The winemaking process begins with the pressing of grapes, in order to extract the must inside the berry. After that follows the decantation by gravity, to remove impurities contained in the liquid, and subsequently the start of the first fermentation.
The first fermentation begins with the inoculum of yeasts, which are used to transform the sugars contained in the grapes into alcohol. This chemical process allows to convert the must containing only sugars, to a finished wine (also called base wine) which contains only alcohol.
The wine obtained is stable, as the absence of sugar and the presence of alcohol prevent any anomalous fermentations. This could already be marketed as still Prosecco. Historically though, with Prosecco it's normal to proceed with a second fermentation, called sparkling process.
The sparkling process takes place when the base wine has become limpid. This second fermentation is done with the Italian Martinotti method (also called Charmat) in which pressure-tight tanks, called autoclaves, are used.
With the Martinotti method, sugar and yeast are added, in order to spark a second fermentation. This process is similar to the first fermentation, and produces CO2 as well. In this second phase, however, this pressure is maintained inside the autoclave, so that it combines with the liquid, forming the typical perlage of sparkling and fizzy Prosecco.
The wine has to stay at least 30 days in the autoclave. At the end of this period, it is filtered to remove all the impurities, and is then ready to be bottled.
The chemical process: the alcoholic fermentation takes place in two stages: first, the yeast breaks down the complex sugars (disaccharides) into simple sugars (monosaccharides). In the second stage, there is the formation of ethanol (or ethyl alcohol) starting from the simple sugars, together with the formation of carbon dioxide (CO2). The latter is released into the air during the first fermentation, while it is held into the liquid during the sparkling process (or second fermentation).