Phenolic Composition of Vinegars over an Accelerated Aging Process

Article pubs.acs.org/JAFC

Phenolic Composition of Vinegars over an Accelerated Aging Process Using Different Wood Species (Acacia, Cherry, Chestnut, and Oak): Effect of Wood Toasting Ana B. Cerezo, M. Antonia Á lvarez-Fernández, Ruth Hornedo-Ortega, Ana M. Troncoso, and M. Carmen García-Parrilla* Á rea de Nutrición y Bromatología, Facultad de Farmacia, Universidad de Sevilla, C/P García González no. 2, Sevilla 41012, Spain ABSTRACT: Wood shavings are widely employed in vinegar making to reduce aging time. Accordingly, this study aims to evaluate the effects of using shavings from different wood species (acacia, cherry, chestnut, and oak) and of toasting on the release of phenolic compounds into vinegar during the aging process. The study involved aging vinegars using previously toasted shavings and untoasted ones, at 0.5% and 1% (w/v), and collecting samples at 15 and 30 days. The phenolic compounds were analyzed by LC-DAD during the aging process. As a result, wood markers naringenin and kaempferol (cherry), robinetin and fustin (acacia), and isovanillin (oak) were identified for the first time in vinegars. The results also showed that toasting wood shavings decreases the concentration of most flavonoid wood markers (e.g., (+)-taxifolin, naringenin, and fustin) in vinegar, but that it is essential for the highest releases of aldehyde compounds (syringaldehyde, protocatechualdehyde, and vanillin). Remarkably, 15 days was sufficient to obtain the highest increases of most polyphenol compounds in the vinegar. Statistical analysis (linear discriminant analysis) proved that the phenolic compounds identified in vinegars are useful for discriminating vinegars regarding the wood species of the shavings used to accelerate aging. KEYWORDS: acacia, cherry, chestnut, oak, vinegar, polyphenol, toasting

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been identified in vinegars aged in toasted oak barrels.24 These studies have recorded that vanillin took 7 months longer to be released in untoasted oak barrels than in toasted ones.22,24 Further to this, it has also been shown that toasting led to the formation of benzoic acids (protocatechuic acid, p-hydroxybenzoic acid, vanillic acid, and syringic acid) in cherry and chestnut heartwood, and cinnamic acids (caffeic acid and ferulic acid) in acacia and chestnut heartwood, due to lignin degradation,19−21 while it resulted in the degradation of flavonoid markers.19−21 To be specific, (+)-dihydrorobinetin concentration decreased 20-fold in vinegar that was aged with toasted acacia shavings.10 What is more, the concentration of (+)-taxifolin in cherry heartwood was reduced by 97% when it was toasted.19 Previous studies have further reported that condensed and hydrolyzable tannin concentrations diminished remarkably (70−95%) or even disappeared in toasted cherry, acacia, and chestnut heartwood when compared to seasoned heartwood, due to their heat-sensitive nature.19−21 WINEGAR European Project (COOP-CT-2005-017269) identified certain phenolic compounds as markers in highquality vinegar aged in different untoasted wood barrels, such as acacia (dihydrorobinetin), cherry ((+)-taxifolin), and chestnut (high concentration of gallic acid and gallic ethyl ester). This paper aims to evaluate the effect of toasting on the release of the wood markers previously established by WINEGAR and polyphenols, in general, from acacia, cherry, chestnut, and oak shavings into vinegar. Besides this, we intend to assess the use

INTRODUCTION Wood chips are used extensively in enology to accelerate aging.1−4 However, there are scarce data available for vinegar making, and the majority is fairly recent.5,6 Most studies have focused on oak chips, since this is the most commonly used species.5,7,8 Even though other wood species, such as acacia, chestnut, cherry, mulberry, and ash, have all been crucial in producing traditional high-quality balsamic vinegars,9 the use of chips from these woods to diversify products remains virtually unexplored in the literature concerning vinegar making.10 The release of polyphenol compounds from wood chips, staves, and shavings into wine and vinegar is a matter of interest in scientific literature.5,6,11−15 The main points of interest as regards the release of polyphenol from wood chips into beverages are wood species, wood treatment in chip making, and the beverage matrix. Several recent studies have evaluated the release of polyphenol compounds from different heartwoods, such as cherry, chestnut, acacia, ash, etc., for their further use in cooperage.16−21 Certain phenolic compounds have been established as wood markers, such as naringenin, isosakuranetin, eriodictyol, aromadendrin, and (+)-taxifolin in cherry wood;19,21,22 (+)-dihydrorobinetin, fustin, robinetin, robtin, butin, and leucorobinetinidin in acacia wood;10,20,21 gallic and ellagic acid and hydrolyzable tannins of both the ellagitannins and gallotannins in chestnut wood;21,23 and oleuropein, ligstroside, olivil, verbascoside, isoverbascoside, oleoside, tyrosol, syringaresinol, and cyclolovil in ash wood.21 The thermic treatment of wood is a determining factor in the release of polyphenols. Indeed, aldehydes, such as protocatechualdehyde and coniferaldehyde, have not been reported in vinegars aged in untoasted oak barrels,22 while they have © 2014 American Chemical Society

Received: October 18, 2013 Accepted: April 17, 2014 Published: April 17, 2014 4369

dx.doi.org/10.1021/jf500654d | J. Agric. Food Chem. 2014, 62, 4369−4376

4370

34.14 ac ni 2.54 4.02 ac 27.2 28.0 ni 10.44 a ni 2.67 ac ni 7.4 ac ni 3.70 1.63 19.64 a 9.7 ac

30 d 43.6 ad ni 3.32 6.95 ad 33.1 26.9 ni 10.3 a ni 5.70 ad ni 12.7 ad ni 3.85 1.49 19.6 ae 12.79 ad

15 d

1% 44.14 ad ni 2.19 6.98 ad 28.6 25.8 ni 9.4 a ni 5.2 ad ni 12.4 ad ni 3.22 1.49 17.2 af 12.7 ad

30 d

15 d

30 d 43.97 be ni 2.33 ni b 28.78 23.70 ni 4.8 be ni 17.3 bde ni 40.44 bd ni 3.54 1.13 14.25 be 9.7 bd

15 d

untoasted

46.9 bf ni 2.93 ni b 36.90 27.24 ni 5.1 b ni 9.59 bcf ni 21.7 bc ni 3.94 1.29 13.77 bf 5.9 bc

0.5% 38.33 be ni 2.92 ni b 33.9 19.38 ni 4.9 b ni 8.78 bce ni 21.9 bc ni 3.21 1.06 15.7 be 6.2 bc

cherry

1% 55.1 bf ni 2.82 ni b 39.2 32.1 ni 3.11 bf ni 26.46 bdf ni 40.5 bd ni 4.09 1.32 9.51 bf 10.57 bd

30 d

30 d 35.8 ac 1.11 2.9 ni 32.52 30.6 0.71 a 10.9 a 8.04 ac ni 8.12 ac ni 109.9 ni 2.01 19.2 a ni

0.5% 34.1 ac 1.05 3.04 ni 32.60 27.4 0.89 a 10.67 a 7.94 ac ni 8.04 ac ni 114.5 ni 2.06 22.7 a ni

15 d 54.9 ad 0.76 2.55 ni 32.6 31.1 0.86 a 10.6 a 13.82 ad ni 10.51 ad ni 126.63 ni 1.75 20.9 a ni

15 d

toasted 1% 54.9 ad 1.06 2.79 ni 31.9 34.9 0.89 a 11.39 a 14.70 ad ni 11.37 ad ni 114.8 ni 1.84 18.1 a ni

30 d 98.2 bc 1.20 2.45 ni 33.2 22.1 0.51 b 12.06 b ni b ni 5.63 b ni 122.0 ni 1.81 12.27 b ni

15 d

chestnut

30 d 107.6 bc 0.98 2.51 ni 28.1 26.0 0.55 b 11.65 b ni b ni 4.91 b ni 117.11 ni 1.44 13.86 b ni

0.5%

1%\ 157.3 bde 1.13 2.20 ni 33.38 21.4 0.58 b 14.6 b ni b ni 7.77 b ni 126.18 ni 1.69 13.9 b ni

15 d

untoasted

219.1 bdf 1.58 2.38 ni 28.24 25.0 0.46 b 12.7 b ni b ni 5.73 b ni 108.4 ni 1.64 14.4 b ni

30 d

a %, w/v. bDays. cLetters a and b in the same column indicate significant differences (p < 0.05) between different treatments (toasted and untoasted) within the same wood. Letters c and d in the same column indicate significant differences (p < 0.05) between different shaving % within the same treatment and wood. Letters e and f in the same column indicate significant differences (p < 0.05) between different aging time within the same treatment, shaving %, and wood. dNot identified.

32.0 ac nid 3.01 3.88 ac 30.8 25.87 ni 11.3 a ni 2.77 ac ni 7.23 ac ni 3.40 1.96 19.30 a 10.19 ac

gallic acid protocatechuic acid 3-O-methylgallic acid protocatechualdehyde tyrosol syringic acid vanillin gallic ethyl ester syringaldehyde (+)-taxifolin ellagic acid naringenin caftaric acid caffeic acid p-cuomaric acid quercetin-3-glucoside kaempferol

c

0.5%

15 db

compounds

a

toasted

concentration (mg/L)

Table 1. Quantitive Evaluation of Phenolic Compounds in Vinegars Aged with Cherry and Chestnut Shavings

Journal of Agricultural and Food Chemistry Article


4371

28.8 ac 3.02 33.9 ac 32.65 11.6 ac 26.50 1.51 a ni 10.89 2.44 ac 4.28 ac 3.55 61.74 acf 99.6 1.8 18.50

30 d 30.6 ad 3.06 52.1 ad 32.9 20.06 ad 25.13 1.40 a ni 9.9 3.65 ade 6.68 ade 3.6 88.9 ade 88.2 1.71 18.2

32.8 ad 3.42 58.8 ad 35.6 23.50 ad 29.8 1.66 a ni 11.57 4.08 adf 7.19 adf 4.2 102.89 adf 71.7 2.06 14.2

30 d

1% (%, w/v) 15 d 20.48 be 3.09 19.6 b 33.66 388.5 bd 21.73 0.85 b ni 10.3 12.18 be ni b 4.06 62.70 bde 72.54 1.79 17.74

28.16 bf 3.1 19.6 b 30.95 384.8 bd 23.70 0.81 b ni 8.4 29.00 bf ni b 3.7 73.17 bdf 84.92 1.46 18.29

30 d

1% (%, w/v) 15 d

untoasted

27.3 bf 3.44 19.45 b 32.6 246.8 bc 25.70 0.90 b ni 11.78 12.16 bf ni b 3.7 54.3 bc 78.4 1.89 19.1

30 d

0.5% (%, w/v) 15 d 17.3 be 3.03 18.92 b 32.9 234.6 bc 22.31 0.88 b ni 11.50 11.1 be ni b 4.3 44.5 bc 72.4 1.78 16.15

acacia

44.7 a nid ni 38.5 ni 25.9 2.36 a 0.76 ac 6.3 c ni 2.89 ac 6.13 ac ni 55.3 2.2 21.6

45.59 a ni ni 36.5 ni 29.5 2.27 a 1.11 ac 6.31 c ni 3.03 ac 5.65 ac ni 67.7 1.53 19.73

30 d

0.5% (%, w/v) 15 d 39.7 a ni ni 34.72 ni 29.6 2.6 a 2.07 ad 8.63 d ni 4.63 ad 8.66 ad ni 92.3 1.58 21.02

35.9 a ni ni 30.3 ni 32.6 2.60 a 2.75 ad 10.86 d ni 4.64 ad 8.33 ad ni 99.2 1.64 17.98

30 d

1% (%, w/v) 15 d

toasted

47.4 bc ni ni 39.4 ni 26.7 2.40 b 0.94 b 9.40 c ni ni b 4.69 bc ni 102.4 1.52 18.23

50.07 bd ni ni 38.2 ni 22.60 2.18 b 0.32 b 7.1 d ni ni b 6.65 bd ni 84.9 3.72 20.4

50.2 bd ni ni 37.12 ni 26.1 2.24 b 0.69 b 7.0 d ni ni b 6.59 bd ni 74.8 1.55 17.8

30 d

1% (%, w/v) 15 d

untoasted

30 d

0.5% (%, w/v) 15 d 47.91 bc ni ni 29.4 ni 23.1 2.12 b 0.61 b 8.6 c ni ni b 4.8 bc ni 86.3 1.44 20.8

oak

a %, w/v. bDays. cLetters a and b in the same column indicate significant differences (p < 0.05) between the different treatments (toasted and untoasted) within the same wood. Letters c and d in the same column indicate significant differences (p < 0.05) between the different shaving % within the same treatment and wood. Letters e and f in the same column indicate significant differences (p < 0.05) between the different aging time within the same treatment, shaving % and wood. dNot identified. ePreviously reported (Cerezo et al., 2009).

29.0 ac 3.16 30.8 ac 33.8 11.1 a 23.5 1.15 a ni 11.3 2.45 ac 3.7 ac 3.42 55.28 ace 98.2 1.0 14.9

gallic acid 3-O-methylgallic acid protocatechualdehyde tyrosol dihydrorobinetine syringic acid vanillin isovanillin gallic ethyl ester fustin syringaldehyde ellagic acid robinetin caftaric acid p-cuomaric acid quercetin-3-glucoside

c

0.5%

15 db

compounds

a

toasted

concentration (mg/mL)

Table 2. Quantitive Evaluation of Phenolic Compounds in Vinegars Aged with Acacia and Oak Shavings

Journal of Agricultural and Food Chemistry Article


Journal of Agricultural and Food Chemistry

Article

Figure 1. LC-DAD chromatograms (280 nm) of vinegars aged with 1% of cherry (A), chestnut (B), acacia (C), and oak (D) shavings along 30 days: 1, gallic acid; 2, protocatechuic acid; 3, 3-O-methylgallic acid; 4, protocatechualdehyde; 5, tyrosol; 6, dihydrorobinetin; 7, syringic acid; 8, vanillin; 9, isovanillin; 10, gallic ethyl ester; 11, fustin; 12, syringaldehyde; 13, (+)-taxifolin; 14, ellagic acid; 15, robinetin; 16, naringenin. 4372



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fustin) changed depending on toasting, shaving percentage, and aging time. None of the other compounds changed under the different conditions. Polyphenol Compounds Identified for the First Time in Vinegars. Naringenin and kaempferol were both identified for the first time in vinegars aged with cherry wood, confirming previous studies that have described them as a marker in cherry heartwood.19,21,27 Recently, prunin and eriodictyol were established as markers of wine aged with cherry wood because they were determined after 2 months aging with cherry chips.28 The values reported were low: concentrations ranged between 0.24 and 0.65 mg/L (eriodictyol) and between 0.31 and 0.97 mg/L (prunin). The shorter length of our experiment may account for the absence of the above-mentioned compounds in our samples. It would be worthwhile verifying whether these compounds could be a marker of the time in contact with this wood, in addition to their value as cherry species markers. This is the first time that robinetin and fustin compounds have been determined in vinegars aged with acacia wood. They have been previously identified in acacia heartwood20,29 and in red wine aged in acacia wood barrels,30 but not in vinegars. Isovanillin was detected only in vinegars aged with oak shavings. Thus, isovanillin can be proposed as a marker of vinegars in contact with oak wood. As far as we know, this is the first time isovanillin has been identified in vinegars in contact with oak wood. Hydroxycinnamic aldehydes such as coniferaldehyde and sinapaldehyde have been detected in previous studies in wood extracts using methanol/water (1:1), diethyl ether, and ethyl acetate as solvent.19−21 They have not been not detected either in wine aged with these woods28 or in vinegars that likely involved a matrix effect. Furthermore, in these studies, wood extracts were subsequently treated to concentrate minor compounds.19−21 Conversely, our samples were only filtered before injection, to prevent any artifact formation during sample treatment. The absence of the sample treatment and concentration step in our experiment explains the absence of these compounds in our samples. In summary, further to (+)-taxifolin and (+)-dihydrorobinetin, which have previously been identified in vinegars aged in cherry and acacia wood,10,22 naringenin and kaempferol are also good markers of vinegars aged with cherry wood, and robinetin and fustin are wood markers of acacia. Effect of Wood Toasting on Phenolic Composition. In general, toasting was crucial for decreasing the concentrations of flavonoids ((+)-taxifolin, naringenin, and fustin) and a certain phenolic acid (gallic acid). Furthermore, wood toasting is essential for the release of certain compounds, such as aldehydes, into the vinegar, which is crucial for quality and sensory purposes.31 Indeed, vinegars that have no contact with wood maintain their polyphenolic composition with no changes.32 In our results, (+)-taxifolin concentration was 4−5 times higher in vinegars aged with untoasted shavings compared to vinegars aged with toasted shavings (Table 1). (+)-Taxifolin has been previously described as a marker of vinegar aged in untoasted cherry wood barrels,22 but no studies have yet been performed to elucidate on the effect of toasting on its release into the vinegar. Sanz et al.19 found that the (+)-taxifolin content in seasoned cherry wood was more than 35 times higher than in toasted wood. In this study, toasting showed similar effects on naringenin, where its concentration was 3 times higher in samples aged with untoasted cherry wood shavings (Table 1). Some authors have also shown toasting to

of shavings as an option to shorten aging time, in order to produce differentiated high-quality products more economically.

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MATERIALS AND METHODS

Woods and Vinegar. A young, monovarietal, Cabernet Sauvignon red wine, a 2007 vintage, produced under controlled experimental conditions by the “Rancho La Merced” research center (Jerez de la Frontera, Spain), was used to produce the vinegar samples under conditions described in a previous study.25 The resulting vinegar had 8.97 g/100 mL of acetic acid and an alcoholic degree of 1.33° (%, v/v). Acacia (Robina pseudoacacia), cherry (Prunus avium), chestnut (Castanea sativa), and oak (Quercus sps.) wood shavings (5−10 mm) were provided by Boteria Torner S.L. (Barcelona, Spain). Vinegar samples were aged for 15 and 30 days using previously toasted (at 180 °C for 3 h in the oven) and untoasted shavings at 0.5% and 1% (w/v). A total of 32 samples were analyzed in duplicate. Samples were filtered through a Millex-LCR 13 mm filter before injection. LC-DAD Identification and Quantification of Phenolic Compounds. LC analyses of phenolic compounds were performed using an Agilent series 1100 system equipped with a quaternary pump (series 1100 G1311A), automatic injector (series 1100 G1313A), and degasser (series 1100 G1379A). Detection was carried out using a UV/vis diode detector (series 1100 G1315B) coupled to a Chemstation HP A.10.02 (HP/Agilent). The column was an Agilent Zorbax SB-C18, 4.6 × 250 mm and particle size 3.5 μm. The chromatographic conditions had previously been used for vinegar analysis.5 The method uses a binary gradient, A (glacial acetic acid/ water pH 2.65) and B (20% A + 80% acetonitrile), programmed for the following gradient: 0 min 0% B; 5 min 2% B; 10 min 4% B; 15 min 10% B; 30 min 20% B; 35 min 30% B; 40 min 100% B; 45 min 0% B. The sample volume injected was 50 μL. The flow rate was 1.5 mL/ min, and the temperature was set at 40 °C. Quantification was performed by external calibration with respective standards, using detection at 365 nm (robinetin, quercetin-3-glucoside, and kaempferol), 320 nm (hydroxycinnamic acids and syringaldehyde), and 280 nm (the remaining compounds). Once successful spectral matching had taken place, the results were confirmed by spiking with respective standards to achieve a complete identification by means of the peak purity test. The peaks that did not fulfill these requirements were not quantified. The standards of the identified phenolic compounds were purchased from Fluka [caffeic acid, gallic acid, p-coumaric acid, gallic ethyl ester, quercetin-3-glucoside, isovanillin, and tyrosol], Sigma [(+)-taxifolin, ellagic acid, syringic acid, protocatechualdehyde, kaempferol, protocatechuic acid, and syringaldehyde], Merck [vanillin], Extrasynthese [fustin and robinetin], and Chromadex [3o-methylgallic acid, caftaric acid, and naringenin]. Dihydrorobinetin was isolated from vinegar.10 Statistical Analyses. Statistical analyses were performed using Statistica software.26 One-way analysis of variance (ANOVA) was used to test significant differences between (i) samples aged with toasted and untoasted shavings, (ii) vinegars aged for different periods, and (iii) vinegars aged with different shaving proportions. Linear discriminant analysis (LDA) enabled functions to be constructed to classify samples according to the type of wood or wood toasting with which they were in contact. The forward stepwise method achieved the best results.

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RESULTS AND DISCUSSION A total of 21 phenolic compounds were detected in vinegars aged with cherry, acacia, chestnut, and oak wood shavings (Tables 1 and 2). Figure 1 shows the chromatograms of the vinegars aged with each wood. The concentrations of phenolic acids (gallic acid and ellagic acid) and their esters (gallic ethyl ester), phenolic aldehydes (protocatechualdehyde, vanillin, isovanillin, and syringaldehyde), and flavonoids ((+)-taxifolin, naringenin, kaempferol, quercetin-3-glucoside, robinetin, and 4373



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has also been described for wine aged with 3 and 6 g/L of cherry chips.28 However, other phenolic compounds such as protocatechualdehyde, kaempferol, fustin, robinetin, and isovanillin significantly increased their concentration at 1% of shavings but not in the same proportion as the shaving amount (Tables 1 and 2). On the other hand, the concentration of other phenolic compounds, vanillin for instance, did not significantly change in accordance with shaving amount (Tables 1 and 2). Aging time has no remarkable influence on most of the phenolic compounds released from wood (Tables 1 and 2), which is in line with previous data reported, in vinegars aged with chips.5 Fifteen days was sufficient to obtain the highest polyphenol concentrations in the vinegar, except for gallic acid, (+)-taxifolin, syringaldehyde, robinetin, and fustin (Tables 1 and 2). Gallic acid concentration significantly increased at 30 days, when aging was performed only with untoasted chestnut (1% shaving) and cherry and acacia (0.5 and 1% shaving) wood, the highest increase being with chestnut (40%), due to the high gallic acid content present in chestnut wood species.22 The concentration of (+)-taxifolin also increased (>50%) after 30 days of aging with untoasted cherry shavings (Table 1). Additionally, we observed that the longer the time, the higher the concentration of syringaldehyde in vinegars aged only with toasted acacia shavings (1%) (Table 2). Remarkably, (+)-taxifolin, protocatechualdehyde and vanillin showed a higher concentration in vinegars aged with shavings (Tables 1 and 2) than in vinegars aged in barrels that were made specifically using the same untoasted wood.22 For instance, (+)-taxifolin content was more than 7 times higher in vinegars aged with untoasted cherry shavings at 1% (v/v) for one month (Table 1) compared to vinegars aged in an untoasted cherry wood barrel for six months.22 Additionally, vinegars aged with untoasted acacia shavings at 0.5% (v/v) for 15 days (Table 2) presented a protocatechualdehyde concentration 3 times higher than in vinegars aged in untoasted acacia wood barrels for 180 days.22 Moreover, vanillin concentration was 4 times higher in vinegars aged with untoasted oak shavings at 0.5% (v/v) for 15 days (Table 2) compared with vinegars aged in an untoasted oak wood barrel for a year.22 On comparing robinetin concentration in vinegars aged with toasted acacia shavings at 1% (v/v) for 15 days (Table 2) to its concentration in wines aged in toasted acacia wood barrels for 6 months,30 its concentration was observed to be 3 times higher in the former. These data therefore support that shaving is a useful practice to accelerate the aging process and release the highest quantity of certain polyphenol compounds, including wood markers. Statistical Analysis. Phenolic compounds are a powerful tool for discriminating vinegars produced in different origins34 and with different acetification processes35 and woods.22,24 They are chemical markers for authentication purposes.36 Since this paper found phenolic compounds in vinegars for the first time, we analyzed their capability to be used for these purposes. In order to achieve this goal, a linear discriminant analysis (LDA) was performed on the whole set of samples, grouping the vinegars according to the species of wood shavings used for aging. By means of the forward stepwise method, 16 variables were included in the model (robinetin, kaempferol, (+)-taxifolin, naringenin, vanillin, isovanillin, syringaldehyde, gallic ethyl ester, gallic acid, ellagic acid, caffeic acid, caftaric acid, protocatechuic acid, 3-O-methylgallic acid, quercetin-3-glucoside, and tyrosol) and 3 factors obtained. Table 3 displays the

be crucial in the release of naringenin from cherry wood, its content being 9 times higher in seasoned heartwood compared to toasted heartwood.19 A matrix effect should explain the differences in these concentration values obtained. Differences are greater if a solvent such as methanol/water (1:1), diethyl ether, or ethyl acetate19 is used rather than wine28 or vinegar (Table 1). In fact, red wine aged with toasted cherry chips has shown double the concentration of (+)-taxifolin28 compared to our vinegar aged with toasted cherry chips in a similar proportion (Table 1), as a consequence of the higher solubility of this compound in an ethanol matrix. However, in similar conditions (toasting and chips content) naringenin showed similar concentrations in wine28 and vinegar (Table 1). The release of fustin into the vinegar was also affected by toasting; it presented a concentration 6−7 times higher in vinegars aged with untoasted acacia wood shavings (Table 2). Previous studies have reported that fustin is highly sensitive to thermic treatment; acacia heartwood toasted at medium plus conditions (185 °C for 45 min) did not show fustin content, while there was a high concentration in the seasoned one.20 Similar thermic sensitivity has been previously described for (+)-dihydrorobinetin, which presented a concentration 20 times higher in vinegars aged with untoasted acacia shavings compared to the toasted ones.10 In contrast, toasting had the reverse effect on robinetin content, which presented concentrations between 12 and 29% higher in vinegars aged with toasted acacia shavings (Table 2). Previous studies state that robinetin does not change significantly with toasting in acacia heartwood20 and wine aged with acacia wood barrels,30 appearing to be quite stable when heated. A high concentration of gallic acid has been described as a marker of vinegars in contact with chestnut wood.23 The present study shows that gallic acid was significantly influenced by toasting, showing concentrations more than 3 times higher in vinegars aged with untoasted shavings (Table 1). Sanz et al. agrees that gallic acid is extremely sensitive to heat treatment.21 On the other hand, phenolic aldehyde concentrations were higher in vinegars aged with toasted shavings, in accordance with previous data reported comparing toasted and untoasted heartwoods.16,21 In particular, syringaldehyde was restricted to vinegars aged with toasted shavings (Tables 1 and 2). Wood species determined the release of aldehydes and their concentration in the vinegar. Specifically, protocatechualdehyde was identified in cherry and acacia; vanillin and syringaldehyde in acacia, chestnut, and oak; and isovanillin only in oak wood (Tables 1 and 2). In addition to the aldehyde compounds, toasting significantly increased the release of ellagic acid in vinegars aged with chestnut and oak wood shavings, showing significantly higher ellagic acid concentration in those aged with chestnut wood. Previous studies also agree that toasting favors the release of this compound.33 Kaempferol concentration was also significantly increased with toasting in vinegars aged with cherry shavings (Table 1). In contrast, Sanz et al. showed that kaempferol content was higher in seasoned cherry heartwood than in toasted heartwood.19 Effect of Shaving Amount and Aging Time on Phenolic Composition. It would seem likely that the higher the shaving percentage (w/v) used, the higher the content of phenolic compounds released into the vinegar. This tendency was observed for (+)-taxifolin and naringenin, whose concentrations remarkably doubled, at 1% compared to 0.5% of shavings, regardless of wood treatment (Table 1). This trend 4374



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phenolic compounds are also a useful tool for discriminating vinegars elaborated with shavings from different wood species. Additionally, results show that toasting acacia, cherry, chestnut, and oak wood shavings mostly decreases flavonoid wood markers, such as (+)-taxifolin, naringenin, and fustin, while it is essential for the highest releases of aldehyde compounds into the vinegar. This paper provides data to develop a strategy to produce vinegars with an appropriate phenolic composition in a short time. If phenolic aldehydes are desired for their impact on the sensory properties of the product, toasting shavings is mandatory. Conversely, if the aim is to obtain a product that is rich in flavonoids, no toasting is recommended.

Table 3. Correlation of Variables with the Canonical Roots Using the Forward Stepwise LDA Analysis According to the Species of Wood Used for Vinegar Aging variables

root 1

root 2

root 3

caffeic acid tyrosol quercetin-3-glucoside caftaric acid 3-O-methylgallic acid vanillin robinetin protocatechuic acid naringenin syringaldehyde ellagic acid gallic ethyl ester kaempferol (+)-taxifolin gallic acid isovanillin

0.392 885 0.095 382 0.003 531 0.085 433 0.024 167 −0.128 460 −0.054 338 −0.020 036 0.054 986 −0.013 557 −0.050 510 −0.019 819 0.136 626 0.043 024 −0.001 211 −0.021 178

0.113 208 −0.313 251 0.242 954 −0.132 525 −0.135 065 0.165 642 −0.023 099 −0.206 189 0.015 844 −0.030 086 −0.034 872 −0.054 281 0.039 368 0.012 397 −0.039 380 0.070 516

0.107 229 0.290 785 0.159 681 −0.112 586 0.150 303 −0.089 921 0.274 941 −0.178 797 0.015 007 −0.016 787 −0.231 856 0.006 696 0.037 289 0.011 742 −0.057 710 −0.058 782

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 954233765. Funding

The authors are grateful to the European Commission (Project WINEGAR, COOP-CT-2005-017269) for financial assistance. Notes

correlation of variables with the canonical roots obtained. Figure 2 shows the scatterplot obtained plotting root 1 versus

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Belén Puertas and Emma Cantos (“Rancho de la Merced”, Jerez de la Frontera, Spain) for providing the wine.

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REFERENCES

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Figure 2. Plot of the first two roots issued from linear discriminant analysis (LDA): A, acacia; R, oak; S, chestnut; C, cherry.

root 2. The classification functions achieved 100% correct sample classifications according to wood species. Furthermore, a statistical analysis to discriminate samples according to thermal treatment of shaving was performed. Forward stepwise LDA allowed 100% of samples to be correctly classified; the variables in the model were dihydrorobinetin, kaempferol, naringenin, isovanillin, syringaldehyde, protocatechualdehyde, gallic ethyl ester, ellagic acid, caffeic acid, syringic acid, caftaric acid, protocatechuic acid, 3-O-methylgallic acid, and quercetin3-glucoside. (+)-Taxifolin has been previously used as a wood marker to classify vinegars according to the kind of wood in which they were aged.22 This paper shows for the first time that robinetin, kaempferol, and naringenin are wood markers that are useful for authentication purposes in vinegar making. This paper provides advanced knowledge of polyphenol composition in vinegars by describing naringenin, kaempferol, robinetin, fustin, and isovanillin in vinegars for the first time. The statistical treatment proves that certain newly detected 4375



Article

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Phenolic Composition of Vinegars over an Accelerated Aging Process

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