## 1 Introduction  
Polyphenols are chemical compounds produced by plants as secondary metabolites to be used as self-defenders [1]. Because of self-protection activities against ultraviolet radiation or pathogenic microorganisms among other factors, polyphenols have become a focal point of interest in nutrition and new drug design [2]. Polyphenols have also proved their antimicrobial activity via different mechanisms of action, such as the inhibition of the cell wall synthesis, the disruption of the cell membrane permeability with the subsequent leakage of cytoplasmic content, DNA damage, or the inactivation of key resistant enzymes and surface proteins [3]. Preclinical and clinical studies strongly suggest that the long term consumption of polyphenol-rich diets offers protection against the development of various chronic diseases, due to the crucial role these compounds play in metabolism regulation and cell proliferation [4]. Flavanols, such as catechins and their derivates, are the most abundant polyphenols in human diets, mainly consumed in fruits and beverages [5], with the average total polyphenol intake being approximately 1 g per day [6]. Moreover, one of the biggest challenges related to the consumption of polyphenols is their low bioaccessibility and bioavailability [7]. Polyphenol bioaccessibility depends on the food matrix, basically conformed by dietary fibre [8], which has a fundamental role in the metabolism of the gut microbiota and the beneficial metabolites it produces [9]. On the other hand, low polyphenol bioavailability is not only related to molecular size and chemical composition, but also to interactions with other food components, the effect of the gastrointestinal environment, and degradation by the microbiota [10]. Bioavailability studies on humans have detected very low plasma concentrations of polyphenols after ingestion, rounding to 1 uM after the ingestion of 10-100 mg of an individual phenolic compound [5]. Among dietary sources of polyphenols, grapes and their derivatives are some of the most widely studied [11]. Various studies have proved their bioactivities, which include anticancer [12], anti-inflammatory, antioxidant [13], antimicrobial, and neurodegenerative and antiaging protection [14; 15]. Although fruits and wine are extensively consumed, non-commonly consumed parts of the grape, such as the seeds and skins, contain the majority of polyphenols [16], mainly catechins, proanthocyanidins in all kinds of grapes [9], and tannins and stilbenes in red ones [17]. Because of the large number of polyphenols retained in the usually wasted grape parts, rich polyphenic extracts produced using grape byproducts are becoming very popular. Previous research conducted on the antimicrobial and antioxidant activity of polyphenol-rich grape extracts suggests promising applications as antibiotic coadjutants, foodborne pathogen controllers, and potential sources of nutraceuticalals [18]. A previous study assessed the antimicrobial activity of an Albarino grape marc extract produced using a hydro-organic solvent. This extract, which was reported to contain a high catechin concentration, showed good potential as a bacterial controller in nosocomial environments due to its effectiveness against multidrug-resistant bacteria [16]. Even though plenty of in vitro investigations to assess the bioactivities of grape extracts are being conducted, very few of them consider the behaviour and stability of the polyphenic compounds under digestive conditions. As mentioned above, the polyphenols found in abundance in our diets may not necessarily possess the optimal bioavailability [5; 7; 8]. Therefore, it is important not only to know the quantity of polyphenols in a particular food or extract, but also to understand their potential bioavailability and behaviour once consumed.