## 3 Bioactive Compounds in Wine By-Products  
Bioactive compounds refer to a diverse range of phytochemicals that are produced through secondary metabolism in plants. Plants produce primary metabolites, such as carbohydrates, proteins, and fats, to sustain vital life processes, such as growth, development, respiration, storage, and reproduction. Additionally, they create secondary metabolites, including glucosinolates, carotenoids, or polyphenols, to protect themselves from external abiotic factors, such as light and water scarcity, as well as to ward off pathogens and communicate with other plants [56; 57; 58; 59; 60]. Polyphenols are the primary bioactive compounds found in grapes and their by-products. In recent decades, these compounds have gained significant interest from the scientific community and various industries, such as food, pharmaceutical, and cosmetics (Sections 4.1-4.3). Utilizing winemaking by-products as a source of polyphenols is a promising strategy to elevate the worth of these by-products [61]. The chemical compounds known as polyphenols are intricate structures made up of one or more aromatic rings. They can have different levels of hydroxylation, methylation, and glycosylation, which affect the fruit's color, astringency, and bitterness [62; 63; 64]. There are two classes of grape polyphenols that are distinguished by the number of phenolic rings and the type of structural unit that binds them together. These classes are non-flavonoid and flavonoid [65; 66]. Non-flavonoid includes stilbenes and phenolic acids subclasses, while the flavonoid class is subdivided into flavanols, flavonols, and anthocyanins subclasses. The profile of polyphenols in wine varies greatly depending on the grape variety, aging, ripening stages, climatic and geographical conditions, and cultivation practices [34; 67; 68; 69]. Grape varieties are distinguished by their different properties, including their ability to form specific organic compounds and their more remarkable ability to produce particular amounts of these components [70,71]. Grape is one of the fruits with the highest antioxidant compounds present in large amounts in the skin, pulp, and seeds [40]. Therefore, solid by-products, such as grape skin, grape pomace (skins and seeds), and stems, among others, obtained from industrially processed grapes, are of great economic interest [72]. The grape skin, considered a solid residue from grape processing for juices and wine production, may contain different but significant concentrations of polyphenols and available antioxidants [70,71]. The grape pomace consists of the skins and seeds, considering that the stem is separated in the previous stage, called disengage [73]. Grape pomace is considered of high economic interest due to its alcoholic and tartaric richness but also because of its physical components. Grape pomace is also a rich source of phenolic acids, colored anthocyanins, flavonoids, and tannins. The bioactive compounds extracted from grape by-products tend to differ in composition based on several factors, including the grape variety, processing method, environmental conditions, and the proportions of bark, seeds, and cuttings utilized [74,75] (Table 1). [PERSON] et al. [21] extracted 33.7 mg GAE.g\({}^{-1}\) of phenolic compounds from the stem residue of the TR variety from the Douro region, while [PERSON] et al. [23] obtained a much higher concentration between 78.02 and 103.49 mg GAE.g\({}^{-1}\) for 'Moscatel' grape stem samples of the lower altitude Douro regions. [PERSON] et al. [24] extracted 42.04 to 96.29 mg GAE.g\({}^{-1}\) and 29.46 to 76.20 mg CAT.g\({}^{-1}\) of total phenolics and flavonoids, respectively, also from the stem (Table 1). [PERSON] and [PERSON] [76] compared the content of the phenolic compounds and flavonoids in wine lees from the Douro region of two different varieties.