# Polypphenols: Natural Antioxidants to Be Used as a Quality Tool in Wine Authenticity
## 1 Introduction  
Polyphenols constitute a diverse group of bioactive compounds occurring in both grapes and wines [1]. In plants, they have been found to exhibit key roles in growth, fertility, and reproduction. They present protective properties against abiotic stress conditions such as UV-light and biotic stresses such as pathogen and predator attacks [2, 3]. Polyphenols exhibit a significant role in modern food technology and human nutrition [4, 5] and are frequently key ingredients in functional foods. The benefits derived from the moderated wine consumption for human health have been well elaborated [6, 7] with several groups of phenolic compounds including stilbenes [8], flavonoids [9], and proanthocyanidins [10] found to exert various health-promoting properties [6].  
Wine production is regulated by OIV (International Organization of Vine and Wine), global wine policies, and national governments. Provision of authentic wines to the market has always been a prerequisite meaning that the declarations on the wine label should mirror the composition and provenance of this intriguing product [11]. However multiple cases of intentional or unintentional wine mislabeling [12, 13, 14, 15] have been recorded, alarming wine consumers who demand for strict controls safeguarding wine transparency.  
Polyphenols play a fundamental role in the determination of wine sensory attributes contributing greatly to its color, flavor, and taste. In wine trade, the organoleptic characteristics are the majordeterminants of wine consumers' acceptance and preference. It is generally accepted that high-quality red wines exhibit sensory properties that reflect their chemical composition.  
Wine is a complex matrix composed of molecules of diverse nature, significantly influenced by environmental factors, as well as viticultural and oenological management approaches. Concerning the latter, various winemaking practices are implemented that play a key role in the composition of the final product. As a result, wine fraud detection can become a challenging task. It has been reported that specific phenolic compounds can be employed as markers in authenticity verification [16].  
It is important to investigate the parameters that affect wine composition and to develop reliable methods for wine authenticity. Common applications of wet chemistry or basic chromatographic applications are widely used in routine analysis of phenolic compounds in wine. Nowadays, emerging platforms including mass spectrometry (MS)-based or nuclear magnetic resonance (NMR) metabolomics are considered the current trend in wine authenticity studies. The use of instrumentation of exceptional selectivity and sensitivity combined with advanced multivariate methods of analysis (MVA) for efficient data mining have permitted a thorough characterization of the wine chemical profile often employing polyphenic compounds as discriminant markers among studied groups.  
The aim of this review is to provide updated knowledge regarding the contribution of polyphenols in wine authenticity rotating around three axes. Initially, we present updated data on the polyphenic composition of grape and wine according to current and earlier studies. Basic structures as well as novel findings are highlighted. Subsequently, we briefly describe the technological approaches implemented for enhanced polyphenol extraction and stability in modern winemaking. This section is included in order to increase the understanding in relation to the extent of the influence of these practices toward wine authenticity. In the last section, studies are presented employing emerging analytical platforms aiming in the provision of in-depth information relating to intra-/inter-varietal aspects as well as vii/viticultural process monitoring.