# From Vine to Wine: Non-Colored Flavonoids as Fingerprints
## 1 Introduction  
Flavonoids are a group of secondary plant metabolites belonging to the polyphenol family. They are commonly present in fruits, vegetables, flowers, and beverages such as tea and wine, as they play biological roles in the plant kingdom. Its basic chemical structure is based on a 15-carbon skeleton composed of two aromatic rings connected by an oxygenated bridge. The coloration of flavonoids depends on their degree of conjugation and the pH of the environment [1]. Non-colored flavonoids, such as flavanols and flavonoids,generally lack extensive conjugation between their aromatic rings, which limits their ability to absorb visible light. Instead, they primarily absorb ultraviolet (UV) radiation due to their aromatic structure and the presence of hydroxyl groups. Their electronic transitions occur at wavelengths below the visible spectrum (typically between 250 and 350 nm), making them appear colorless to the human eye [2]. However, non-colored flavonoids can sometimes exhibit a slight yellowish tone, although in red wines this subtle coloration is usually masked by the intense red pigments of anthocyanins. In contrast, colored flavonoids have extended conjugation systems that absorb visible light, thus providing distinctive coloration to plants and derived products [3]. Flavonoids are typically distinguished by the functional groups attached to their core structure. They are primarily classified into seven major families based on their structural variations: chalcones, flavones, flavonoids, flavandiols (or flavan-3-ols), anthocyanins, condensed tannins (or proanthocyanidins), and aurones. While aurones and chalcones are rare in wine, the rest are commonly found in wines [3]. Most representative flavonoid groups in red wines can be divided into two groups: (1) Anthocyanins, which are visible pigments responsible for the red, purple, and blue colors in flowers and fruits; and (2) Non-colored flavonoids, mostly flavan-3-ols, commonly referred to as flavanols, and flavonols. Flavonols are highly related to tannins and thus are fundamental to the sensory quality characteristics of wines, as they influence bitter taste and mouthfeel, while flavonoids are more related to color persistence [4]. Flavonoid compounds in grapevines are synthesized during grape development. These compounds play an important role in the plant physiology, as they are involved in defense mechanisms against pathogens and radiation [5]. Flavanols, including catechin and epicatechin, are also synthesized through the flavonoid biosynthetic pathway [6]. Key enzymes involved in this synthesis are leucanthocyanidin reductase and anthocyanin reductase, which reduce leucanthocyanidins and anthocyanins to produce catechins and epicatechins [7]. Flavonols are synthesized via the flavonoid biosynthetic pathway from the amino acid phenylalanine, through the action of phenylalanine ammonia-ylase and other enzymes [8]. Common flavonoids in grapes include quercetin, kaempfer, and myricetin [9]. They are primarily accumulated in the grape skin and serve as protectors again ultraviolet radiation preventing damage to the plant tissues [10]. The flavonoid profile of wines is shaped by factors such as grape cultivar, terroir, environmental conditions, and viticultural practices [1; 11]. Therefore, the flavonoid composition of each wine can contribute in the authentication of grape cultivars, geographical origins, and even production methods [12]. This is increasingly important in the global wine market to prevent fraud and ensure consumer confidence [13]. Scientific studies have demonstrated the potential of using these types of profiles for wine authentication and classification [14]. In a previous study we used the anthocyanin composition to characterize red Canary wines [15].