The Way of the Yeast
Yeast, what is it? How is it different from prokaryotic (bacteria) cells? Yeast are aerobic and unicellular organisms, and they are composed of animal cells (also called eukaryotic cells). The life of a yeast cell starts with budding, which involves asexually reproducing offspring from a single parent. This then transitions into immediate absorption of available nutrients to create stores of ATP (Adenosine Tri-phosphate). The main difference between yeast and bacteria is that yeast has this ability to store nutrients, whereas bacteria requires a host which continually supplies the nutrients.
Aerobic or Anaerobic?
While converting the oxygen and nutrients into useable ATP, the yeast is taking advantage of the aerobic environment. However, once the yeast begins to use up the oxygen, the environment becomes much more anaerobic. When this happens, the yeasts need to rely on their energy stores to get them through the tough times. This process initiates the yeasts to begin to producing alcohol. It is my belief that this happens as a defense mechanism.
Defense Mechanism?
If you think about it, most of the microbes associated with wild fermentations in winemaking are dying off at a particularly low level. Brettanomyces (an extremely common wild yeast found in many strains) will usually start dying off around 7%-10% ABV, making them slightly edged-out in the competition for survival. Kloeckera Apiculata is a highly acid tolerant wild yeast that is also tolerant to low amounts of sulfites. Kloeckera dies off at a low percent of alcohol (usually around 2%-5%); however, if it continues unnoticed, it may devastate the final outcome of your wine.
This defense mechanism of producing alcohol will kill off the other organisms that occupy the remaining nutrients. This allows the yeast to remain dominant, ensuring that no surprise smells or off flavors will be produced.
The Importance of the Punch Down!
Punching down the cap is largely important for a few reasons. One reason is that the surface of the cap does not recirculate back into the must, so it tends to dry out if left unpunched for more than 8 hours (depending on humidity of the area). A dry cap leads to microbial growth in the form of mold and vinegar producing organisms called “acetobacter”. Another reason to punch-down is to reintroduce oxygen back into the environment. The cap is air tight and since carbon dioxide is heavier than oxygen, the oxygen has no chance of reintroducing itself on its own. By punching down, you are expelling carbon dioxide (making room for the good stuff). This is important because the yeast cells have likely depleted their energy stores as the oxygen became scarce. In order to keep the fermentation going, you must give the yeast a breath of fresh air. Lastly, you should punch-down to keep the temperature down. High fermentation temperatures (above 86 degrees F) may create “cooked” or jammy flavors that may be undesirable.