# An Overview on Management and Valorisation of Winery Wastes
## 2 Winery Waste Composition and Source of Bio-Active Compounds
In this respect, the grape pomace corresponds with this trend due to its polyphenic content (70% grape polyphenols) that remains after the vinicification process. Phenolic compounds represent the main secondary plant metabolites, having one or more aromatic rings and they usually appear as glycosides, esters, and methyl esters which can be conjugated with mono-, oligo-, or polysaccharides [23]. The main polyphenic compounds from grape pomace are the anthocyanins (only in red grape pomace), catechins, flavonoid glycosides, or phenolic acids [51]. Phenolics are the most valuable compounds from grape marc, along with dietary fibre which have health benefits, such as the prevention of chronic diseases or cancer [52]. Additionally, polyphenols have an increased antioxidant potential and they can be used for food preservation, by inhibiting lipid oxidation and through their antibacterial effects [53]. According to the literature, 6 L of wine generates 1 kg of grape pomace [27]. One tonne of grape pomace contains 249 kg stalks, 225 kg grape seeds, 425 kg grape skins, and water [54]. The grape pomace can be regarded as an unconventional pectin source [55]. The physico-chemical composition and main components of grape pomace are presented in Table 3.  
It is worth mentioning that grape pomace represents an important source of fibre (43-75%), such as cellulose, hemicellulose, lignin, and pectin [27]. However, few available products from grape pomace extracts are commercialized in various countries, according to the local regulations [29]; however, considering the low cost for marc processing and integrated usage, more products must be developed. It is well-known that grape pomace presents high antioxidant properties, but the difference between grape pomace resulted from white or red wines must be established in order for the appropriate valorisation [59]. The strong antioxidant activity of grape pomace is due to its phenolic compounds and it is related to their chemical structure. The differences between the grape pomaces resulted from white or red whites are influenced by the terror [14] and by the winemaking technology [60]. It was demonstrated that red grape pomace is rich in phenolic acids (such as gallic acid and protocatechuic acid), flavanols (such as myricetin-3-O-hammoside), stilbenes (mainly resveratrol), and anthocyanins (cyanidin-3-O-glucose, delphinidil-3-O-glucose, malvidin-3-O-glucose, penoidin-3-O-glucose, and petunidin-3-O-glucose) [61]. White grape pomace contains high amounts of phenolic acids (such as homovanillic acid, gentisic acid, p-hydroxyphenylacetic acid, syringic acid, vanillic acid, 3- and 4-O-methylgallic acid, hydrocaffeic acid, hydroferulic acid, and isoferulic acid), flavanols (catechin, epicatechin, and procyanidin B1) [61], flavonoid glycosides (such as isoquercitrin, quercetin, and rutin), and flavonoid aglycons (such as luteolin) [62]. Overall, it was concluded that red grape pomace possesses a higher polyphenonic content than white ones [63]. Various studies have revealed a beneficial impact of grape pomace polyphenols on metabolic syndrome, which represents a key factor in several health-related problems [64; 65; 66], for example, by reducing cardiovascular disease risk factors [67], and decreasing hypertension and hyperglyaemia [68].