3.1. Phenolic Composition of Oak Phenolic Extracts
The total phenolic contents of oak phenolic extracts from shavings are shown in Table 1
The oak phenolic extract from UOW and AOW shavings had similar TPC (665.47 ± 80.98 and 556.58 ± 87.94 mg GAE/L, respectively), and significant differences were not found (p > 0.05).
In a previous work, the extracts from American woods (Quercus alba
) had lower total phenolic content than the extracts from two European woods (Quercus robur
and Quercus petraea
]. In addition, other authors indicated that American oak woods have a content of total ellagitannins lower than that of European oaks [27
]. The major compounds in woods are ellagitannins, which constitute a complex class of polyphenols characterized by one or more hexahydroxydiphenoyl moieties esterified to a sugar [16
The detailed phenolic profiles of oak phenolic extracts from AOW and UOW shavings are shown in Table 1
. A total of ten compounds were determined. Regarding phenolic acids, only gallic acid and ellagic acid were detected. Among ellagitannins, four monomers (vescalagin, castalagin, grandinin, and roburin E) and four dimers (roburins A, B, C and D) were identified. These compounds were reported in the literature as important components of oak wood [20
As is shown in Table 1
, the qualitative phenolic profile is the same for the two types of oak phenolic extracts. Vescalagin and castalagin were the most abundant compounds found in both oak phenolic extracts (28.5% and 26.4% for AOW, and 29.2% and 29.7% for UOW, respectively). The rest of ellagitannins showed lower relative proportions, between 3% and 11%. Considering phenolic acids, these compounds were in a lower proportion than all ellagitannins evaluated (<3%). Gallic acid had a higher proportion than ellagic acid in the oak phenolic extracts (2.7% and 3% vs. 0.5% and 0.4%, respectively).
3.2. Antioxidant Activity of Oak Phenolic Extracts
Results showed that the antioxidant activity was in accordance with the TPC of each oak phenolic extract, according to the type of wood (Table 2
). Antioxidant activity for the oak phenolic extracts from UOW shavings was 7713.61 ± 411.34 and 3809.98 ± 508.80 μmol TE/L, for ABTS and FRAP methods, respectively. These values for oak phenolic extracts from AOW shavings were 6733.79 ± 319.35 and 2385.29 ± 406.12 μmol TE/L, respectively. Significant differences (p
< 0.05) were found between the antioxidant activity of oak phenolic extracts from UOW and AOW shavings for the FRAP data. However, the antioxidant activity of UOW and AOW determined by the ABTS assay was not significantly different (p
The correlation between TPC and antioxidant activity values was evaluated and a high positive and significant correlation (p
< 0.05) was found: r = 0.87 for the FRAP assay, and r = 0.81 for the ABTS assay. Therefore, the lower FRAP values of the oak phenolic extract from AOW shavings may be due to the TPC of this type of wood. According to other authors, phenols, especially ellagitannins, are mainly responsible for the antioxidant capacity of wood extracts [17
3.4. Antioxidant Activity of Model Solutions
Results of antioxidant activity of the control solution and the model solutions with different TPC are shown in Table 4
As can be observed, differences in the antioxidant activity of the model solutions were found depending on the type of wood (from UOW and AOW) and the phenolic concentration level (100, 400, and 500 mg/L). Antioxidant activity was higher for all concentration levels than for control (Table 4
Regarding ABTS assay, the same pattern was observed for model solutions from UOW and AOW shavings. Control model solution (TAC = 200 mg/L) had the lowest antioxidant activity (1300.89 ± 37.33 μmol TE/L). The antioxidant activity for the 100 mg/L model solutions was not significantly different (p > 0.05) from that of the control solution (1396.81 ± 33.74 and 1489.61 ± 224.16 for UOW and AOW, respectively). An increase in the antioxidant activity for the 400 and 500 mg/L model solutions was observed (2037.66 ± 33.74 and 1839.07 ± 150.59 for UOW, respectively; 2248.57 ± 0.97 and 1707.33 ± 274.98 for AOW, respectively), showing significant differences with the control solution and the 100 mg/L model solution (p < 0.05).
Antioxidant activity measured by FRAP method also was higher for the 400 and 500 mg/L model solutions than for the 100 mg/L model solution and the control solution (Table 4
< 0.05). A correlation analysis between TPC and antioxidant activity confirmed that the greater antioxidant effects in the 400 and 500 mg/L model solutions than in 100 mg/L and control solutions are due to their higher TPC.
Significant differences were not found between model solutions from AOW and UOW shavings for the antioxidant activity.
3.5. Color and Copigmentation Effect in Model Solutions
A color analysis (CIELAB space) of the model solutions and control solutions was performed. Figure 1
depicts the location of the samples within the (a*b*) diagram and lightness values (L*).
The CIELAB color parameters of the solutions were influenced by the type of wood (AOW and UOW) and the TPC of the model solutions. Lightness, chroma and hue values (L*, C*ab, and hab, respectively) of the model solutions decreased with the increase of the TPC. In general, solutions in the presence of the copigments exhibited darker and more bluish color than the control solution. These results show that an important part of the expression of the color of the anthocyanins could be due to the copigmentation phenomenon in which they are involved, leading to changes at both quantitative and qualitative levels in the color of the wine.
The effect of the type of wood (AOW and UOW shavings) and TPC on color characteristics of the model solutions was evaluated. Significant differences between the two types of wood were found (p < 0.01), as well as between model solutions with different TPC (p < 0.001). The results indicated a strong influence (p < 0.001) of TPC on all colorimetric parameters (L*, C*ab, hab). However, the type of wood influenced (p < 0.05) all colorimetric variables except L*.
In the model solutions from AOW, the colorimetric parameter values (L*, C*ab
) were lower for all concentration levels (100 mg/L: 14.9, 47.4 CIELAB units, 26.5°; 400 mg/L: 11.7, 42.5 CIELAB units, 23.6°; 500 mg/L: 1.2, 5.7 CIELAB units, 14.7°, respectively) than for the control solution (21.6, 58.5 CIELAB units, and 28.5°, respectively) (Figure 1
a). These differences were significant (p
< 0.05) between the control solution and the 500 mg/L model solution for L*, and between the 500 mg/L model solution and all the other model solutions for C*ab
Differences were also found between the control solution and UOW model solutions. UOW model solutions showed lower chroma (C*ab
), lightness (L*) and hue (hab
) values than the control solution (Figure 1
b). The decrease in C*ab
was more noticeable for the 400 and 500 mg/L model solutions (400 mg/L: 0.1 CIELAB units and 14.2°; 500 mg/L: 2.6 CIELAB units and 16.0°, respectively), and presented significant differences with the control solution (p
The total color and the effect of copigmentation of the model solutions with increasing concentrations of phenolic compounds are shown in Figure 2
Intermolecular copigmentation was observed in the model solutions: The control solution showed a hyperchromic shift of its λmax
(520 nm) and an increase of its initial total color (97.8 CIELAB units) (Figure 2
The copigmentation magnitude evaluated by the absorbance at 520 nm and by tristimulus colorimetry was different depending on the type of wood and the TPC (Figure 2
b,c). It was observed that the copigmentation effects of model solutions from UOW (15.93%, 95.20%, and 80.32%, for 100, 400 and 500 mg/L, respectively) were higher than of solutions from AOW (8.90%, 19.90%, and 26.82%, for 100, 400 and 500 mg/L, respectively) (Figure 2
b). The effect was significantly different (p
< 0.05) for the 400 mg/L model solution.
The color changes that provide color information related to visual perception were carried out by means of color differences calculation between the control solution and model solutions with different TPC (Figure 3
). Moreover, the relative contribution of each colorimetric parameter (%∆L
) to color difference permits an objective comparison of the colorimetric effects among samples. Results showed that the color changes of the model solutions from AOW and UOW shavings at all the concentrations were visually perceptible (∆E
> 3, [33
]). It can be observed that, in general, the relative contribution of lightness and chroma to color differences was greater than that of hue (%∆L
= 23% and 16%; %∆C
= 75% and 74%; %∆h
= 2% and 9%, for AOW and UOW model solutions, respectively).
Our results indicate that phenolic compounds extracted from oak byproducts improve the color characteristics and copigmentation effect of red anthocyanins in model solutions. These results are in accordance with the ones reported by some authors about the influence of some compounds from wood, like ellagic acid and ellagitannins, on the copigmentation and color of wine [34
]. UOW shavings are a better source of colorless copigments than AOW shavings. The difference between both kinds of wood (AOW and UOW) could be due to American oak wood is characterized by a lower total ellagitannin content than the European oak [39