Yeasts are microorganisms that intervene during the fermentation phase, a process by which, through enzymes, they transform sugars into ethyl alcohol and carbon dioxide: this is an extremely important phase in the winemaking process, as anomalies in the fermentation process they can lead to a deterioration in the quality of the fermented product, with consequent compromise in the quality of the final product. In the wine sector, the most important yeasts are attributable to the genus Saccharomyces, species cerevisiae.
Yeasts are single-celled mushrooms having a spherical, oval or elliptical shape; sometimes they come in the form of filamentous hyphae called pseudomycelium. Their dimensions are around 5-30 μm in length and 1-5 μm in width. According to the current taxonomy, yeasts are located in the Mushroom Kingdom complex inside the phylum Mycota and according to the mode of sexual reproduction they are divided into the classes of Ascomycetes and Basidiomycetes. In general there are few yeasts found on immature grapes, 10-10 3 CFU / g, but with ripening and until the harvest, when sugars spread on the surface, the population reaches 10 4 -10 6 CFU / g. Torulopsis, Cryptococcus, Rhodotorula and Candida predominate over immature bunches, as well as Aureobasidium, Sporobolomyces, Filobasidium which are generally present in the vineyard environment (soil, leaves, bark). These are also found in the ripe bunches, but in less quantity than the apiculated yeasts with oxidative metabolism Hanseniaspora and Metschnikowia, which seems to dominate also on damaged fruits, together with the genera Saccharomyces and Zygosaccharomyces. The main fermentation agent, Saccharomyces cerevisiae, is not present or is detected at very low concentrations on the bunch.
Breathing and fermentation
In the initial phase of the wine production process, the yeasts placed in the culture substrate perform aerobic respiration (i.e. they use oxygen from the air), transforming sugars into CO2 and H2O. This metabolic pathway is extremely important as it allows microorganisms to obtain a greater quantity of energy and therefore their rapid growth. Fermentation, which can be defined as the second phase, takes place within the fermenting mass when the yeasts, due to lack of oxygen, change from an aerobic to an anaerobic metabolism.
The set of reactions that lead from sugars to 6 carbon atoms (glucose and fructose) up to pyruvic acid, is common both to the aerobic route (breathing) and to the anaerobic route (fermentation) and is called glycolysis. Glycolysis occurs in the cytoplasm of microbial cells for several purposes:
- produce ATP, high energy molecules exploitable by the cell;
- produce biosynthesis intermediates including glycerol;
- produce pyruvic acid, the starting molecule for further degradation pathways.
Under anaerobic conditions, the yeasts degrade pyruvic acid into carbon dioxide and ethyl alcohol through the reactions thus schematized, which together with glycolysis conclude the alcoholic fermentation.
Alcoholic fermentation reactions (source: www.wikipedia.org)
The global energy balance is: C6H12O6 → 2 C2H5OH + 22 Kcal / mol
As secondary products, glycerin, acetic acid and succinic acid are obtained, which together with other by-products help to characterize the aroma of the finished product.
The Saccharomyces cerevisiae yeast is scarcely present in the environment and, generally, also inside the musts: it can happen, therefore, that sometimes other yeasts take over the fermentation, giving rise to a process which is not very controllable and whose results are unknown. Therefore, over the years, the selected yeasts have increasingly been used, that is, particular strains of Saccharomyces cerevisiae that have specific peculiarities, known a priori, expressed and highlighted with their activity in the fermenting must: their use allows to achieve a qualitatively result Interesting. Many commercial products are available which are applied on the basis of the winemaking technology and the type of product: there are selected yeasts for the production of common wines, for quality white wines, for sparkling and sparkling wines, for damned or valuable red wines.
Dry yeasts are used, which must be properly prepared and activated: first of all, the yeasts must be dispersed in small doses in a container of water at 35-40 ° C, leaving them to hydrate spontaneously; after which, the yeasts are placed in the must to be fermented; better yet, it would be to put the rehydrated yeasts into a small container full of must, in order to activate their fermentation activity, and then insert them in the whole must mass.
Activation of yeasts for the fermentation of wines (source: www.enotre.com)
- Plant Research and Production Center (CRPV), Viticulture and Biological Enology, Edagricole, 2004
- Dandrea Maura, Characterization and evaluation of the potential of use of lactic acid bacteria in the oenological field, Doctoral Thesis, 2010