When thinking about winemaking, most of us look to the grape varietal, vineyard location and pedigree, the winemaker’s skill and a committed vintner’s ability to blend it all together for the “ultimate” enjoyment of the finished product.

But without yeast, wine would be merely grape juice. Its sole role in the winemaking process is to convert the sugar so carefully nurtured throughout the growing season to alcohol through fermentation that, in great measure, defines wine as a unique product.

Yet, while wine has been produced and enjoyed by countless civilizations for roughly 9,000 years, the fermentation process was not really understood and merely taken for granted. During this time, fermentations were observed and often referred to as “boiling,” which described the frothy bubbling juice in the vat.

The actual study of fermentation dates back only to the mid-19th century when the French government told Louis Pasteur to research wine spoilage. During his studies, he discovered that yeast converts (metabolizes) sugar to alcohol, carbon dioxide and other molecular components. But the exact mechanism of this conversion was only isolated by others in the early 20th century.

Yeast strains break down into two major categories with both serving necessary roles in winemaking, the understanding of fermentation and many of the stylistic differences we observe. Wild yeast (often referred to as ambient, natural, native or indigenous) are complex blends of various strains that exist throughout the vineyard and in the winery. Commercially cultured yeasts are typically isolated strains that are available from production laboratories around the globe to serve specific functions during fermentation.

So let’s first take a brief look at fermentation. Then we can explore the various roles of yeast in the process and their effect on the finished wine.

Yeast, whether wild or cultured, loves sugar and will devour it in an anaerobic environment through the metabolic process of glycolysis until it dies and settles to the bottom of the vessel. This process results primarily in the production of ethyl alcohol and carbon dioxide. The level of alcohol produced is directly related to the amount of sugar in the grape, the efficiency of the yeast or any interference by the winemaker to limit the process.

Port and other fortified wines see the addition of brandy to halt fermentation, thus retaining residual sugar and bolstering the level of alcohol. Cooling the tank as fermentation begins to wind down will also halt the process and leave some residual sugar as seen in many of today’s table wines despite being marketed as “dry.” And many dessert wines are harvested at sugar levels so high that the yeast strains cannot fully metabolize it leaving wines of enticing sweetness, texture and complexity.

Fermentation science in winemaking is a broad and multifaceted study where a myriad practical applications exist in real time. The ultimate effect depends not only on the choice of indigenous yeast versus cultured or a mix of both, but more directly on the winemaker’s knowledge and experience along with the stylistic impression he wants to exhibit in the finished wine.

Wild yeast exists throughout the vineyard and is naturally transported to the winery on the grape’s skins to flourish and multiply in the air and on various equipment. Under proper conditions, and left on its own, the complex mix of wild yeast strains can initiate and continue fermentation to dryness. But the process is not always perfect and may require the winemaker’s skilled intervention along the way.

Many winemakers elect to take advantage of wild yeast to begin fermentation and follow with the inoculation of specific cultured strains that take over and finish the process. These specific strains are chosen by the winemaker to better control fermentation and willingly serve to express his stylistic goals.

Others let the native yeast fermentation proceed on its own hoping to achieve more complexity and enhanced textural feel even though this tack is a bit riskier and far less predictable. In case the wild yeast fermentation gets “stuck” or begins to show negative characteristics, the winemaker does have the option of inoculating with cultured yeast, allowing the process to proceed to completion in a more predictable manner.

When it comes to fermentation protocols, the treatment of different varietals or grapes from different vineyard sites does not fit a linear pattern A winemaker may elect to treat Chardonnay very differently than Pinot Noir or Cabernet Sauvignon regarding the choice of native vs. specific strains of cultured yeast or a combination of the two. If cultured yeast is the choice at any point during fermentation, there are many specific strains available, each with individual characteristics to best complement the winemaker’s objectives.

In researching this column, I contacted several respected winemakers, both locally and abroad, to gain a better understanding of their differing practices with yeast and its role in various fermentation regimens. Their individual opinions and experiences were most informative, and I look forward to sharing them with you in a couple of weeks as we further explore several diverse modalities of yeast driven fermentations by the talented and dedicated people who know best.

Share your experiences with other readers by commenting on this article with an email to me at allenbalik@savorlifethroughwine.com.

Allen Balik, a Napa resident, has been a wine collector, consultant, author, fundraiser and enthusiast for more than 35 years.