HOW DOES FERMENTATION WORK? Wine yeast have the ability to convert sugars into alcohol (with carbon dioxide and heat as by-products). Fully ripened wine grapes tend to be very sweet, upwards of 25% sugar, and we in the wine industry refer to this percentage as degrees-Brix, or 25 Brix. Generally speaking the yeast can convert about one-half of the sugar into alcohol, thus a 25% sugar grape juice can yield about 12.5% alcohol wine. The fermentation process creates a lot of carbon dioxide gas!
Once fermentation takes off, with the addition of a specially selected commercial yeast strain, the grape skins rise to the surface because of all that CO2 gas. Because the grape skins contain important properties, such as anthocyanins (color pigments), tannin, and other good stuff, we want to make sure it is incorporated into the fermenting wine. Additionally, it makes for good practice to avoid any bad micro-organisms.
So, two or three times daily we "punch down the cap" to make sure all that goodness is in the wine! :) The following video shows the process in action.