Carbon dioxide gas (bubbles) is given off by the yeast during an active fermentation The CO2 bubbles rise and stick to the skins of the fruit you are fermenting. The bubbles make the fruit lighter and they begin to float to the top of your must. This group of fruit is called your “cap”. As the layer of fruit rises, liquid slowly drains away causing the cap to becomes dry, and left this way is an open invitation for bacteria to begin to oxidize your fruit.
Vinegar bacteria convert alcohol into acetic acid, and the acetic acid will spoil your wine. If your cap becomes dry, in a short amount of time vinegar bacteria will set in, grow and convert the alcohol into vinegar. This is called “acetification”. By breaking up and punching down your fermentation cap at least twice a day you will heed off bacteria, and keep your fruit moist and allow for more color and flavor to be extracted from the fruit.
Placing your fruit into a straining bag or cheese cloth will make punching down the cap and the extraction easier. Once the specific gravity reaches 1.020 to 1.015 it is recommended that the fruit be removed from the must. Punching down your cap more than twice a day will not harm your wine.
It is also important to mention that the removal of CO2 gas will help your yeast in the fermentation process and you will have that much less to degas later on. Excess build up of CO2 will stress your yeasts and in some cases stall a fermentation. By punching down your cap you will also be introducing much needed oxygen the yeasts need to survive.
Photo A. Chunks of pineapple lifted to the top of the primary by CO2 gas.
Photo B. As the “cap” is broken CO2 gas begins to be separated from the fruit.
Photo C. Fruit begins to sink back into the must as CO2 gas is released.
You may use a sanitized spoon to punch down small caps. Larger primaries may need specialized equipment.