What Are The Basics Of Fermentation? Ask The Expert!

Just how does grape juice become wine anyway?

In today’s edition of Ask The Expert we examine the core process behind the transformation of grapes into the product we love so much – wine! For our expert answer we turn to winemaker David Kuhlken of Pedernales Cellars in the Texas Hill Country.

Here is David’s fascinating answer to the question: What Are The Basics Of Fermentation?

Fermentation is the fundamental process in winemaking through which grapes are made into wine. Yeast fermentation is key to making a wide range of foods and drinks we know well including cheese, bread, and beer just to name a few. It is an entirely natural process that will take place under a wide range of conditions and in a wide range of circumstances (for example the apple juice in the back of your refrigerator that is suddenly fizzy). For our purposes we will just focus on the role of fermentation in traditional winemaking.

When we are talking about fermentation in regards to wine, we are referring specifically to the conversion of sugar to alcohol. Though a wide range of yeasts exist in our natural environment, those associated with the fermentation in winemaking primarily belong to the genus and species “Saccharomyces cerevisiae”. This particular species is well suited for this purpose given its affinity for sugars and ability to thrive under the conditions common to a wine fermentation. These yeasts, as well as other strains, are pervasive in nature, and it is likely that early wines made thousands of years ago simple relied on the “wild” yeast already living in the vineyard and in the vessels used for fermentation.

Ancient winemakers had no specific knowledge of the role yeast played converting grape juice into wine, but were well aware of how to manage the conditions in order to produce wines to their tastes within the limits of technology at the time.

In today’s winery, most fermentations are managed so as to create the ideal conditions for directing a yeast fermentation in line with a winemaker’s goal for a wine from the grapes harvested. This includes inoculating grape must with a specific strain of saccharomyces cerevisiae rather than relying on the wild yeasts that might collect on the fruit.

A typical red wine fermentation starts with a harvest decision considering aspects such as sugar level, acidity, and flavor development. Once harvested, the fruit is brought to the winery where it will typically be sorted, “de-stemmed” (removing berries from the stem), and gently crushed before being dropped or pumped into a fermentation vessel with skins, seeds, and juice mixing together (a “must”).

White wines differ primarily in that the juice is pressed away from the skins at this stage. Assuming a natural or spontaneous fermentation has not already begun (again, by way of “wild” or “native” yeasts), most modern winemakers will then add a re-hydrated yeast strain to the must or juice.

Different strains of yeast have been isolated over the years from different winemaking regions, and each winemaker often develops a set of preferences for one strain over another based on its compatibility with the type of winemaking they prefer. Strains can vary in terms of aromatics they produce, temperature tolerance, alcohol production, and many other factors that may suit the particular winemaking.

In the fermentation, the yeast feed on natural sugars from the grape and produce a range of bi-products in the metabolism. The most important for winemaking are alcohol, CO2, and heat. Additionally, yeast metabolism can produce a range of favorable and unfavorable compounds affecting the aromatics and flavor of a wine.

A critical part of winemaking is thus controlling and managing a fermentation to maximize positive characteristics and avoid or limit the negative ones. A wide range of factors play a part including the control of temperature, oxygen, and mixing of skins and juice to name a few. This latter is essential in age-worthy red wines for the tannins and color that are incorporated from the skins during fermentation.

Once the yeast have consumed all of the sugars, they will die and settle to the bottom of a tank where a winemaker will then separate or “rack” the wine away from the settled lees. At this stage, primary fermentation has been completed and a wine has been born, ready to begin its next stage of cellaring or finishing depending on the wine style.

Thanks to David Kuhlken of Pedernales Cellars for this fascinating science lesson answering the question What Are The Basics Of Fermentation?

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