According to the main aim of this study, discussion of the obtained results will be focused on the effect of wood extractable components on the chromatic characteristics of red synthetic wine and on the anthocyanins, which are the compounds with more influence on red wine color.
3.1. Effect on Chromatic Characteristics of Red Synthetic Wine Solutions
It is well known that during the storage and aging period, red wine color commonly changes. Thus, a decrease of color intensity occurs together with a color tonality increase. Obtained results of chromatic characteristics agree with these usual changes of wine color (Figure 1
and Table 2
). The decrease of color intensity detected in the different experimental synthetic wine solutions was intense in all cases (an average value reduction ranged from 21.7 to 38.3%, respectively, after 15 and 30 aging days), although it was quicker when wood extracts of 30 extraction days were used. In these cases, significant decreases were observed after the first 15 aging days, while extracts of 15 extraction days produced significant changes only after one month of aging (Figure 1
Cherry wood extracts seemed to induce a slight decrease in color intensity, although quantitatively, this fact was only statistically significant after 30 aging days (ChExt30 + Anth sample). Several authors [24
] demonstrated that the use of cherry wood provides an environment favoring oxidative reactions, and therefore increases the red color loss, making it less suitable for longer wine aging stages. In addition, according to the same authors, cherry wood is also characterized by a very low level of ellagitannins, which provides a reduction of the antioxidant protection of anthocyanins providing a higher oxidative environment than other wood species, and consequently, it potentially further reduces color intensity.
With respect to color tonality (Figure 1
B), significant and similar increases were detected for all synthetic wine solutions containing the wood extracts with similar extraction time and the anthocyanin extracts. In agreement with the changes in color intensity, tonality increased quicker and more intensively when wood extracts obtained after 30 extraction days were used. In this case, no significant effect of wood species was observed. Tonality results agree with previous data obtained in red wines aging with oak chips [10
Regarding to CIELab* parameters (Table 2
), lightness (L*
) values showed the usual increase tendency, which correspond to color losses, mainly with the reduction of absorbance at 525 nm [41
]. Thus, L
* values showed the same tendency that color intensity, being the synthetic wine solutions containing wood extracts of 30 extraction days, those that induced quicker L
* value changes. In addition, the general decrease in the tendency observed for a*
(redness) agree with the observed results for color intensity showed in Figure 1
A. The a*
values decreases were particularly intense when wood chip extracts were obtained after 30 extraction days were used. In addition, the lower a
* values were detected when wood chip extracts from French oak and cherry wood were used. Previously, Jordão et al. [17
] also reported, for model wine solutions containing malvidin-3-glucoside, a decrease of this anthocyanin and a*
values more pronounced when in the presence of oak wood extracts. Furthermore, for b*
values (yellowness), in general, an increase of the values was detected. Corresponding with the increase of color tonality, b*
values in general increased after mixed anthocyanin and wood extracts (Table 2
). This fact pointed out a clear increase of the yellow color that was more intense when extracts of oak wood species (French, American, and Iberian species) were used. Besides this, the results of 15 days of maceration extracts wrote down a significant increase of b
* values after the first 15 aging days. This point, together with the stability of a
* values, could point out a possible protective color of these extracts, but only during this period. It is important to note that the extraction of several wood phenolic compounds could explain an increase in b*
values (yellowness) that were already detected in red and white wines aged in contact with different wood chips [28
Finally, the values obtained for color difference (ΔE) between control and the other synthetic wine solutions showed that, in all cases, ΔE values were much higher than 3 CIELab units (values ranging from 8.0 to 35.4 CIELab units, Table 2
) and then all chromatic modifications were potentially detected by human eyes [38
]. According to previously commented results, ΔE values showed intense increase after 15 aging days when wood extracts of 30 days of extraction were used, and in general, they had longer aging time and higher values of ΔE were observed.
Considering all chromatic results obtained, it is possible to assert that, as higher levels of extractable wood components (Table 3
), higher modification of color was observed, and this fact could have negative consequences on quality, especially due to the drastic reduction of the color intensity. In fact, it was clear that synthetic wine solutions containing extracts obtained with higher extraction time (30 days) showed, in general, a significant increase of total phenolic content.
3.2. Effect on Total Anthocyanin and Phenolic Levels
Results showed an intense and quick reduction of the global level of total anthocyanins in synthetic wines solutions with wood extracts of 30 days of maceration, and similar results were observed until one month of aging of synthetic wines containing wood extracts of 15 days of maceration (Figure 2
). These results explain the evident decrease of color intensity and a
* values commented previously. Furthermore, it is well correlated with the increase of L
* values, since losses of red pigments produce lighter solutions.
No significant differences among wood species were detected in any case, and after 30 days of aging, all the synthetic wines with wood extracts showed similar levels of global anthocyanins, which were drastically lower than levels of the control synthetic wine. These results showed a clear effect of extractable wood components on the modification of the anthocyanin fraction. Therefore, results point out that extractable wood compounds can constitute a destabilizing factor for the anthocyanins, yielding colorless compounds, with lower absorbance to 520 nm (red color) and higher absorbance in the visible region around 400–460 nm, which correspond with yellow-brown tones, justifying the previously commented increase of tonality and b*
values. These results are contrary to previous works in which wood contact was described as a stabilizing wine color process [33
]. However, it is interesting to have in mind that the cited studies were carried out in wines, where many other compounds can interfere the reactions occurring between anthocyanins and wood compounds, and where anthocyanins can be in more stable structures (co-pigmented and condensed forms) than the “free anthocyanins” extracted from red grapes skins.
From the other point of view, results also showed that the effect of extractable wood components seems to be independent of the quantity and type of extractable wood compounds since all the extracts, independent of their global phenolic content (Table 3
), produced similar final effect.
Levels of total phenolic of the wood extracts were significantly different with respect to both factors, wood species and maceration time. In general, as the longer the extraction time higher the quantity of total phenols extracted until reaching a maximum, followed by a decreasing of the phenolic content because of their participation in different reactions. Jordão et al. [42
] reported, in model wine solutions, that there is an initial period, during which supply of ellagitannins and ellagic acid to the oak wood chips/model wine solutions contact layer is high, due to the solubility of the compounds, and a second period during which there is a clear and continuous decrease in their values. Presumably, this decrease is a consequence of their participation in oxidation reactions, causing them to degrade and consequently leads to a decrease in their values. However, according to our results, the increase ratio of total phenols was very different among wood species. Thus, while American oak extracts showed very low increment with the time of extraction, this was not statistically significant in fact, French oak extracts showed the highest increase around 42%, followed by Acacia extracts (around 28%) and cherry and Portuguese oak extracts with an increment between 15% and 18%, respectively. These results agree with those of previous works that pointed out that each type of wood showed particular extraction kinetics [43
]. For example, the anatomical structure of the American oak wood itself, including its porosity, makes ease the extraction of wood components. This fact may explain that most extractable compounds of American oak were extracted during the first 15 days of maceration. Furthermore, results agree with previous studies that reported a variability of total and individual extractable phenolic compounds between oak and other non oak wood species and indicate a higher total phenolic composition of oak woods in comparison with cherry wood [27
]. In addition, the slight influence of the wood species factor on the final levels of total anthocyanins of the synthetic solutions agree with previous work [33
] carried out with chips of different types of oak and with diverse toasting degree.
3.3. Effect on Individual Anthocyanin Levels
The analysis of the levels of some individual anthocyanins gave more information about the anthocyanin transformation globally tested by the decrease of total anthocyanins levels.
Levels of free monoglucoside anthocyanins, which includes -3-0-glucosyl derivatives of cyanidin, delphinidin, malvidin, peonidin, and petunidin, were significant lower in all synthetic solutions containing wood extracts than in control solution, containing only grape skin anthocyanin extract (Figure 3
A). In general, the lowest levels were measured under 30 days of storage. The mean loss of monoglucoside anthocyanins level was around 45%, indicating a drastic reduction of free anthocyanin pigments. Barrera-García et al. [9
] also reported 30% lower levels of malvidin-3-monoglucoside after 20 days of contact with wood extracts in model wine solution. No remarkable differences were detected for the extracts with 15 and 30 days of extraction, nor with respect to the wood species factor. These results are contrary to other published works. Thus, Del Álamo Sanza et al. [10
] reported a greater decrease of monomeric anthocyanins in red wines aged with French than those aged with American oak. In addition, other authors [33
] reported a significant effect of oak wood origin on the individual anthocyanin level of a wine macerated with chips, however the effect was variety wood dependent. Once more, the cited differences could be attributed to the effect of other compounds present in wines and none in the synthetic solutions, which can interfere in the reactions that occur between anthocyanins and wood compounds. In addition, it is important to note that some anthocyanins (co-pigmented and condensed forms), formed during fermentation and others during the winemaking process are in more stable structures in wines.
Similar to monoglucoside anthocyanins, levels of the main p
-coumaroyl derivatives (Figure 3
B) were also significantly lower in all synthetic wines containing wood extracts than in the control synthetic wines and no remarkable differences among species were detected. In addition, the decrease ratio of p
-coumaroyl derivatives was lower than that of monoglucosyl derivatives (30%). These results agree with the higher stability of acyl-anthocyanins with respect to non-acylated derivatives already reported by other authors [46
]. According to Smart [49
], wines made from red grapevine cultivars with high proportions of acylated anthocyanins can have greater color stability compared with those from red varieties with no acylated anthocyanins, such as cv Pinot Noir.
New condensed pigments, not present in control synthetic wine containing only grape skin anthocyanin extract, were detected in model wine solutions with wood extracts. Some of them showed retention time and UV-Vis spectrum, like condensed catechin-anthocyanins, while others eluted in time very close to monoglucoside anthocyanins, but showed UV-Vis spectrum clearly different, and in fact allow to different ones to the others. In general, all the new pigments showed UV-Vis spectrum with the maximum absorbance in visible zones lower than 520 nm. None of all the new pigments detected (chromatographic peaks) could be identified and named. Besides this, those “new” chromatographic peaks that were well defined were considered together, being named new pigments group (Figure 3
The levels of new pigments were higher when wood extracts of 30 days of extraction were used, and new pigment levels increased along the time of storages. Previously, Jordão et al. [18
] reported the formation of new compounds detected in model wine solutions containing malvidin-3-monoglucoside and oak wood extracts after a short storage period. According to these new compounds, it showed a slight increase during 64 storage days.
Significant effects of wood species and maceration time factors on new pigment formation were detected. Wood extracts of 30 days of maceration produced higher and quicker increases than those of 15 days. After 15 days of aging, levels of these pigments were between four and eight times higher than in control wine, and after 30 days of aging, raised increase ranged between 7 and 13 times. The extracts of American oak and 30 extraction days induced the quickest and maximum formation of this type of new pigments, and only synthetic wines with wood extracts of acacia and Portuguese oak, obtained after 30 extraction days, showed levels of new pigments like them.
Observed results could be explained by the ability of anthocyanins to interact with wood components. Among extractable wood components, ellagitannins are easily extracted from wood by water-alcohol and water-acetone mixtures [17
], and ellagitannins can indeed react with flavanols and anthocyanins to provide condensation products [51
]. Then, there was a dynamic evolution from several interaction reactions and subsequent transformation of the original pigments results. The loss of free anthocyanins and the new compounds formed contribute to the color differences (ΔE), which were commented previously. Several authors [24
] reported that the use of cherry wood barrels in wine aging induces a faster evolution of wine pigments with a fast increment of derived and polymeric compounds formation with a consequential decrease of anthocyanin content. However, for the different synthetic wine solutions studied, it was not evident that a more marked increased in new pigments formed in solutions containing cherry wood extracts compared to the others. Other compounds probably different than the anthocyanins themselves may play an important role in new pigments formation. For example, condensed tannins present in wines may help to explain a greater evolution in the formation of new compounds during the wine aging process in contact with the cherry wood, in comparison to the verified one in the synthetic solutions studied.