# An Overview on Management and Valorisation of Winery Wastes
## 7 Valorisation of Wine Lees  
Wine lees are materials that look like sludge, containing dead and living yeast cells, and yeast debris that gradually precipitate at the bottom of wine tanks [130].  
Wine lees (\"heavy\" or \"light\" depending on the decanting stage) were defined as \"_the residue that forms at the bottom of recipients containing wine, after fermentation, during storage or after authorized treatments, as well as the residue obtained following the filtration or centrifugation of this product_\" [131] and represent 2-6% of the total volume of the resulting wine.  
The high values of chemical oxygen demand (COD \(\cong\) 30,000 mg/L) and organic matter content (OMC \(\leq\) 35,000 mg/L) make the lees environmentally friendly [132]. Despite the various methods that were proposed to recover and valorise ethanol, polyphenols, and tartaric acid from wine lees [133], few studies investigated the solid fraction from wine lees, which mainly consists of yeast biomass [134].  
Three extraction protocols were applied in order to extract glycol compounds from wine lees resulting from the alcoholic fermentation of a white wine [130]. The purpose of this study was to investigate whether the less could be used to increase wine stability and sensorial characteristics. It was observed that none of the extracts was equally efficient for all the tested characteristics, although some of them influenced various properties. For example, lees extracted by autoclaving influenced the wine tartrate stabilization, wine foaming, and tartrate recovery from the insoluble fraction. Contrarily, lees extracted by ultrasonication or enzymatic techniques aided the protein stabilization in heat-unstable wines [130].  
The wine lees usage as nutrient supplement for fermentation was investigated. For example, lees resulting from red or white wines from the first or second decanting stage were applied as a nutrient supplement for lactic acid production in the presence of _Lactobacillus_ and glucose as the carbon source [111]. In 2010, a process for tartaric acid recovery was proposed, with the remaining wine lees being applied as fermentation medium for xylitol production [135]. Nevertheless, all the above-mentioned procedures did not take full advantage of the wine lees potential. In a biorefinery perspective, wine lees could be applied to produce antioxidants, ethanol, and tartaric acid, the remaining fraction rich in yeast cells could be transformed into a generic fermentation feedstock [121]. Wine lees hydrolysates and crude glycerol were applied as nutrient and carbon sources, respectively, to produce poly(3-hydroxybutyrate). Its production was significantly influenced by the free amino nitrogen content of the wine lees hydrolysates [121].  
The only actual commercial use of wine lees is for ethanol extraction by distillation, as required by European Regulations [18]. Nevertheless, this is not sufficient to reduce the chemical oxygen demand of wine lees or for a complete valorisation. In this respect, future investigation is needed to efficiently recover valuable compounds. For example, an integrated procedure for polyphenol extraction together with ethanol and tartaric acid extraction should be designed. The remaining biomass could be applied for obtaining fermentation media supplements, thus targeting a complete exploitation of the lees. One issue for industrial scaling is represented by the variability in wine lees composition. In this respect, the recovery of yeast cell wall polysaccharides may be an interesting option for the biomass valorisation. Indeed, the extraction of mannoproteins (MPs) and \(\beta\)-glucans (\(\beta\)-G) with potential applications was reported [136].