I just finished my sommelier training, and during that time we tasted some 300 wines. In quite a few instances I asked our lecturer: "Has this been fermented with wild yeast?" In each instance it had. What does this mean? Can I taste (or smell) wild yeast?
The short answer is "no".
What I noted was the same "funky" aromas that lambic(*) are famous for, the same aromas the come from brettanomyces. What I smelled was, in my opinion, poorly made wines. As it turns out I'm particularly sensitive to brettanomyces(**), which has led me to a few discussions with some of the natural wines fans.
Brettanomyces comes from the skin of the grapes and develop much more easily in natural wines for two reasons:
1. The cultivated yeasts that most wine producers use (natural wines is still a very small niche in a niche (biodynamic wine)) overpower brett and thus makes sure it never develops.
2. The low to none sulfur additions that are also part of natural wines also aids brettanomyces as the sulfur, being an anti-oxidant, would inhibit brettanomyces growth.
Wild yeasts, as has been mentioned, are the yeasts that later have been cultivated so that one can choose which properties one wants the most. Also, a wine maker in Austria once told me that lab tests show that wild yeasts will eventually, during fermentation, develop into the same yeast strains as the cultivated ones, and that they are 70% identical. The difference isn't that big, according to him (he's a biodynamic producer, but not a natural wine producer), but he preferred the predictability of the cultural yeasts.
Also; a lot of cultured yeasts are cultured from yeasts strains from that area, meaning that they do originate from the same terroir as the wild yeasts would.
(*) Lambic are the wheat beers from Belgium, named after the town Lambeek, that are yeasted with wild yeast.
(**) Fascinatingly enough we all have different levels of sensitivity to different aromas. For example the studies that are being done by Centro de Aromas show that the difference in sensitivity to rotundon (which is the smell of black pepper often found in shiraz/syrah) varies a lot. Some people can detect levels as low as 0,000000016 nanograms per litre and others still can't pick it up at 4000 nanograms per litre (one in five, in their tests, actually). I am not sensitive to pepper, rather below average, but very sensitive to brett and about average on TCA. We are all different, which makes smelling wine both fascinating and frustrating.