What is a ferment? Fermentation is a process by which microorganisms convert one chemical to another in a way that releases energy for the use of the fermenter, and this occurs in the absence of oxygen.
Bacteria that ferment and break down fruits naturally occur on the fruit skin, just waiting to infiltrate into a crack or bruise in the fruit and to take over and explode. A community of microbes converts sugars to alcohol and releases bubbles of carbon dioxide as a by-product, and this is why champagne is bubbly. The very origin of the word ferment is fervor and to boil.
While one community of yeast is converting sugar to alcohol, another community is converting alcohol into acetic acid– making vinegar. These communities help each other, metabolizing foods where waste products of one species become foods to another.
A bruise or split in the fruit skin allows fermenting bacteria to enter and explode in population, protected from the environment in the resilient shelter. Any fruit with lots of sugar will undergo this process on the vine– apples, grapes and blueberries.
There is an amazing degree of diversity in the different fruits fermenting on the same vine. Different wild yeasts specific to geographic regions will make a wild ferment in New York taste different from that in California. Different types of alcohol may be produced in different ferments. If the fruit gets overexposed to the atmosphere while it is still wet, mold bacteria may take over. Otherwise, the fruit may dry in the sun–preserving the sugars, alcohols and seeds intact by making an environment inhospitably dry for any further fermentation.