Truth be told, although we've been making wine for thousands of years, we didn't figure out the mechanics of fermentation until fairly recently.
Antonie van Leeuwenhoek, known as the Father of Microbiology, lived from 1632 to 1723. He was the first to see microorganisms with a magnifying glass. For lack of a better term, he called them "cavorting wee beasties."
It took more than 100 years before science first linked the wee beastie known as yeast to fermentation … and other wee beasties to spoilage and sickness.
Now yeast is once again a hot topic. And there are quite a few reasons why.
As an extension of the modern-day search for authenticity, courtesy of the locavore movement, many winemakers are opting to use indigenous yeast populations to start their fermentations vs. inoculating with yeasts that have been cultured in a laboratory. This precept is part of the "natural wine movement" which, among other things, eschews agrochemicals and frowns upon the use of sulfur.
What is so significant about yeast?
Although yeasts are the same the world over, their specific populations are not. For this reason, local yeast populations have become synonymous with a true expression of "terroir," whereas cultured yeasts, for some, indicate an introduction of non-native flavors.
Yes. Yeasts do change the flavor profile of wine. Professor Thomas Henick-Kling, director of viticulture and enology at Washington State University, proved this conclusively after 20 years of research.
This said, why do some winemakers prefer cultured yeasts over local yeasts?
Cultured yeasts often give a rapid start to the fermentation process and are super-effective at converting sugar into alcohol. Their lab-developed traits enable them to ferment at lower temperatures and to higher alcohol levels. They also produce clean wines that are true to varietal with little or no off-aromas or by-products.
Indigenous or wild yeasts are less-efficient with regard to alcohol production and are much slower in kick-starting fermentation. In addition, they produce a host of by-products such as acetic acid (adds a rustic note), ethyl acetate (contributes hints of pear in small quantities), glycerol (an odorless, sweet-tasting alcohol), phenyl ethanol (imparts rose, floral and honey aromas), SO2, acetaldehyde (contributes a hint of sherry) and higher alcohols (which add a pungent or fusel note).
Although a tad off-putting when listed on paper, the by-products of wild yeast fermentations are produced in small quantities and end up as "complexing" agents. They do not usually dominate the flavor profile; they simply add to it and produce a softer, rounder, fuller wine.
Unfortunately, local yeasts are not often reliable, especially in challenging vintages and it takes skill to manage them correctly.
The use of indigenous yeast, although trendy these days, is nothing new under the sun. Some wine regions, such as Burgundy, have used indigenous yeasts as part of their preferred winemaking traditions from the very beginning and never wavered off the mark while other New World winemakers have chosen this path out of principal or preference and not fad.
Are wines made from cultured yeast any less "natural" than wines made from indigenous yeast?
No. But yeasts do affect a wine's flavor profile.
The good news is, as a consumer, you can rest assured that cultured or indigenous, the winemaker is making his choice based on one goal: to make the best wine possible so that we can become the cavorting beasties.