**3. Types of aroma**

Despite the massive amount of aromatic compounds present in wine, not all of them contribute to the perceived aroma, since some of them are found in concentrations below the perception threshold. Compounds that exceed this concentration are called active compounds. Aromas can be classified in different ways according to the parameter considered. In this way, a classification can be made based on its presence (basic, subtle and special) or according to its origin or sequence of wine production as described in **Figure 3**. In the last case, the classification based in the sequence of wine production allows to differentiate the aromas accordingly to its process as primary, secondary and post-fermentative aroma [9, 45], facilitating the inference of wine makers. The following paragraphs will describe the different stages of wine production and the aromas originated during the process.

varietal character, notable difference between varieties have been reported. This difference is therefore due to the overall aromatic profile [48]. Among the compounds that determine the free varietal aroma, determined by volatile substances linked to the aromatic typicity of the variety, two chemical families are distin-

These compounds (**Table 2**) were first identified in Cabernet Sauvignon grape variety. They are nitrogen compounds derived from the catabolism of some amino acids such as leucine, isoleucine, valine and glyoxal. They are usually found in concentrations below the perception threshold. Its concentration has a positive correlation with the herbaceous note of some wines such as Cabernet Sauvignon and Sauvignon Blanc [46, 49]. The concentration of pyrazines has been estimated in different grape varieties, such as Sauvignon Blanc (3 ng/L), Semillon (2 ng/L), Cabernet Sauvignon (2–24 ng/L) [50]. Regarding wine, pyrazines content has been estimated at about 1 ng/L in white wines, while in red wines, the concentration reached 10 ng/L [51]. Several factors have been described to influence the pyrazines

guished: pyrazines and terpenes [46].

*Classification of wine aroma according to its origin along wine production.*

**formula**

*Management of Wine Aroma Compounds: Principal Basis and Future Perspectives*

CH2CH3

3-Isobutyl-2-methoxypyrazine CH2CH(CH3)2 Green peppers

**Chemical structure Olfactory**

CH(CH3)2 Green peppers

**descriptor**

Green peppers

**Compound Chemical**

*DOI: http://dx.doi.org/10.5772/intechopen.92973*

2-Methoxy-3-sec-butyl-pyrazine CH(CH3)

2-Methoxy-3-(2-methylpropyl)

*Principal pyrazines found in wines.*

*3.1.1.1 Pyrazines*

**55**

pyrazine

**Table 2.**

**Figure 3.**

## **3.1 Primary aroma**

The primary aroma is formed by the varietal aromatic constituents. Three large groups of compounds can be distinguished: the free varietal aroma, the precursors of varietal origin (non-volatile or non-odorous precursors and odorous volatile compounds) and the substances that are formed from the precursors [46, 47].

#### *3.1.1 Free varietal aroma*

This type of aroma distinguishes the different varieties of grapes. Although relatively few studies have been able to identify a compound responsible for the *Management of Wine Aroma Compounds: Principal Basis and Future Perspectives DOI: http://dx.doi.org/10.5772/intechopen.92973*

#### **Figure 3.**

(white and red musts, young and aged wines, sweet wines) made from different varieties of grapes such as Chardonnay, Riesling, Gewürztraminer, Merlot, Cabernet Sauvignon, Grenache, Tempranillo, Zalema, Palomino Fino, Touriga Nacional, Aragonez or Trincadeira. It can also be applied to study the sensory profiles of wines produced with sound and sour rot affected grapes and compare them

Recently, the electronic nose has been introduced in the wine industry. It consists of an instrument equipped with chemical sensors and a chemometric model recognition program, capable of identifying and comparing individual or complex odors. As its main objective is to obtain results comparable to those from the human olfactory system, the aim of this method is to relate the perceived aroma with a response that, after being stored in memory, will serve as a model in further analysis. It has been displayed as a useful tool due to the advantages it offers: short analysis time in chromatography position (5–10 min), continuous control, it is a non-destructive method and it does not require qualified personnel. However, it is limited by the effectiveness of the detectors [41]. Most of its applications are related to the discrimination of wines to prevent their adulteration or detection of disagreeable odors, but only a few of them consider the identification of the quality of wine aromas. Despite all, this system allows a good classification of typical red and white wine aromas [42]. Finally, along the scientific literature, it has been described as an innovative technique of aroma determination. It consists of an array of conducting polymer sensors coupled to a selective solid-phase micro-extraction (SPME) fiber. This assay allows carrying an analysis of the principal components, differentiating the aromas of the sample, even for wines with very similar sensory characteristics. Moreover, the response is fast and consistent. The selective adsorption of the fiber provides a better distinction, increasing the concentration of the minor compounds of an aroma [43, 44].

Despite the massive amount of aromatic compounds present in wine, not all of them contribute to the perceived aroma, since some of them are found in concentrations below the perception threshold. Compounds that exceed this concentration are called active compounds. Aromas can be classified in different ways according to the parameter considered. In this way, a classification can be made based on its presence (basic, subtle and special) or according to its origin or sequence of wine production as described in **Figure 3**. In the last case, the classification based in the sequence of wine production allows to differentiate the aromas accordingly to its process as primary, secondary and post-fermentative aroma [9, 45], facilitating the inference of wine makers. The following paragraphs will describe the different stages of wine production and the aromas originated during the process.

The primary aroma is formed by the varietal aromatic constituents. Three large groups of compounds can be distinguished: the free varietal aroma, the precursors of varietal origin (non-volatile or non-odorous precursors and odorous volatile compounds) and the substances that are formed from the precursors [46, 47].

This type of aroma distinguishes the different varieties of grapes. Although relatively few studies have been able to identify a compound responsible for the

to understand the role of sour rot in the odor nuances of wines [40].

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

**3. Types of aroma**

**3.1 Primary aroma**

*3.1.1 Free varietal aroma*

**54**

*Classification of wine aroma according to its origin along wine production.*


#### **Table 2.**

*Principal pyrazines found in wines.*

varietal character, notable difference between varieties have been reported. This difference is therefore due to the overall aromatic profile [48]. Among the compounds that determine the free varietal aroma, determined by volatile substances linked to the aromatic typicity of the variety, two chemical families are distinguished: pyrazines and terpenes [46].

## *3.1.1.1 Pyrazines*

These compounds (**Table 2**) were first identified in Cabernet Sauvignon grape variety. They are nitrogen compounds derived from the catabolism of some amino acids such as leucine, isoleucine, valine and glyoxal. They are usually found in concentrations below the perception threshold. Its concentration has a positive correlation with the herbaceous note of some wines such as Cabernet Sauvignon and Sauvignon Blanc [46, 49]. The concentration of pyrazines has been estimated in different grape varieties, such as Sauvignon Blanc (3 ng/L), Semillon (2 ng/L), Cabernet Sauvignon (2–24 ng/L) [50]. Regarding wine, pyrazines content has been estimated at about 1 ng/L in white wines, while in red wines, the concentration reached 10 ng/L [51]. Several factors have been described to influence the pyrazines content in grapes, especially grape variety and maturation. The degree of grapes' ripeness influences their content, being pyrazines' content inversely proportional to this factor. Thus, its content decreases appreciably from summer and disappears practically under optimal conditions of maturation. Other factors have been described to influence pyrazine content, such as temperature and irradiation of vineyard [46, 50]. The soil also plays a significant role in pyrazine levels. A higher amount of pyrazines has been found in vineyards grown in limestone and clay soils than in sandy soils [49].

processes that involve an increase in the exchange of solid and liquid parts

*Management of Wine Aroma Compounds: Principal Basis and Future Perspectives*

of the wine [46].

phenols [55].

*3.1.2.2 Diols*

*3.1.2.3 Carotenoids*

others in smaller quantities [63].

*3.1.2.4 Glycosylated precursors*

**57**

*3.1.2.1 Monoterpenes*

*3.1.2 Precursors of varietal origin*

*DOI: http://dx.doi.org/10.5772/intechopen.92973*

(e.g. maceration) have important implications on the final aromatic characteristics

Although some precursors do not possess odoriferous characteristics, they can

This group of compounds is one of the most studied in wine and includes a wide

These compounds are characterized by transforming at relatively acidic pH, such as those found in musts or wines. Some of these compounds thus obtained are fragrant, but may be the cause of transfers of strange aromas to the wines [46]. Red grapes are not very rich in these compounds, but their action on the aroma is not improbable if the olfactory threshold is taken into account, since it is very low in

Carotenoid content decreases throughout ripening, with a higher content in the grapes exposed to the shade than those exposed to the sun. In grapes grown in high altitude, the content is also lower, due to the low temperatures and higher humidity [60–62]. Its content in grapes ranges between 15 and 2.000 μg/kg. Lutein and β-carotene stand out as the most abundant, as well as neoxanthin, flavoxanthin and

Carotenoids are not found in grape juices and in wines made without maceration, as they are degraded during the breaking of the grape and the vinification. Light and oxidases are capable of degrading carotenoids into smaller fragments, more soluble and more fragrant. Among the compounds that are formed in the decomposition of carotenoids, norisoprenoids are worth mentioning because they have low perception thresholds that make them play an important role in the aroma of wine. This degradation can be direct or with an intermediate step that is the formation of glycoconjugates, which can then release their volatile aglycone during

fermentation through enzymatic and acid hydrolysis processes [64, 65].

in the pulp or juice. These compounds are four types of glycosides: one

monoglycoside (β-D-glucopyranoside) and three diglucosides

All grape varieties have the same glycosylated derivatives, being Moscatel varieties the most concentrated. They appear in greater content in the skin than

give rise to odoriferous substances. These are monoterpenes, diols or terpene polyols, fatty acids, carotenoids, glycosylated precursors of aroma and volatile

variety of compounds. The formation of these molecules is mainly due to the oxidative metabolism of linalool in grapes [56]. Monoterpenols are sensitive to the hydration and oxidation reactions that occur during winemaking and cause the transformation of one into another [57, 58]. Some yeasts can increase the content of

this type of compound. This is the case of *Issatchenkia* spp. [59].

some of its derivatives, either isolated or well mixed [8].

#### *3.1.1.2 Terpenes*

Within this group, very abundant in the plant kingdom, are monoterpenes (formed by 10 carbon atoms), sesquiterpenes (15 carbon atoms) and the corresponding alcohols and aldehydes [52].

They are the most studied odoriferous compounds found in *Vitis vinifera*, having identified around 70 compounds in both wines and grapes. They can be found as free forms or as odorless precursors, mainly glycosylates. The most odoriferous monoterpenes are monoterpenic alcohols like linalool, α-terpineol, nerol, geraniol, citronellol and ho-trienol (**Figure 4**), which provides floral aromas (rose, lily, citronella, linden, etc.). These floral attributes are characteristic of white wines [49, 53]. In addition, the content in terpenoles is determined by the state of the grape since they are very sensitive to the attack of *Botrytis cinerea* [54].

Both monoterpenoles and sesquiterpenes are synthesized from isopentyl pyrophosphate (IPP) and dimethylalkyl pyrophosphate (DMAPP). IPP and DMAPP precursors are produced through the cytosolic mevalonic acid (MVA) pathway (from three molecules of acetyl-CoA) or through 2-C-methyl-D-eryritol-4-phosphate plastidial (MEP) pyruvate and glyceraldehyde-3-phosphate [48].

Regarding the location of these compounds in grapes, more than 50% are concentrated in the solid part (pulp and skin), reaching concentrations of 90% of geraniol and nerol in these parts. In contrast, half of the linalool is in the juice; thus,

**Figure 4.** *Structure of the main monoterpenic alcohols found in wine.*

*Management of Wine Aroma Compounds: Principal Basis and Future Perspectives DOI: http://dx.doi.org/10.5772/intechopen.92973*

processes that involve an increase in the exchange of solid and liquid parts (e.g. maceration) have important implications on the final aromatic characteristics of the wine [46].
