Definition: Yeasts

There are many different types of yeast, which differ in terms of their morphology, properties and mode of reproduction (vegetative reproduction by budding, or reproduction by spore formation). Wine yeasts belong to a dozen different genera. Yeasts found in winemaking can take one of four forms: elliptical or ovoid, spherical, or apiculate. Saccharomyces ellipsoideus (or cerevisiae): the most common elliptical yeast, Kloeckera apiculata Hanseniaspora uvarum saccharomyces chevalieri (its fermentation properties are indistinguishable from saccharomyces ellipsoideus. White harvests: saccharomyces oviformis (or bayanus), capable of reaching a high alcohol content. Torulopsis stellata (specific to grapes affected by noble rot); Schizosaccharomyces pombe: has the property of removing malic acid and deacidifying the must. The development of this yeast is only significant in the absence of elliptical yeasts. The different species of yeast take turns during the fermentation of the must. Apiculate yeasts start the process. Then, very quickly, saccharomyces invade the environment. Towards the end of the fermentation of sugar-rich musts, the dominant species is Saccharomyces oviformis, which is less sensitive to alcohol. There is no good spontaneous uniformisation of the yeast population in the fermenting harvest. The highest proportion of yeasts is found in the marc. This explains why pumping over has such an important effect on the oxygenation of the yeast population. Yeasts are made up of 25 to 60 per cent nitrogenous matter. In order to form their cells and reproduce, they must therefore find sufficient nitrogen in the environment, in a form that is easy to use. This is not the case, for example, in harvests affected by Botrytis cinerea. The addition of ammoniacal nitrogen or ammonium salt is essential in certain cases. Yeast selection has led to the creation of yeast strains that perform very well in terms of fermentation kinetics. The strains are mainly Saccharomyces cerevisiae var. cerevisiae and Saccharomyces cerevisiae var. bayanus. Cerevisiae strains have shorter latency phases than bayanus strains and are more resistant to alcohol due to their specific membrane combination.