Yeasts exist naturally in vineyard soils, cling to grapes as they grow, and are present in the air of wine cellars. Some winemakers allow these ambient yeasts to carry out fermentation, believing that they impart desirable characteristics and more complexity to the final wine than cultured yeasts would. Ambient yeasts, however, are unpredictable in the alcohol levels they can achieve and in the aromatic components they form, some of which may be disagreeable.
In addition, ambient yeasts can be slow, even sluggish, about getting the fermentation to begin. As a result of these factors, many winemakers prefer to use a strain of cultured yeast, which can be depended on to multiply actively at a given temperature. There are many strains of cultured yeasts. A winemaker’s choice depends on how fast and intense he or she wants the fermentation to be. This in turn may subtly affect the flavours and aromas of the wine.
Fermentation is a furious chemical reaction, during which carbon dioxide gas and heat are thrown off. As the yeasts begin to convert the grape sugar into alcohol, carbon dioxide bubbles up from the fermenting mass and pushes the skins to the surface. Unattended, the skins will float like a shag carpet on top of the wine. But the winemaker does not want them to float there. This dense cap of skins is critical to the eventual character of the wine, for as we know, the skins contain the wine’s potential colour and tannin, as well as compounds that become aromas and flavours. The more the wine is in contact with the cap, the more colour, tannin, flavour, and aroma can be extracted.
Winemakers, therefore, gently break up the cap and submerge it in the wine. Sometimes this is done by pushing the skins under the surface of the liquid using a rake-like pole or a mechanical plate that acts like a plunger.