## 4 Discussion
Other studies have been carried out regarding the quantification of phenolic compounds and flavonoids from seeds in red varieties, such as Pinot Noir, Negro Amaro, Cabernet Sauvignon [38], and Marselan [39], among others, and the values of these compounds vary greatly from each other [38]. For example, studies carried out by [PERSON] et al. [39] showed that extracts from Cabernet Sauvignon grape seeds, crushed to powder, in a 70% aqueous ethanol solution, had TPC values of 88.22 \(\pm\) 0.72 mg GAE/g of dry extract. For the Marselan variety, the TPC values were 103.24 \(\pm\) 1.28 mg GAE/g of dry extract and for Pinot Noir, 79.06 \(\pm\) 0.65 mg EAG/g of dry extract. In the case of TFC, the values varied between 45.95 \(\pm\) 0.14 mg CE/g of dry extract (Cabernet Sauvignon) and 52.01 \(\pm\) 0.34 mg CE/g of dry extract (Marselan), lower than the respective values of TPC, which agrees with our results, except for the aqueous extract of Touriga Nacional. However, considering that the grape varieties and the method of extraction are different from those used in our work, it is not possible to make a direct comparison with the results we obtained. [PERSON] et al. [40] compiled values for the content of phenolic compounds in stem extracts of several red grape varieties, namely Cabernet Sauvignon, Manto Negro, Mandilaria, and Tempranillo, among others. According to this review article, for example, TPC values for the Cabernet Sauvignon variety ranged between 348 and 7076 mg GAE/100 g of dry extract; for Mandilaria, between 1057 and 1434.3 mg GAE/100 g of dry extract; and for Tempranillo, between 4679 and 7622 mg GAE/100 g of dry extract, depending on the harvest year. In addition to the grape harvest year, which is very dependent on climatic factors, the composition of phenolic compounds depends on cultivation conditions and practices, geographic location, and stage of maturation [17; 18], which is also reflected in the composition of the by-products. As mentioned above, for seeds, it is not possible to make a direct comparison with the results we obtained. According to some studies, red wine varieties have higher levels of flavonoids and phenolic compounds [41; 42; 43; 44]. This is in line with what was found by us for seeds, but not when comparing the Touriga Nacional (red) and Viosinho (white) grape varieties stems, which is in accordance with [PERSON] et al. [27]. The quantitative differences found in the phenolic composition of the by-products, regardless of the extracting solvent, prove that each type of red or white grape by-product has a particular phenolic composition, which can be related to different winemaking practices [18; 33; 36; 45]. For example, according to [PERSON] et al. [45], the by-products obtained from the carbonic maceration of grapes contain smaller amounts of phenolic compounds than those obtained through conventional fermentation. The breathing process and, in general, all oxidative reactions that occur in cells and use oxygen lead to the formation of free radicals, which are unstable molecules or ions that can cause cellular damage and consequently be harmful to the body. These damages are associated with various health problems, such as inflammation, tumors, Alzheimer's disease, and cardiovascular diseases. The antioxidants present in plants can act as reducing agents and scavengers of these free radicals, as well as enzyme inhibitors and metal chelators. The main antioxidants present in vegetables are phenolic compounds, especially flavonoids, as well as vitamins C and E and carotenoids [46; 47]. Therefore, antioxidant activity can be related to the content of polyphenols, that is, the higher the concentration of these compounds, the greater the antioxidant activity [18; 27; 37]. In the present study, this correlation is verified in seed extracts, regardless of the variety and solvent used.