Carbon dioxide produced during fermentation causes the grape solids (skins, seeds, stems) to rise, forming a cap that floats upon the wine. Unmanaged, this cap can create problems in the wine. For example, with a floating cap, there is little contact between the juice and the skins, which inhibits the extraction of color and flavors. Also, the cap behaves like an insulator, and can trap dangerous levels of heat that can kill the yeast and halt fermentation. Finally, the surface of the cap is in contact with the air, providing an environment for aerobic bacteria to produce vinegar.
Over the centuries, winemakers have developed methods to manage the cap, increasing flavor extraction while reducing the potential for spoilage. Today, there are many strategies for cap management, all with French names like délestage or remontage. I teach all of these methods in my class. However, most artisan producers rely on variations of just two time honored practices: punch down, where a tool is used to submerge and break up the cap, and pump over, where wine is circulated over the top and through the cap. These techniques are not limited to a certain type of wine; often, during the course of fermentation, we will use both punch down and pump over on the same wine.
The frequency of punch down (or pump over) is at the winemaker’s discretion. At Occasio, we begin with three punch downs a day. Later in fermentation, we might reduce this to twice a day depending on the saturation and developing structure of the wine. Cap management is where the “art” of winemaking begins.
Though I spend but a single lecture on the science and techniques of “cap” management with my students, next to harvest this period is the most important phase in winemaking. It is during fermentation and cap management that the winemaker has the ability to release the full potential of the wine. Fail at cap management, and you have failed the wine. This is why I wince when I hear the slogan, “great wines are made in the vineyard.” As an old mentor of mine used to say, “great wines might begin in the vineyard, but they are made in the winery.” I can drink to that.