Yeast is round to oval microscopic single-celled fungi. They convert sugar in a solution to alcohol and carbon dioxide. Yeast evolved from antiquity by genome multiplication, re-arrangement, and deletions over millennia.
Saccharomyces cerevisae family yeast have been, and still are widely employed by winemakers, and brewers. Saccharomyces ovarum family yeasts are popular with distillers, and some brewers. There are now more than 600 yeast strains and counting. Scientists are now investigating to determine which yeast strains are best for different grape varieties.
I have asked several North American winemakers which yeast they prefer. Most indicated their preference for cultivated yeast, versus ambient yeast (natural strains on the skins).
Cultured yeast is selected from ambient strains and more tolerant to high alcohol and temperature variations. They tend to be predictable, whereas ambient yeast representing a range of strains can stop the fermentation. Winemakers dislike “stuck” fermentations, and once the juice is pressed, add sulphur to kill ambient yeast. They then add cultured yeast of their preference. This renders the fermentation process more even and predictable, but the wine with a less intriguing flavour profile.
Cultured yeast strains are selected by scientists from ambient yeast populations for different characteristics, and then multiplied in laboratories. Ambient yeast populations contain a range. Some strains die off at low ( 4 – 5 per cent ABV) alcohol levels, others continue fermenting, others within the population resulting in more appealing aromas and flavours.
Artisan European winemakers stick with ambient yeast. They pay more close attention to the fermentation process (i.e temperature), and can do it better, due to small volumes involved.
Large wineries produce huge quantities, and are more interested in uniformity of flavours vintage after vintage not withstanding harvest characteristics. The problem is that yeast change over time and even cultivated yeast populations will eventually become more varied. Laboratories try to avoid this undesirable condition as much as possible.
European winemakers are concerned enough to maintain ambient yeast strains on their vineyards; they dump the skins of spent grapes on their vineyards to re-vitalize ambient yeast. Now several North American winemakers believe in using ambient yeast for their more upscale wines and sell them at higher prices.
If you are lucky enough to taste the wines made of the same grape from the same vineyard using ambient yeast, and another batch using cultivated yeast you can discern distinct flavour differences.