3.1. Physicochemical Analysis of Wine at the Time of Aging in Bottles and Oak Barrels
shows the enological characteristics of wines obtained with 3 and 6 days of skin maceration of untreated and PEF-treated grapes. Data correspond to the wines at the time of bottling and aging in oak barrels. Parameter values lay within the range usually observed in Grenache
variety wines [18
]. However, the wines obtained in this study had higher alcohol content because the grapes were harvested at the end of the campaign with high sugar concentration.
No statistically significant differences were found in the pH, nor in the total acidity of the four wines, and the differences in ethanol content between the wine with the lowest value and the highest value were less than one unit of ethanol (%v.v−1
). These differences can be attributed to the varying fermentation processes brought about by the yeast in the separate tanks rather than to the PEF treatment. These results agree to previous studies that showed that PEF does not show any significant effect on wine alcoholic content [20
For the enological characteristics that depend on phenolic extraction during skin maceration, values increased when grapes were treated with PEF, or when skin maceration was extended from 3 to 6 days. Statistically significant differences were found between the wines obtained from untreated and PEF-treated grapes after 3 or 6 days of skin maceration. Greater differences were found for color intensity (51%) and total anthocyanins (37%) between the wines obtained from untreated and PEF-treated grapes when maceration time was shorter (3 days). The total polyphenol index and the tannin content were 30 to 40% higher for the wines obtained from grapes treated with PEF with the two maceration periods. These effects can be attributed to the electroporation caused by PEF that facilitates the release of intracellular compounds [21
3.2. Evolution of Color Intensity, Anthocyanin Content, Total Phenolic Content, and Tannin Content during Aging in Bottles and Oak Barrels
and Figure 3
show the evolution of wine characteristics depending on the polyphenols extracted throughout the maceration stage during aging in bottle for 24 months, and aging in oak barrels for 6 months plus subsequent aging in bottle for 18 months, respectively.
As previously reported during aging of Cabernet Sauvignon wine obtained with grapes treated by PEF [14
], in general, the application of a PEF treatment prior to the maceration-fermentation stage did not affect the subsequent evolution of color intensity, anthocyanin content, total phenolic content, or tannin content: neither during bottle aging, nor during oak aging followed by bottle aging. In all cases, at the end of the aging process the values for those indexes were lowest for the wine obtained from untreated grapes with 3 days of maceration. It has been reported that wines obtained with techniques such as thermovinification or flash-expansion, which greatly accelerate polyphenol extraction, produce wines that often have poor stability and little structure [23
]. This effect has been explained by the fact that these techniques promote the extraction of anthocyanins, but not the extraction of other polyphenols that provide wine structure and anthocyanin stabilization [24
]. According to our results, this effect was not observed in the wines obtained with grapes treated by PEF. The evolution of the wine obtained with grapes treated by PEF followed the typical pattern for wine aging reported in the literature [10
Color intensity values of wines aged in bottles (Figure 2
A) or aged in oak barrels plus bottles (Figure 3
A) did not change significantly after 24 months of storage. However, aging caused a significant decrease in total anthocyanin content for all four wines (Figure 2
B or Figure 3
B). In all cases, the reduction in anthocyanin content was more rapid during the first six months of aging. The decrease in anthocyanins during wine aging has been attributed to precipitation and oxidation reactions [25
]. These reactions seem to occur to the same extent in wine obtained from untreated grapes as in wine obtained from grapes treated with PEF. Although anthocyanins are the compounds that mainly account for the red and purple color of wine, the reduction of those compounds during aging did not affect the color intensity. This preservation of color during aging is a consequence of the formation of polymeric pigments lying between anthocyanins and other wine components such as tannins, and of the formation of derived pigments by condensation. Condensation consists in non-covalent links of anthocyanins with colorless molecules or with other anthocyanins [28
]. Therefore, similarly to the wine obtained from untreated grapes, the wine obtained with PEF-treated grapes contained the molecules that participate in the reactions that are responsible for color stabilization.
The total phenol index (TPI) for the wines aged in bottle obtained with untreated and PEF-treated grapes with 3 days of maceration remained practically constant (Figure 2
C). In the case of the wine obtained from PEF-treated grapes with 6 days of maceration, a decrease in TPI was observed after the 3 first months of aging, after which it remained practically constant (Figure 2
C). This decrease in TPI could be due to the precipitation of a proportion of polyphenols as a consequence of their high initial concentration at the point of bottling. The evolution of TPI during aging in oak barrels was similar to those in bottle. (Figure 3
C). However, for the remainder of the wines, the TPI increased during aging in barrels (6 months), after which it slightly decreased during aging in bottle. Therefore, the extraction of phenolic compounds from the wood responsible for the TPI increment occurred both in wines obtained with untreated grapes as in those with PEF-treated grapes. In the case of the wine obtained with PEF-treated grapes after 6 days of maceration, an increment in TPI was not observed. This was probably because the precipitation of polyphenols exceeded the degree of phenolic extraction from the wood.
Tannins represented in Figure 2
D or Figure 3
D are formed by the polymerization of the polyphenolic flavan-3-ol monomers catechin and epicatechin [28
]. An increment in tannin content up to 12 months of aging was observed as a consequence of the formation of polymer chains with a different degree of polymerization for the four wines aged in bottle or in barrel. After 12 months, this index tended to decrease slowly. Similarly to TPI, no differences in tannin content were observed at the end of aging between the wine obtained from untreated grapes with six days of maceration and the wine obtained from grapes treated by PEF with 3 days of maceration. These results indicate that the concentration of alcohol after 3 days of fermentation was high enough to encourage an elevated rate of extraction of the polyphenols that would form the tannins by polymerization. These compounds, which have a low degree of water solubility, require the presence of ethanol in order to be extracted [30
3.3. Evolution of the Content of Phenolic Families and Individual Phenolics during Aging in Bottles and in Oak Barrels
The concentration of phenolic families (anthocyanins, hydroxycinnamic acids, flavonols, and flavanols) and the individual polyphenols of the four wines after 6, 12, and 24 months of bottle aging, or oak aging followed by bottle aging, are shown in Table 2
and Table 3
In the course of the entire 2-year aging period, the total content of phenolic families tended to decrease, independently of PEF treatment or maceration time. Similar results have been observed in the aging of Cabernet Sauvignon
wine obtained from grapes treated with PEF [14
As in the evolution of the characteristics described above, a higher concentration of individual phenolic compounds was generally observed in the wines obtained from PEF-treated grapes than in those obtained from untreated grapes after an identical maceration period. The differences between the wines obtained from untreated grapes after 6 days of maceration and the wines obtained from PEF-treated grapes with 3 or 6 days of maceration tended to level out in the course of aging, whereby the polyphenolic content of the wines obtained from untreated grapes with 3 days of maceration was always lower. In all cases, no evidence of a particular effect of PEF treatment on the extraction of a specific family or individual phenolic compound was observed.
Monomeric anthocyanins were the predominant polyphenols in all the wines. Among all polyphenolic families, anthocyanins were considerably more reduced in all four wines, either due to reactions associated with the formation of new stable polymeric pigments, or due to degradation reactions. As the color of all four wines remained stable during aging, the loss of monomeric anthocyanins seems to be due to their transformation into more stable pigments in terms of color, rather than to their degradation. Anthocyanin decrease was more pronounced in the wines aging only in bottle than in the wines aging in oak barrels. After 24 months of aging, total individual anthocyanins were 20 to 40% higher for wines aged in barrels.
Malvidin-3-glucoside was the principle anthocyanin, representing practically half of all monomeric anthocyanins in all wines. As in other studies on wine aging, the observed decrease in total monomeric anthocyanins was mainly due to this compound’s notable decrease [14
]. After 24 months of aging, the concentration of malvidin-3-glucoside decreased significantly in all wines, representing approximately one-third of all monomeric anthocyanins. This decrease was observed in the same proportion in the wines obtained from untreated grapes as in those obtained from PEF-treated grapes. Thus, after the same maceration period, wines obtained with untreated grapes had a lower amount of monomeric anthocyanins compared with the wines obtained from PEF-treated grapes after 24 months in both aging processes.
Glucoside, acetylated, and coumarylated anthocyanins evolved in a similar way, decreasing in the course of aging in wines obtained with untreated and PEF-treated grapes.
A total of three hydrodynamic acids, five flavonols, and two flavanols were identified and quantified in all wines. The evolution of these polyphenolic families in wines obtained from untreated and PEF-treated grapes was similar in the course of aging, either in bottles, or in oak barrels. In all cases, a progressive decrease throughout aging was observed. In general terms, by the end of the aging process, the highest value in these families was observed in the wine obtained from grapes treated with PEF after 6 days of maceration, and the lowest values thereof in the wines obtained after 3 days of maceration with untreated grapes.
Similar results as those discussed regarding different polyphenol families were observed for the individual polyphenols of each family as well. The evolution of individual polyphenols was similar in the two wines obtained from untreated and PEF-treated grapes after aging in bottles, or oak barrels with subsequent bottling. In all cases, a decrease in the concentration of these compounds was observed through time. The wine obtained from grapes treated with PEF after 6 days of maceration presented the highest amount of hydroxycinnamic acids in the course of aging, mainly due to a higher amount of t
-caftaric acid. This wine also presented the highest amount of flavonols, whereby myricetin-3- glucoside was the most abundant flavonol. In the case of flavanols, after 6 months of aging their content tended to be higher in the wines in oak barrels than in the wines exclusively aged in bottles. This higher content is related to the extraction of flavanols from oak wood [10
]. Whereas after 6 months of aging the content of (+)-catechin was higher than the content of (-)-epicatechin in all wines, after 24 months of aging the content of both flavanols was similar.
3.4. Sensory Evaluation
shows the percentages of correct responses identifying the odd sample in the triangle test and the results of the preference test. Significant sensory differences were detected by the panelists in the wines obtained with untreated or PEF-treated grapes after 3 and 6 days of maceration when aged either in bottles or in oak barrels. All panelists were able to differentiate the wines obtained with untreated or PEF-treated grapes after 3 days of maceration for both types of aging (bottles vs. oak barrels plus bottles). In both cases, a majority of panelists (86%) preferred wines elaborated with PEF-treated grapes. When the wines obtained with untreated and PEF-treated grapes with 6 days of maceration were compared, panelists had more difficulty in differentiating them (71% success) when they had aged in bottles. However, all seven panelists were able to differentiate them when they had aged in barrels. In both cases, panelists likewise preferred the wine obtained from grapes treated with PEF. Finally, independently of the type of aging, panelists were able to differentiate the wines obtained with grapes treated by PEF with 3 days of maceration from the wines obtained with 6 days of maceration with untreated or PEF-treated grapes with a success rate of 86%.
In the preference test, panelists preferred the wines obtained from grapes treated with PEF with longer maceration times when they had aged in bottle (71%) or in oak barrels (57%). Smaller differences were observed in the panelists’ preferences between the wine obtained from grapes treated with PEF and 3 days of maceration (57%) and the wine obtained from untreated grapes and 6 days of maceration (43%), but 71% of the panelists preferred the wine obtained from PEF-treated grapes after six months of aging in oak barrels.
In summary, these results indicate that the improvement in polyphenolic extraction brought about by the application of a PEF treatment prior to maceration permits to obtain wines that are sensorially different from those obtained with untreated grapes. In all cases, panelists preferred wines obtained from grapes treated with PEF after aging in bottles, or in oak barrels. These results support conclusions previously reached in the comparison of physicochemical wine characteristics. The application of a PEF treatment to the grapes permitted to reduce maceration time from 6 to 3 days without negatively affecting the wines’ physicochemical and sensory characteristics. When comparing wines obtained with untreated and PEF-treated grapes after longer maceration periods, smaller differences were observed in characteristics depending on polyphenol extraction, but from a sensory point of view the wine obtained from grapes treated by PEF was preferred by panelists, especially after it had aged 6 months in barrels.
displays the sensory profiles of the wines obtained from untreated and PEF-treated grapes with 3 and 6 days of maceration after six months of oak aging and 6 months of bottle aging. This evaluation confirmed the differences among the wines already observed through physicochemical analysis. Wine obtained from untreated grapes and 3 days of maceration was clearly distinct from the remaining wines. It had a lower intensity in flavor, and lower descriptors directly related with polyphenol content such as color intensity, body, astringency, and persistency.
On the other hand, smaller differences in sensory descriptors were obtained between the other three wines, thereby confirming the potential of PEF for the reduction of maceration time without impairing physicochemical characteristics and sensory properties of wine, even after aging.