Abstract
Yeast mannoproteins are proteoglycans mainly located in the outer layer of the cell wall. They are naturally released into wines during fermentation and aging on lees. They can also be added in the form of yeast-derived products. Although they have many interesting properties in enology (impact on tartaric stability, colloidal stability, color, etc.), the structure-function relationships of mannoproteins are not yet clearly understood. This is mainly due to the wide diversity of composition and molecular structures of these macromolecules, which is itself linked to the diversity of origin of the mannoproteins studied. With the aim to progress in the identification of these structure-function relationships, this thesis work focused on the characterization of these macromolecules and the study of the role played by the characteristics of their polysaccharide part in their interactions with tannins and anthocyanins, in relation to the colloidal stability, astringency, and color of red wines.The mannoproteins studied were extracted from four yeast strains selected for this purpose: a commercial oenological strain, a laboratory strain, and two of its mutants, Mnn2 (absence of branching of the N-glycosylated chains) and Mnn4 (absence of mannosyl phosphate groups). Extraction was achieved by the action of -glucanases only, to preserve as much as possible their native structure. The structural characterization of the mannoprotein pools confirmed the impact of genetic mutations and revealed differences in conformation in solution. The interactions between mannoproteins and tannins were studied at the molecular level by isothermal titration calorimetry. This technique shows the existence of low-affinity interactions between mannoproteins and tannins, which temperature dependence suggests the predominance of hydrogen bonds. These interactions are not affected by the structure of the polysaccharide moiety. Their consequences at the colloidal scale were studied by dynamic light scattering and NTA (Nano-particle Tracking Analysis). They lead to the formation of colloidal aggregates, in small numbers and of finite size (function of the Tannin/MPs ratio), which remain stable over time. Here again, the structure of the polysaccharide moiety has no major impact on the stability of the aggregates formed. By contrast, this structure influences the capacity of mannoproteins to interfere in the aggregation and precipitation kinetics that result from tannin-protein interactions (BSA). This ability is highly dependent on its density and degree of branching. The polysaccharide structure also affects the ability of mannoproteins to interact with Malvidin-3-O-glucoside and act as weak copigments.