During fermentation, thousands of different identifiable flavors are created in wine. Understanding how to choose and use yeast to manage this process can give winemakers an impressive amount of control and creativity in determining their wines' profiles. Today's finest winemakers employ yeast as a key tool to add character to their wines and to explore new sensory experiences.

THE BASICS

Wine is the result of the fermentative activity of yeasts and bacteria. The microbiota of grape juice fermentation can vary significantly, as over 40 genera and 100 different species of yeast have been isolated from grapes or wine. Saccharomyces is the genus most winemakers are familiar with. The S. cerevisiae and S. bayanus species have been found to efficiently dominate and execute alcoholic fermentation. However, other yeast, collectively known as non-Saccharomyces yeast, along with bacteria, may also contribute to the aroma and flavor profile of wine.

There are two basic types of wine production practices with respect to management of microbial populations: indigenous and inoculated (the deliberate addition of pure cultures of selected microorganisms).

There are two main reasons for the inoculation of selected yeasts:

1. The rapid dominance of the fermentation by a high population of Saccharomyces spp., which minimizes the effect of the non-Saccharomyces yeast and bacteria and maximizes the chance of successful fermentation.

2. The desire to accent the fruit component of the wine aroma and flavor profile, and minimize the influence of wild microflora.

Use of pure cultures in winemaking arose only within the last 70 years of the 7000-year history of wine production!

THE AROMATIC IMPACT

The primary intent of using selected yeasts is to successfully complete alcoholic fermentation, avoiding stuck or sluggish ferments. But their impact on a wine's aromatic profile is so significant, it is also an essential criterion of choice. Yeast is a flavor engine responsible for up to 80% of all aroma-active compounds in wine (Ribereau Gayon 2006, Meier-Dörnberg 2017).

There are three different types of wine aromas, depending on their origin:

• The grape's derived or varietal aromas. These aromas depend on the type of grapes used and are mainly represented by polyfunctional thiols (Sauvignon Blanc, Colombard, etc.), terpenols (Muscat, Viognier, etc.) and C-13 norisoprenoids (Chardonnay, Cabernet Sauvignon, etc.).
  
• Fermentation-derived or fermentative aromas. Two of the most important compounds for winemakers are higher alcohols and esters. But this is where we also find ethanol, sulphur compounds, acetaldehyde or acetic acid.
  
• Aging aromas. These aromas come from the wine aging in barrels or tanks. They are represented by oxidative notes and/or oak flavors, etc.

The two first types of aromas depend strongly on the selected yeast according to their metabolic and enzymatic characteristics.

Fermentative (positive or negative) aromas are generated through the metabolism of the yeast or bacteria during the fermentation process. The three sources of all these aromas are the fermentable sugars (glucose and fructose), the assimilable nitrogen compounds (amino acids and ammonium), and the sulphur compounds (sulphates and sulphites).

Depending on their genetics, yeasts have more or less abilities to assimilate and biotransform these compounds and to generate corresponding aromas. When we talk about fermentative aromas in wine, it generally refers to esters, as these molecules have the strongest aromatic impact.

ACETATE ESTERS are derived from the esterification of their corresponding higher alcohols. The main ones are isobutyl acetate imparting fruity flavors; 2-phenylethyl acetate for floral flavors, and isoamyl acetate, for its strong banana flavors and its ability to enhance other flavors at low concentration.

ETHYL ESTERS are derived from the esterification of ethanol with fatty acids. Medium chain ethyl esters are produced inside the cell; branched chain ethyl esters are produced after the autolysis of the yeast. These ethyl esters are mainly responsible for fruity flavors like pear, strawberry, pineapple, etc.

Yeast used to be considered a simple commodity for facilitating fermentation and safeguarding yields. But when employed with craftsmanship and creativity, yeast significantly influences the elegance of aromas, the prominence of flavors, and a wine's distinct final notes. At Fermentis, yeast strains are designed to reveal the highest potential of wine, giving winemakers greater choice, confidence and control to achieve their goals.