**1. Introduction**

Port wine is a traditional fortified wine produced in the Douro Demarcated Region (Northeast of Portugal in the Douro Valley, **Figure 1**) under very specific conditions. There are several Port wine styles being related to the winemaking and ageing process and also to the ageing time, which enhances uniqueness to the wines and recognition throughout the world. The Douro Demarcated Region is located within the Douro River basin, surrounded by mountains, having a total area of approximately 250,000 hectares. This region has singular climate and soil characteristics for the production of selected grape varieties for the Port winemaking that contributes to the distinctive characteristics of Port wines and guarantee that these wines are irreproducible elsewhere. This area is divided into three naturally distinct sub-regions (**Figure 1**) concerning the climatic as well as socio-economic factors, "Baixo Corgo" (Lower Corgo), "Cima Corgo" (Upper Corgo), and "Douro Superior" (Upper Douro) [1].

Different grape varieties are used in the production of Port wine, being usually produced by their blend. There is evidence that the grape varieties determines the wine character, even after the extended ageing process. The grape varieties that may be cultivated in the Douro Demarcated Region are regulated by Decree-Law n°104/85, 10th of April, 1985 [2]. Accordingly, Port wine is produced only from the authorised grape varieties, being the principal red grape varieties recommended for Port wine production "Touriga Nacional", "Mourisco Tinto", "Bastardo", "Tinta

**Figure 1.**

*Location of the Douro Demarcated Region in the northeast of Portugal and of the three sub-regions: "Baixo Corgo", "Cima Corgo" and "Douro Superior" according to [1].*

Roriz", "Tinta Cão", "Tinta Amarela", "Tinta Barroca", "Touriga Franca" and "Tinta Francisca", since these grape varieties produce wines with stable colour, fruity aroma and sugar content, characteristics required to produce good quality Port wines. The white grape varieties used for White Port wine production are "Malvasia Fina", "Viosinho", "Donzelinho Branco", "Gouveio", "Rabigato", and "Códega" [3].

In the Port wine vinification process, the alcoholic fermentation is stopped according to the desired residual sugar content by the addition of a wine spirit known as "aguardente vínica" (with an alcohol content of about 77% (v/v)), to an alcohol content up to 18–22% (v/v) of the final product. Therefore, Port wine is a naturally sweet fortified wine since the natural sugar from the grapes is not completely transformed into alcohol. After the vinification process, Port wine is usually stored and aged in wood barrels of different sizes, from 2 years to many decades in accordance with the intended Port wine style. The Port wine ageing process can take place either in the Douro Valley or in Vila Nova de Gaia (Porto), in order to qualify for a Certificate of Origin from the "Instituto dos Vinhos do Douro e Porto" (IVDP).
