501-538 Contributi 3-4.qxp:117-186 Contributios 3-2.qxp

Yeasts, bacteria and filamentous fungi all contribute to the microbial ecology of wine production and the chemical composition of wine, although yeasts play the dominant role because they drive alcoholic fermentation [18]. Many factors affect the microbial ecology of wine production, of which the chemical composition of the grape juice and the fermentation processes are the most significant. In complex microbial ecosystems containing mixtures of different species and strains, interactions between microorganisms may occur and these may affect the final ecology [12, 18, 41] Grape juice presents extreme conditions for the growth of microorganisms; a low pH (between 3 and 4) and a high sugar concentration. During alcoholic fermentation the sugar content is transformed into ethanol by yeasts, which supposes an additional restriction on the development of microorganisms. Such environmental changes are responsible for differences in the microbial ecology throughout the process; tolerance to high concentrations of ethanol and low pH are the main factors that select species occurrence in wine ecosystems [18]. Furthermore, the coexistence of different microorganisms in the medium generates competition for nutrients. The early growth of yeasts in grape juice decreases the nutrient content, thus making the resulting wine less favourable as an environment for any further microbial growth. In addition, such growth releases different metabolites into the medium, some of which could be toxic to other species. Another factor that affects the development of some microorganisms is carbon dioxide production which strips the medium of oxygen, thereby limiting the growth Paraules clau: vi, vinagre, acetobacter, gluconobacter, gluconacetobacter