbed by the solution, as well as the efficiency with which photon absorption resulted in iron(III) carboxylate degradation. Before irradiation, the 18 mmol/L tartaric acid, malic acid, and succinic acid solutions exhibited absorbances at 366 nm of 0.086, 0.070, and 0.034, respectively. Hence, it appears that the greater absorbance of the tartaric acid solution and the more efficient photodegradation of iron(III) tartrate contributed to the greater rate of DO consumption in the tartaric acid solution compared to that in the malic acid and succinic acid solutions. In deaerated acidic (pH 3.1) aqueous solutions containing citric acid and iron(III), decreasing the ratio of citric acid to iron(III) from 50 to 1 increased the iron(II) quantum yield from approximately 0.21 to 0.44.35 In the current study, decreasing the citric acid concentration from 18 to 2.6 mmol/L decreased the citric acid:iron(III) ratio from about 200 to 30. In addition, before irradiation, the 18 and 2.6 mmol/L citric acid solutions exhibited an absorbance at 366 nm of 0.053 and 0.061, respectively. The greater absorbance of the 2.6 mmol/L solution, and the more efficient photodegradation of iron(III) citrate species in this solution, most likely contributed to its lower DO3.5 concentration. The structure of the organic acid will also determine the strength of its interaction with iron(III), which in turn will affect the reactivity of iron(II) and thus the consumption of oxygen and the oxidation of iron(II) back to iron(III) (reactions 5−7, Figure 1). Previously it was shown that in the absence of light, whereas iron(II) was rapidly oxidized in airsaturated model wine with tartaric acid (33.3 mmol/L) and copper(II), it was not oxidized in an equivalent solution containing acetic acid (66.6 mmol/L) instead of tartaric acid, and this was attributed to the stronger interaction between tartrate anions and iron(III) compared to that between acetate anions and iron(III).14 It appears that the most fundamental factor affecting the ability of an organic acid to bind iron(III) at
wine-like pH values is the strength of the acid. The pKa1 values of the organic acids increase in the order tartaric acid, citric acid, malic acid, lactic acid, and succinic acid, and for those with more than one carboxylic acid group, the pKa2 values increase in the same order.38 From these values, it is expected that the strength of the interaction between the organic acid and iron(III) will decrease in the same order; however, other factors, such as the ability of the ligand to chelate iron(III), will also contribute to the overall stability of the complex. For tartaric acid, malic acid, and succinic acid, the pKa1 and pKa2 values increase as the number of α-hydroxyl groups decreases, consistent with the ability of the hydroxyl group to draw electron density away from the carboxylic acid. For these organic acids, it appears that an increasing number of α-hydroxyl groups not only will increase the efficiency of iron(III) carboxylate photodegradation, as discussed above, but also will increase the affinity of the organic acid for iron(III), which in turn will increase the rate of reaction of iron(II) with oxygen and hydrogen peroxide and, thereby, increase DO consumption and/or the re-formation of iron(III). In the 18 mmol/L combined organic acid solution, the organic acids competed for iron(III), and considering the strengths of the acids, it is likely that a sizable proportion of the iron(III) was bound to tartrate anions. Decreasing the concentrations of succinic acid, citric acid, and lactic acid from 18 mmol/L to the wine-like values was expected to increase the amount of iron(III) in the form of iron(III) tartrate to some degree; however, this had a relatively minor impact on the average rate of DO consumption over the first 2 h and the final DO concentration. Photochemical Reduction of Iron(III). As stated earlier, before irradiation, the iron(III) concentrations of the solutions varied between 4.4 and 5.1 mg/L (Table 2). After storage, the iron(III) concentrations of the dark controls were essentially identical to the initial values (Table 2). The iron(III) concentrations of the irradiated 18 mmol/L tartaric acid, 18 mmol/L lactic acid, 2.6 mmol/L citric acid, and 2.2 mmol/L lactic acid solutions were significantly lower (p < 0.05) than that of the dark controls (Table 2). For the other irradiated solutions, the iron(III) concentrations were similar to that of the dark controls (Table 2). As discussed above, the increased rate of DO consumption in the light-exposed solutions implies that under these conditions iron(III) was reduced to iron(II) via iron(III) carboxylate photodegradation and possibly also by organic acid-derived radicals (reactions 1 and 3, Figure 1). The iron(II) produced could be oxidized back to iron(III) by oxygen, the hydroperoxyl radical, or hydrogen peroxide (reactions 5−7, Figure 1) and it is expected that iron(III)/iron(II) redox cycling will continue until oxygen and hydrogen peroxide are almost entirely consumed, leading to the accumulation of iron(II). After irradiation, in the time before UV−visible absorption analysis (approximately 1 h), the iron(II) present may have been oxidized by oxygen and hydrogen peroxide remaining in the solution, as well as oxygen introduced when the bottles were opened. All irradiated solutions with an iron(III) concentration considerably lower than that of the dark control had a DO3.5 concentration ≤0.3 mg/L, whereas those with an iron(III) concentration close to that of the dark control had a DO3.5 concentration ≥0.7 mg/L (Table 2). In the solutions with DO3.5 concentrations ≤0.3 mg/L, the low amount of oxygen, and presumably also hydrogen peroxide, present at the end of the irradiation period most likely restricted the subsequent G
DOI: 10.1021/acs.jafc.6b04669 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry
Organic Acid Degradation and Carbonyl Compound Production. In the dark controls, the concentrations of the organic acids did not change appreciably. This was also the case in the irradiated solutions, except for the irradiated 2.6 mmol/L citric acid and 2.2 mmol/L lactic acid solutions, in which the organic acid concentration was significantly lower (p < 0.05) than that of the dark control. (In the irradiated 2.6 mmol/L citric acid and 2.2 mmol/L lactic acid solutions, the organic acid peak area decreased by 12 and 13%, respectively.) However, organic acid-derived carbonyl compounds were detected in all of the irradiated solutions, except the irradiated 2.2 mmol/L lactic acid solution, and these compounds were not detected before irradiation or in the dark controls. In the irradiated tartaric acid, malic acid, succinic acid, and citric acid solutions, the carbonyl compounds produced were the same as those previously identified in oxygen-saturated solutions with iron(II) exposed to light from a xenon arc lamp24 (Figure 2, Table 3, and Table S1). The amount of glyoxylic acid detected in the irradiated tartaric acid solution (0.47 ± 0.04 mmol/L) was comparable to the amounts produced in similar model wine systems containing tartaric acid and iron(II) exposed to light from a xenon arc lamp,13,23 but lower than the amount produced in an oxygen-saturated solution with iron(II) irradiated using a xenon arc lamp.24 This is consistent with the observed effect of the initial DO concentration on glyoxylic acid production under controlled irradiation conditions.13 A greater amount of 3-oxopropanoic acid was detected in the malic acid solution than in the succinic acid solutions, and this is generally consistent with the faster rate of DO consumption observed in the malic acid solution. Similarly, acetoacetic acid was present at a higher concentration in the 2.6 mmol/L citric acid solution than in the 18 mmol/L citric acid solution, in agreement with the faster rate of DO consumption in the former solution, and possible contributing factors were discussed earlier (see DO Consumption).
Table 3. Concentration of Glyoxylic Acid (mmol/L) and Peak Areas (×103) of Other Carbonyl Compounds Detected in the Irradiated Model Wine Solutions by HPLC-DADa,b solution tartaric acid, 18 mmol/L malic acid, 18 mmol/L succinic acid, 18 mmol/L succinic acid, 6.8 mmol/L all, wine-like concns citric acid, 18 mmol/L citric acid, 2.6 mmol/L all, 18 mmol/L all, wine-like concns lactic acid, 18 mmol/L
products 2,3-dioxopropanoic acidc
