I remember reading some where, (just not sure where), that you can capture a wild yeast from the skins of fruit, that will successfully ferment a batch of wine or beer. Wine makers as well as Belgian beer brewers have been capturing and using wild yeast for many years. This was at one time the only way to make wine and beer. The concept is very simple. If you expose the skin of fruit into unfermented, fresh wort or grape must, chances are you will inoculate the beer or must with some wild yeast. Consider that yeast exists on EVERYTHING! It only makes sense that Saccharomyces Cerevisiae ,(the yeast strain used in baking and brewing), exists on grape skins and all types of other fruit. In the old days when Vinters used a grape press to make wine, I’m sure the skins of the grapes made their way into the concentrated must. After a while the veteran wine makers introduced the grape skins on purpose, to inoculate their soon to be wine with the wild yeast on the grape skins. In the old days Belgian farmhouse ales were often fermented by leaving the unfermented beer (wort), exposed to the open air. If anyone has ever made a sourdough culture for sourdough bread making, they will know that when preparing a media such as unfermented beer, grape juice, or a flour and water mixture, in the case of a sourdough culture, the microorganisms best suited for that media will eventually reproduce and take over. As is the case of sourdough, the yeast and lacto-bacteria take about 5 days to get up enough numbers to overtake the mixture, consume it, and take control. The first few days usually smell like you’ve made a mistake or something is wrong with it,however good things come to those who are patient. The lacto bacteria proceed to drop the PH of the flour/water mixture to the point of killing off the “other” less desirable bacteria and microorganisms. It is the lactic acid produced naturally by these bacteria that acidify the culture, and make it much less hospitable for the undesirables. See how to make your own sour dough culture here: http://www.io.com/~sjohn/sour.htm
In the case of brewing yeast, it is essentially the same concept. Wort (unfermented beer) normally has an acidic PH which prohibits reproduction of most spoilage microorganisms. In the case of the picture, a lime skin was used. First a subdued fermentation occurred. Acting very similar to normal yeast fermentation. However a second more vigorous fermentation followed a few days after the first seemed to calmed down. My best guess in this scenario was that there were possibly two yeast strains on the lime peel. The behavior and aroma of the fermentation was a very tell-tale sign Saccharomyces cerevisiae. The picture shows the second starter used in this experiment with a volume of approx 1 1/2 quarts. The first starter, which contained the lime peel used for inoculation, measured approx 1 quart. After that first fermentation took place, the now somewhat fermented “beer” was then transferred to a new container and the peel was strained out completely. When this secondary fermentation without the peel was complete, I then placed it into the fridge to encourage the yeast to “drop out” of suspension or flocculate. ( See link here) http://www.crc.dk/flab/newpage4.htm The next step will be decanting the spent beer above the yeast slurry and pitching the slurry into a small test batch of beer.