*3.1.2 Aroma composition and sensory development*

The volatile compounds present in Port wines have their origin on the grapes used, are produced during the alcoholic fermentation and being also added as part of the wine spirit used for Port wine production that contains trace volatile compounds such as esters (ethyl hexanoate, ethyl octanoate, ethyl decanoate) and terpenes (α-terpineol, linalool) that can affect the quality of the Port wines, contributing to a fruity, balsamic and spicy aroma [94]. In addition, wine spirits are rich in aldehydes such as acetaldehyde, propionaldehyde, isovaleraldehyde, isobutyraldehyde, and benzaldehyde [94]. The volatile profile of young Port wines is significantly different from that of aged Tawny Port wines or bottle-aged Port wines. Producers blend wines from several vintages and vineyards to produce wines with a consistent character. The final aroma character of the Port wine is to a considerable extent determined by the processes that take place during the oxidative ageing process of these wines, such as oxidation, carbohydrate degradation, formation and hydrolysis of esters, formation of acetals and to a lesser extent extraction of components from wood [11]. More than 200 volatile components have been detected in Port wines, 141 of which have been entirely or partially identified, however, the sensory importance of the various groups of volatile compounds does not entirely explain the sensory properties of Ruby or Tawny Port wines [95]. For the Ruby Port wine sensory profile, the attributes are 'Ruby', 'Persistence', 'Red fruits', 'Fruity flavour', 'Astringency' and 'Floral' were dominant, whereas in the White Port wine attributes like 'Honey', 'Sweet taste', 'Alcoholic sensation', 'Balance', 'Acid taste' and 'Moscatel' are the ones that better characterise these wines, Tawny Port wines are characterised by the attributes 'Dried fruits flavour', 'Dried fruits', 'Spices', 'Wood' and 'Sweet/Honey' [96, 97]. The Pink Port wines sensory attributes are characterised by the attributes 'Red fruit aroma', 'Body', 'Fruit aroma', 'Fruity flavour', 'Spicy sensation' and 'Persistence' [98].

Norisoprenoids have been found to contribute significantly to the aroma of young and aged Port wines [76, 99–101]. In a one year aged Port wine produced from Touriga Franca and Touriga Nacional grape varieties the norisoprenoid, 2,6,6-trimethylcyclohex-2-ene-1,4-dione, described as having sweet honey aroma, was identified by Rogerson et al. [102]. In a young Port wine produced from Tinto Cão and Tinta Barroca grape varieties, Rogerson et al. [103] identified the 1,3-dimethoxybenzene

and 2-ene-1,4-dione. Falqué-Lopez et al. [104] characterised a one year aged Touriga Nacional monovarietal Port wine as having 'plum brandy', 'mulberry', 'cherry', 'wild fruits' and 'dry raisin' aromas and Guedes de Pinho et al. [105] identified linalool and linalyl acetate as being the responsible for the bergamot descriptor.

Ferreira et al. [100] have studied the influence of several factors on the levels of norisoprenoid in Port wines such as dissolved oxygen levels, free sulphur dioxide concentration, pH, and time/temperature of ageing. These authors observed that temperature and pH had a major influence on norisoprenoids levels and oxygen saturation reduced these compounds.

The concentration of several norisoprenoids increases during ageing, as for example β-ionone and β-damascenone in Vintage Port wines, and vitispirane, 2,2,6-trimethylcyclohexanone (TCH) and 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN) in Tawny Port wines [99]. Ferreira and Pinho [99] showed that the occurrence of β-damascenone, β-ionone, TCH, TDN, and vitispirane was distinct in young or aged Port wines. It was observed that in wood barrel ageing TDN, vitispirane, and TCH increased, however, the concentration of β-ionone and β-damascenone decreased. Freitas et al. [106] described that TCH was responsible for the "rock-roselike" aroma. According to several authors, in Port wines due to the short fermentation time precursors of norisoprenoids such as carotenoids, β-carotene, lutein, neoxanthin and violaxanthin can be present [104, 107, 108]. Carotenoids are the precursor of norisoprenoids and in Port wine carotenoids persist after the vinification process [108]. Grape varieties used for Port wine production are rich in certain carotenoids and viticultural practices, such as bunch shading [108, 109] and grapevine water status [110] can influence the concentration of carotenoids in the grape berries.

Acetals, derived from glycerol and acetaldehyde, also appear to be involved in the flavour of aged Tawny Port wines [111]. The levels of aldehydes and methyl ketones increase during the oxidative ageing of Port wines. The major aliphatic aldehyde is acetaldehyde with a clear trend of increasing with the time of storage in wood barrels. Glycerol is present in wines in large amounts, in particular with concentration from 4 to 8 g/L in Port wines and therefore the formation of acetal can be high. At wine pH, four isomers are formed by condensation of glycerol and acetaldehyde: *cis*- and *trans*-5-hydroxy-2-methyl-1,3-dioxane and *cis*- and *trans*-4-hydroxymethyl-2-methyl-1,3-dioxolane. These four acetals have been studied in more detail in order to understand their impact on wine aroma and if these substances can be used as indicators of Port wines age with oxidative ageing conditions. These four isomers are found in Port wine at high concentrations. Nevertheless, this reaction is strongly dependent on free sulphur dioxide levels. When there is no free sulphur dioxide, the level of four isomers increases with the extent of ageing. On the other hand, when sulphur dioxide combines with acetaldehyde, the acetals cannot be formed because of the formation of the acetaldehyde-bisulphite adduct. The concentrations of the four acetals increases consistently with age due to the constant increase of acetaldehyde content and the nonexistence of free sulphur dioxide during Port wine storage. The acetal with the highest intensity aroma described as sweet and Port-like is *trans*-5-hydroxy-2- methyl-1,3-dioxane and the aroma threshold limit of the total concentration of the four acetals was determined as 100 mg/L [111]. Many acetals have been isolated from Tawny Port wines, but their contribution to the oxidised character of the wine is unclear [8].

Port wines with extensive wood-ageing have higher concentrations of diethyl and other succinate esters that contribute to the Port wine bouquet. Oak lactones (β-methyl-γ-octalactone isomers) and other oxygen-containing heterocycles have also been isolated. Some of the latter are furan derivatives, such as dihydro-2-(3H)-furanone and may contribute to a sugary oxidised bouquet [8]. Esters of 2-phenylethanol may contribute to the fruity, sweet bouquet of Port wines, and diacetyl can contribute

**133**

**Figure 4.**

*to [115–118].*

*Port Wine: Production and Ageing*

*DOI: http://dx.doi.org/10.5772/intechopen.94900*

to its caramel odour [99]. Some aldehydes and ketones are associated with the oxida-

However, in wood barrel aged Port wine the 3-hydroxy-4,5-dimethyl-2(5H) furanone (sotolon) seems to be the most significant volatile compound [112, 113]. Some works suggested that sotolon contributes to the characteristic barrel aged aroma of Port wines [114], being the fundamental molecule to understand the "perceived age" of Port wines. The levels of sotolon were measured in "Colheita" and Tawny categories and were shown that it increases with ageing time being present in a range of concentration of some dozen μg/L in a young wine, to about 100 μg/L in wines with 10 years ageing, and to about 1000 μg/L in Port wines older than 50 years [112–114]. Albeit being a compound with an apparently important role in Port wine aged aroma, the mechanism of sotolon formation in wine is not yet fully understood. However, different pathways have been proposed, such as aldol condensation between α-ketobutyric acid and acetaldehyde [115–117] (Pathway 1 – **Figure 4**) and the reaction between ethanol and ascorbic acid [118] (Pathway 2 – **Figure 4**). The formation of sotolon in Port wines is dependent on the temperature and oxygen levels [112, 113], which are crucial parameters during the oxidative ageing of Tawny Port wines. The sensory threshold limit of sotolon was determined as 19 μg/L, which is

tive aged Port wines, conferring "rancio" odour to wines [112, 113].

above the amount present in Port wines older than 10 years [112, 113].

Young Port wines show higher levels of volatile sulphur compounds than aged Port wines [119]. Sulphur compounds, such as 2-mercaptoethanol, 2-(methylthio) ethanol, ethyl 3-(methylthio) propionate, 3-(methylthio)-1-propanol, *cis*-(odourless), and *trans*-2-methyltetrahydrothiophen-3-ol, 3-(ethylthio)-1-propanol, 4-(methylthio)- 1-butanol, dimethyl sulfone, benzothiazole, 3-(methylthio)-1-propionic acid and N-3-(methylthiopropyl) acetamide are not present or are present in lower concentrations in aged Tawny Port wines when compared to young Tawny Port wines [119]. The changes observed in the volatile composition of Port wines during the oxidative ageing results in a complex, oxidised character defined as nutty, nuts,

*Pathway 1: Reaction between* α*-ketobutyric acid (1) and acetaldehyde (2); pathway 2: Reaction between ascorbic acid (3) and ethanol (3). These reactions can lead to the formation of sotolon (8) according* 

#### *Port Wine: Production and Ageing DOI: http://dx.doi.org/10.5772/intechopen.94900*

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

linalyl acetate as being the responsible for the bergamot descriptor.

their contribution to the oxidised character of the wine is unclear [8].

Port wines with extensive wood-ageing have higher concentrations of diethyl and other succinate esters that contribute to the Port wine bouquet. Oak lactones (β-methyl-γ-octalactone isomers) and other oxygen-containing heterocycles have also been isolated. Some of the latter are furan derivatives, such as dihydro-2-(3H)-furanone and may contribute to a sugary oxidised bouquet [8]. Esters of 2-phenylethanol may contribute to the fruity, sweet bouquet of Port wines, and diacetyl can contribute

saturation reduced these compounds.

and 2-ene-1,4-dione. Falqué-Lopez et al. [104] characterised a one year aged Touriga Nacional monovarietal Port wine as having 'plum brandy', 'mulberry', 'cherry', 'wild fruits' and 'dry raisin' aromas and Guedes de Pinho et al. [105] identified linalool and

Ferreira et al. [100] have studied the influence of several factors on the levels of norisoprenoid in Port wines such as dissolved oxygen levels, free sulphur dioxide concentration, pH, and time/temperature of ageing. These authors observed that temperature and pH had a major influence on norisoprenoids levels and oxygen

The concentration of several norisoprenoids increases during ageing, as for example β-ionone and β-damascenone in Vintage Port wines, and vitispirane, 2,2,6-trimethylcyclohexanone (TCH) and 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN) in Tawny Port wines [99]. Ferreira and Pinho [99] showed that the occurrence of β-damascenone, β-ionone, TCH, TDN, and vitispirane was distinct in young or aged Port wines. It was observed that in wood barrel ageing TDN, vitispirane, and TCH increased, however, the concentration of β-ionone and β-damascenone decreased. Freitas et al. [106] described that TCH was responsible for the "rock-roselike" aroma. According to several authors, in Port wines due to the short fermentation time precursors of norisoprenoids such as carotenoids, β-carotene, lutein, neoxanthin and violaxanthin can be present [104, 107, 108]. Carotenoids are the precursor of norisoprenoids and in Port wine carotenoids persist after the vinification process [108]. Grape varieties used for Port wine production are rich in certain carotenoids and viticultural practices, such as bunch shading [108, 109] and grapevine water status [110] can influence the concentration of carotenoids in the grape berries. Acetals, derived from glycerol and acetaldehyde, also appear to be involved in the flavour of aged Tawny Port wines [111]. The levels of aldehydes and methyl ketones increase during the oxidative ageing of Port wines. The major aliphatic aldehyde is acetaldehyde with a clear trend of increasing with the time of storage in wood barrels. Glycerol is present in wines in large amounts, in particular with concentration from 4 to 8 g/L in Port wines and therefore the formation of acetal can be high. At wine pH, four isomers are formed by condensation of glycerol and acetaldehyde: *cis*- and *trans*-5-hydroxy-2-methyl-1,3-dioxane and *cis*- and *trans*-4-hydroxymethyl-2-methyl-1,3-dioxolane. These four acetals have been studied in more detail in order to understand their impact on wine aroma and if these substances can be used as indicators of Port wines age with oxidative ageing conditions. These four isomers are found in Port wine at high concentrations. Nevertheless, this reaction is strongly dependent on free sulphur dioxide levels. When there is no free sulphur dioxide, the level of four isomers increases with the extent of ageing. On the other hand, when sulphur dioxide combines with acetaldehyde, the acetals cannot be formed because of the formation of the acetaldehyde-bisulphite adduct. The concentrations of the four acetals increases consistently with age due to the constant increase of acetaldehyde content and the nonexistence of free sulphur dioxide during Port wine storage. The acetal with the highest intensity aroma described as sweet and Port-like is *trans*-5-hydroxy-2- methyl-1,3-dioxane and the aroma threshold limit of the total concentration of the four acetals was determined as 100 mg/L [111]. Many acetals have been isolated from Tawny Port wines, but

**132**

to its caramel odour [99]. Some aldehydes and ketones are associated with the oxidative aged Port wines, conferring "rancio" odour to wines [112, 113].

However, in wood barrel aged Port wine the 3-hydroxy-4,5-dimethyl-2(5H) furanone (sotolon) seems to be the most significant volatile compound [112, 113]. Some works suggested that sotolon contributes to the characteristic barrel aged aroma of Port wines [114], being the fundamental molecule to understand the "perceived age" of Port wines. The levels of sotolon were measured in "Colheita" and Tawny categories and were shown that it increases with ageing time being present in a range of concentration of some dozen μg/L in a young wine, to about 100 μg/L in wines with 10 years ageing, and to about 1000 μg/L in Port wines older than 50 years [112–114]. Albeit being a compound with an apparently important role in Port wine aged aroma, the mechanism of sotolon formation in wine is not yet fully understood. However, different pathways have been proposed, such as aldol condensation between α-ketobutyric acid and acetaldehyde [115–117] (Pathway 1 – **Figure 4**) and the reaction between ethanol and ascorbic acid [118] (Pathway 2 – **Figure 4**). The formation of sotolon in Port wines is dependent on the temperature and oxygen levels [112, 113], which are crucial parameters during the oxidative ageing of Tawny Port wines. The sensory threshold limit of sotolon was determined as 19 μg/L, which is above the amount present in Port wines older than 10 years [112, 113].

Young Port wines show higher levels of volatile sulphur compounds than aged Port wines [119]. Sulphur compounds, such as 2-mercaptoethanol, 2-(methylthio) ethanol, ethyl 3-(methylthio) propionate, 3-(methylthio)-1-propanol, *cis*-(odourless), and *trans*-2-methyltetrahydrothiophen-3-ol, 3-(ethylthio)-1-propanol, 4-(methylthio)- 1-butanol, dimethyl sulfone, benzothiazole, 3-(methylthio)-1-propionic acid and N-3-(methylthiopropyl) acetamide are not present or are present in lower concentrations in aged Tawny Port wines when compared to young Tawny Port wines [119].

The changes observed in the volatile composition of Port wines during the oxidative ageing results in a complex, oxidised character defined as nutty, nuts,

**Figure 4.**

*Pathway 1: Reaction between* α*-ketobutyric acid (1) and acetaldehyde (2); pathway 2: Reaction between ascorbic acid (3) and ethanol (3). These reactions can lead to the formation of sotolon (8) according to [115–118].*

raisins and crisp apples with a slightly oaky note' giving an impression of dryness. According to Falqué et al. [104] and Ferreira et al. [120] floral, bergamot-like, violet or jasmine notes are present in young Port wines that are changed during the ageing process in wood barrels. Freitas et al. [106] and Ferreira et al. [121] have described the flavours developed during ageing of Port wines as woody, burnt, dry fruit, nutty, and spicy.
