Wine lees as a source of valuable compounds for food applications

Along with grape pomace and grape stalks, wine lees are one of the main by-products of the winemaking process. In the last decade, various studies focused on the valorization of wine lees aiming at the recovery of residual ethanol, tartaric acid, and polyphenols. In contrast, the wine lees’ solid fraction, essentially made of dead yeast cells, has received less attention. This Ph.D. project aims at finding innovative strategies for the valorization of the wine lees’ solid fraction by looking at the possibility to extract the cell wall mannoproteins from them, so polysaccharides already known to have potential to be used as additives in several sectors, including the wine and food industries. Firstly (Study I, published), five protocols for mannoprotein extraction from laboratory grown yeast cells were developed, including a physical treatment (heating and sonication), as well as treatments with different enzymatic (β-glucanase) preparations. After confirming the presence of mannoprotein in the extracts, these latter were tested as emulsifiers in model water/oil emulsions prepared at different pH values (3, 5, and 7). Results showed that the extracts were able to stabilize the emulsions, and the best performances were obtained by extracts produced with the physical treatment of the yeast biomass, mainly at pH 3 and 7. Later (Study II, published), lees recovered after the fermentation of a real white wine were treated with three different extraction methods: two physical (sonication and autoclave treatments), and one involving the use of a commercial, low-cost β-glucanase preparation (Glucanex®). The extracts were then tested for their action in real and model wines to evaluate their impact on protein and tartrate stability and on the foaming properties. The results indicated that the autoclave treatment allowed to obtain extracts showing the highest mannoprotein content and the best functionality in terms of tartrate stability and foam promoters, while no positive effects were shown on protein stability. Finally (Study III, unpublished), two autoclave extraction protocols were used to treat yeast cells recovered after fermentation of a red and a white wine, as well as from pure laboratory-grown cells of the same yeast strains used for the production of the same wines. The extracts were tested as emulsifying and foaming agents in model food system. The best results were obtained by using the extracts from the white wine lees, although they behaved similarly to the extracts obtained from their lab-grown counterpart. In contrast, the extracts of the lees obtained from red wine were much more effective in stabilizing emulsions compared to those obtained by the same strain grown in the laboratory, indicating a major effect of the polyphenols adsorbed on wine lees during winemaking. Overall, the results indicate that wine lees can be used to produce mannoprotein-rich extracts potentially useful in various wine and food applications. If integrated into an optimized process, which considers also the recovery of ethanol, tartaric acid, and polyphenols, the approach here studied can result in a major improvement for both the waste reduction and the production of valuable compounds, thus improving the income of the wine industry and mitigating its environmental impact.