*5.2.2 Discrimination methods based on volatile compounds*

As a matter of fact, GC-MS is successfully used for the characterization and quantitative determination of volatile and semi-volatile compounds directly issued from oak wood [92, 93].

Among the many research studies using GC-MS in order to distinguish wines aged with alternative products or barrels, the work presented by Triacca et al. [94] stands out for the number of samples analyzed. A database made up of 352 new barrel wines, 665 used barrel wines and 600 chip wines, was created in order to verify compliance with laws and regulations prohibiting the use of chips in Switzerland. Wood-related volatiles (xylovolatiles) were elaborated using chemometrics techniques (logistic regression analysis). The authors were able to assign new unknown samples, with good certainty, to the chips or barrel group [94].

A research study carried out in 2008 [95] reported the influence on aroma compounds of adding oak wood chips either in stainless steel tanks or in used barrels, comparing these wines with those aged in new barrels. Both the size of the oak chips and the contact time were considered. To separate the samples according to wood format, three discriminant functions were obtained (81.5% correct classification). Wines in new wood barrels were separated from wines with wood chips, and lactones and 5-methylfurfural were the variables with the highest discriminant power. In a recent comprehensive study [90], 75 volatile compounds were determined by applying GC-MS and flame ionization detection (GC-FID) to a wide set of wines with differing aging processes. The authors found that compounds directly related to wood have greater discriminative power for separating wines aged in barrels from those macerated with oak fragments, but no single compound permits flawless classification. Therefore, they studied the overall effect of the addition of oak fragments on a set of 231 samples and compared them to those same wines aged in oak barrels. Thus, they developed a set of criteria that enables distinguishing, with a high degree of accuracy. The application of these criteria allowed the correct classification in over 90% of cases. It was found that out of the 75 analyzed compounds, those which best enable discrimination are the following: oak lactone isomers, vanillin, acetovanillone, syringaldehyde, furfural, furfuryl alcohol, 5-methylfurfural, 5-hydroxymethylfurfural, eugenol, methyl vanillate, and ethyl vanillate. Vanillin, acetovanillone, and syringaldehyde are the compounds that were present in higher concentrations in wines fermented or macerated with wood fragments than in wines aged in barrels. Eugenol (significantly higher) and oak lactone isomers are the compounds that explain the variance in wines aged in barrels. The authors also point out that the extraction of wood-derived compounds is affected by many factors such as the age of the barrel, the application during

fermentation or maceration and the dose. Nevertheless, the vanillin + acetovanillone/eugenol ratio was an essential marker for discrimination.

A more recent study [96] focused on the characterization of xylovolatile aromatic compounds using GC-MS of wines aged in barrels and those produced using oak chips. Approximately 200 Italian wines aged using oak chips or wood barrels were analyzed and 60 xylovolatile compounds were identified. Wines aged in barrels had a higher concentration of ethylvanillate, 4-ethylphenols, eugenol and whiskey-lactones than wines aged with chips, which were characterized by a generally higher concentration of furanic compounds and hydroxybenzaldehyde derivatives. The presence of 4-ethylphenols at higher concentrations in barrel-aged wines indicated that there is still, in general, a higher risk of contamination from *Brettanomyces bruxellensis* compared with chip refinement. Overall, promising perspectives arose from applying DA (discriminant analysis) to classify wines depending on the aging method (barrels vs. chips) with >96.5% success.

On the other hand, other work has demonstrated that discrimination based on VOCs is not always easy and generalized patterns are hard to establish [97]. The analytical profile of the wood-related volatiles would be expected to exhibit large variations, precluding the detection of a generalized pattern in several cases. Moreover, the aging of red and white wines would certainly follow quite different evolution routes, in terms of the enrichment in wood-related volatiles.

An innovative approach recently proposed in some papers [32, 45] couples VOC analyses with other analytic techniques to overcome the problems described above. Indeed, most previous studies investigating the influence of the type and length of the aging process have dealt with a particular subgroup of compounds (volatile or phenolic), which has recently been considered to be a limiting condition to obtain a more comprehensive view of the subject [45]. Therefore, a study carried out in 2008 on Spanish wines [45] followed the evolution of both aromatic and phenolic composition of wine during the contact time with oak wood: chips and staves, with and without micro-oxygenation. These aging procedures were compared with the traditional oak barrel aging method. Two canonical discriminant analyses were carried out to classify the different treatments based on the volatile compounds from oak wood and low-molecular weight phenols. A good separation was achieved between the three treatments (chips, staves and barrels) for both groups of compounds. In the case of VOCs, 100% of cases were correctly classified. Wines aged in barrels were correlated with a high concentration of 5-hydroxymethylfurfural and 5-methylfurfural; wines in contact with staves were correlated with a high content of vanillin and *trans*-isoeugenol. As for low-molecular weight polyphenols, 100% of the cases were also correctly classified. The variables with the highest discriminant power were syringic acid and quercetin (related to staves) and caftaric acid (associated with barrels). On the other hand, the discriminant function 2 correlates caftaric acid with the wines aged in barrels and with fragments and gallic acid with the wines treated with chips.

The main aim of another research was to evaluate the same wine after 10 years in bottles when aged in barrels and when treated with alternative products and oxygen, by means of volatile compound quantitation and color analyses [32]. Overall, the traditionally aged wines suffered a smaller decrease in color intensity, hue, significance of reds and blues, followed by wines treated with staves + MOX and chips + MOX. Furthermore, the highest concentration of *cis*-oak lactone was found in wines aged in barrels. While, generally, oak-related compounds were found at lower concentrations in barrel-wines than in alternative-products wines, except for eugenol.

It is worth noting that the GC-MS technique and VOCs analyses have been used in several studies on wood alternatives, although without direct comparison with

**109**

*Chemistry and Technology of Wine Aging with Oak Chips*

*5.2.3 Discrimination based on spectroscopy methods*

GC-MS and near infrared spectroscopy (NIR).

oak wood from which alternative products are issued [77, 100].

barrel aging, investigating various technological implications: the impact of time of the addition of chips during winemaking [98, 99]; the effect of wood toasting degree and contact time with oak fragments [31, 44]; and the geographical origin of

In the last decade, vibrational spectroscopy, namely infrared-based techniques (IR), supported by chemometric methods, was found to be a powerful technique because of its widespread use in analytical laboratories, its versatility and low economic impact. Moreover, IR technology requires minimal sample processing

Certainly, IR spectroscopy coupled with multivariate data analysis has been used for the determination of oak volatile compounds [101–103] and for classifying barrels [104]. Subsequently, in recent years, this method has been proposed for discriminating wines aged in different types of wood containers and for different time periods [96, 105, 106], taking advantage on PLS-based calibration between

The first publication appeared in 2012 [107], which investigated two different levels of information on fusion of NIR spectra and midinfrared (MIR) spectra from red wines aged in different ways. A total of 96 red wines, including wines aged in oak barrels, wines aged in stainless steel tanks with oak chips and without, were analyzed. Discriminant models of the three different aged wines were established, and the FDA method was applied to build the classification models of three different aged wines using the NIR, MIR and the merged spectra, reaching up to 98% correct classification with the latter. The results suggest that the spectral fusion of NIR and MIR is a promising technology for discriminating different aged wines. A recent paper aimed to identify if spectroscopic techniques allow discriminating wines aged with alternative oak products (chips and staves) from different oak woods (American, French and Spanish) and floating micro-oxygenation (20 μg/L), compared with those aged in barrels, after 10 years of bottling [108]. The spectral information and analysis were performed in an FTIR-ATR (Fourier-transform IR-Attenuated Total Reflection). The results indicated that with this technique it is possible to clearly separate the wines aged by the three systems (chips, staves, and barrels) in the case of American oak. In the case of French oak, wines aged with chips were clearly differentiated for wines from a single grape variety and with similar oenological features.

The most recent paper on this topic [96] analyzed approximately 90 red wines issued from the same wine appellation in Italy, including commercial barrel-aged wines and wines aged using different types of commercial oak chips. Wines were analyzed in transmittance using NIR. In order to test if combined explanatory variables made it possible to discriminate treatments, an orthogonal partial least squares discriminant analysis (OPLS-DA) was carried out. Several factors were considered, including the aging process, the type of oak used for aging (wood *barriques*, big barrels or chips) and the wine typologies (differing for some oenological parameters). OPLS-DA application reached >96.5% success in classifying wines depending on the aging process (no wood, barrel/*barrique* and chips) in the internal validation check and > 90% in an external test. Certainly, further studies are needed to validate the potential of the technique in order to improve it for wine authentication, enlarging both the array of wine types (e.g., testing different appellations and grape varieties), and the range of oak alternatives (staves, cubes, blocks, and other sizes of

product) to be tested, possibly in the presence of micro-oxygenation.

It is worth noting that IR spectroscopy has been used in other studies on wine aging, although without direct comparison between wood alternatives and barrel

*DOI: http://dx.doi.org/10.5772/intechopen.93529*

prior to analysis [5].

*Chemistry and Biochemistry of Winemaking, Wine Stabilization and Aging*

lone/eugenol ratio was an essential marker for discrimination.

fermentation or maceration and the dose. Nevertheless, the vanillin + acetovanil-

A more recent study [96] focused on the characterization of xylovolatile aromatic compounds using GC-MS of wines aged in barrels and those produced using oak chips. Approximately 200 Italian wines aged using oak chips or wood barrels were analyzed and 60 xylovolatile compounds were identified. Wines aged in barrels had a higher concentration of ethylvanillate, 4-ethylphenols, eugenol and whiskey-lactones than wines aged with chips, which were characterized by a generally higher concentration of furanic compounds and hydroxybenzaldehyde derivatives. The presence of 4-ethylphenols at higher concentrations in barrel-aged wines indicated that there is still, in general, a higher risk of contamination from *Brettanomyces bruxellensis* compared with chip refinement. Overall, promising perspectives arose from applying DA (discriminant analysis) to classify wines depending on the aging method (barrels vs. chips) with >96.5% success.

On the other hand, other work has demonstrated that discrimination based on VOCs is not always easy and generalized patterns are hard to establish [97]. The analytical profile of the wood-related volatiles would be expected to exhibit large variations, precluding the detection of a generalized pattern in several cases. Moreover, the aging of red and white wines would certainly follow quite different

An innovative approach recently proposed in some papers [32, 45] couples VOC analyses with other analytic techniques to overcome the problems described above. Indeed, most previous studies investigating the influence of the type and length of the aging process have dealt with a particular subgroup of compounds (volatile or phenolic), which has recently been considered to be a limiting condition to obtain a more comprehensive view of the subject [45]. Therefore, a study carried out in 2008 on Spanish wines [45] followed the evolution of both aromatic and phenolic composition of wine during the contact time with oak wood: chips and staves, with and without micro-oxygenation. These aging procedures were compared with the traditional oak barrel aging method. Two canonical discriminant analyses were carried out to classify the different treatments based on the volatile compounds from oak wood and low-molecular weight phenols. A good separation was achieved between the three treatments (chips, staves and barrels) for both groups of compounds. In the case of VOCs, 100% of cases were correctly classified. Wines aged in barrels were correlated with a high concentration of 5-hydroxymethylfurfural and 5-methylfurfural; wines in contact with staves were correlated with a high content of vanillin and *trans*-isoeugenol. As for low-molecular weight polyphenols, 100% of the cases were also correctly classified. The variables with the highest discriminant power were syringic acid and quercetin (related to staves) and caftaric acid (associated with barrels). On the other hand, the discriminant function 2 correlates caftaric acid with the wines aged in barrels and with fragments and gallic acid with

The main aim of another research was to evaluate the same wine after 10 years in bottles when aged in barrels and when treated with alternative products and oxygen, by means of volatile compound quantitation and color analyses [32]. Overall, the traditionally aged wines suffered a smaller decrease in color intensity, hue, significance of reds and blues, followed by wines treated with staves + MOX and chips + MOX. Furthermore, the highest concentration of *cis*-oak lactone was found in wines aged in barrels. While, generally, oak-related compounds were found at lower concentrations in barrel-wines than in alternative-products wines, except for

It is worth noting that the GC-MS technique and VOCs analyses have been used in several studies on wood alternatives, although without direct comparison with

evolution routes, in terms of the enrichment in wood-related volatiles.

**108**

eugenol.

the wines treated with chips.

barrel aging, investigating various technological implications: the impact of time of the addition of chips during winemaking [98, 99]; the effect of wood toasting degree and contact time with oak fragments [31, 44]; and the geographical origin of oak wood from which alternative products are issued [77, 100].
