The Cap is a layer of fruit, or other material including spices, herbs, or other ingredients, that floats up and is held at the top of the fermentation vessel by CO2 (and the buoyancy of the material) during active primary fermentation.
This term comes from the conical shape of the heaped-up pomace (in grape winemaking) emerging from the open tanks, which is collected together after having been piagé, that is to say trampled with the feet. (Peynaud 1981) (Or produced from the carbonic maceration of whole fruit included in the fermentation – FRG)
Managing the fruit cap during early fermentation is especially important as there are key factors discussed below that contribute to the “loss” or “burn off” of fruit flavor, yeast stress, stuck fermentation, off flavors/characters, and failure to maximize the fruit character during primary when using fruit (whether fresh, frozen, dried, etc.) if the cap is not broken up regularly throughout the course of the primary fermentation.
Heat, Sugar and Yeast: For every 1 degree of brix reduction by fermentation of your must there is a 2.3o F increase in temperature. The temperature can build up very quickly and kill off the yeast living in the “heat zone” below the cap when the temperature rises above 104-106o F. You’ll literally cook off a lot of the floral, fruity characters from the berries/fruit you’re using. You’ll also lose a good percentage of your yeast and risk a stuck fermentation if the temperature is left unmanaged.
There is also the production of flavors and characters resulting from yeast stress, and fermentation outside of the temperature comfort zone of the yeast. Off flavors resulting from the fruit or spice being fermented “hot” are also very likely.
Finally, by mixing the cap back down into the must you facilitate extraction of color, acid, tannin, aroma, flavor and other significant characters, hence the final mead will present with more of the flavor of the material in the cap. Including more pronounced character, color and flavor, along with acid and tannin balance.
It is also necessary to punch down frequently to redistribute the yeast (the highest concentration of yeast is found in and directly below the cap) throughout the fermentation vessel in order to promote a more uniform fermentation.
This is especially important when one realizes that the yeast population differs in various portions of the vessel, the reduction of sugar through fermentation will be uneven if the cap is not punched down and mixed back into the must. Thus, it is critical that the cap be managed effectively in order to optimize the yeast, sugar and temperature distribution within the fermentation vessel.
(Punch downs can be achieved with a santized, sanitary punch down tool appropriately sized to the vessel your fermentation is occurring within. Stainless steel punch down tools work well and are readily available at wine making supply retailers. – FRG)
CO2: If the cap is not broken up or “punched down” the CO2 buildup under the cap will deprive your yeast of oxygen. Oxygen is required by the yeast at the beginning stages of fermentation for optimal ethanol tolerance. Therefore, we aerate (oxygenate) during the first third of fermentation in order to get O2 into the must for the yeast which enables the synthesis of sterols in order to strengthen the cell walls. One may think of O2 as a rudimentary yeast nutrient during early fermentation for the benefit of the cell walls. Healthy cell walls are needed to:
- Provide resistance to the rapidly rising level of alcohol
- Regulate osmotic pressure across the cell wall
- Effectively transport nitrogen into the cell
- Help regulate pH within the cytoplasm
- Ensure that the yeast cells can reproduce efficiently and proceed unstressed throughout the fermentation
Remember, if the yeast is not performing to full potential due to improper cap management, poor temperature management, and nutrient deprivation, the fermentation may stall or become stuck, and drag on for weeks. When you see a post or comment in a group or forum about how mead fermentations are taking many months, it’s very likely that the cap, temperature and nutrient management being employed are inadequate to the task at hand.
Drying: If the cap isn’t pressed down into the must frequently (this depends on how much fruit you use, along with the type of fruit, i.e. purees will take less punching down than crushed fresh, frozen or rehydrated dried fruit) to keep it moist the top will dry and spoilage organisms can flourish. The spoilage microbes will produce off flavors and characters; worse they may completely spoil the batch. Remember, if the yeast is compromised into a position of fighting for dominance in the must, they will not be working efficiently enough to yield a vigorous and clean fermentation.
Early exposure to oxygen for both the yeast and the fruit during primary is very important in the formation of the overall fruit flavor and character. Sinking the fruit in a grain bag is also practiced widely and yields very good results. Bear in mind that for very large fruit additions (3-4 lbs of fruit/gallon of must) using a grain bag may be impractical based on the size of the vessel, the opening, and the weight of the bag full of fruit. A large plastic bucket (or other vessel with a wide top opening) provides easier management and access to the fermenting must and fruit.