One of the hot topics in the wine making world is the use of natural or indigenous yeast to cause the fermentation process of sugar to alcohol. There are two approaches to working with grapes after harvest to begin fermentation.
The most popular way to start fermentation is by inoculating the grape must with a manufactured yeast. This is popular for many reasons including allowing the wine maker to create flavor and aroma profiles through the use of specific yeasts, causing faster fermentation times, and most importantly guaranteeing the spark that causes fermentation to start. A constant nightmare for wine makers during harvest season is the possibility that after all the work during spring and winter, fermentation does not begin properly and one has to deal with a "stuck fermentation".
Recently, more wine makers have been returning to the traditional method of fermentation....by doing nothing!
Grapes in the field have yeast on them (after all yeast is a bacteria that exists everywhere). The yeast that is already on the grapes at harvest time reflects the ground, climate, and air of the immediate area where the grapes are grown. This yeast can be enough to start the fermentation process on its own without having to add commercial yeast.
The advantage of using natural yeast is that the flavor and aroma profiles of the final product will reflect the natural surroundings of where the grapes were grown and, in doing so, will create a very unique product.
Here is a link to a recent article on winemakers who are experimenting with native yeasts in their winemaking process http://imbibemagazine.com/winemakers-harness-native-yeast/.