**History and Importance of Yeast in Winemaking**:
– French scientist Louis Pasteur discovered the connection between yeast and fermentation.
– Winemakers historically lacked understanding of fermentation converting grape juice into wine.
– Louis Pasteur’s work in the mid-19th century revealed yeast’s role in converting sugars to alcohol.
– Over 700 strains of Saccharomyces cerevisiae have been identified, each with unique characteristics.
– Wild yeast species like Kloeckera and Candida are also present in grape clusters.
– Different yeast species contribute to the fermentation process in winemaking.
– Wild yeasts contribute to the unique expression of terroir in wine.
– The use of wild yeasts influences the character and quality of wines.
– Wild yeasts from vineyards can add complexity and distinctiveness to wine flavors.
– Ambient yeasts play a role in the natural fermentation process of wines.
**Role and By-Products of Yeast in Winemaking**:
– Yeast converts sugars in grape must into alcohol through metabolic pathways.
– Yeast produces glycerol during fermentation, contributing to body and sweetness in wine.
– Diacetyl produced by yeast can be reduced to neutral-smelling 2,3-Butanediol by fresh yeast.
– Yeast metabolism produces compounds like glycerol and fusel oil that influence wine characteristics.
– Methanol, fusel oils, succinic acid, acetic acid, and hydrogen sulfide are by-products of yeast.
– Acetaldehyde concentrations between 50–100mg/L can remain in wine.
– Yeast can convert ethanol back into acetaldehyde in the presence of oxygen.
– Acetaldehyde contributes to oxidized aromas in wine.
**Control and Selection of Yeast in Winemaking**:
– Winemakers have access to a diverse range of yeast strains with distinct characteristics.
– Commercial availability of yeast strains has revolutionized winemaking by offering precise control.
– Yeast strains influence the sensory profile, aroma, mouthfeel, and fermentation kinetics of wine.
– Modern winemaking allows for selecting yeast strains that enhance desirable wine features.
– Specialized suppliers offer a variety of yeast strains for winemakers to choose from.
– Different strains of Saccharomyces cerevisiae can impact the sensory characteristics and aromas of young wine.
– Yeast strains can produce off-flavors, affect fermentation speed, and influence varietal flavors in wines.
– Some strains are selected for specific wine production processes, such as sparkling wine and Sherry.
**Lees, Secondary Fermentation, and Types of Yeasts**:
– Lees formed by dead yeast cells combining with other particles.
– Sur lie technique involves wine spending time on lees.
– Lees contact enhances wine color, aroma, and mouthfeel.
– Sparkling wines like Champagne undergo second fermentation in the bottle.
– Types of yeasts include Brettanomyces, Zygosaccharomyces, and Aureobasidium.
– Saccharomyces cerevisiae is commonly used in brewing, breadmaking, and winemaking.
– Brettanomyces classified by asexual form in winemaking.
**Nutritional Needs and Impact of Yeasts in Winemaking**:
– Diammonium phosphate provides nitrogen and phosphate for yeast.
– Yeast nutritional needs include minerals, vitamins, and other compounds.
– Non-Saccharomyces yeasts begin fermentation in virtually every wine.
– Non-Saccharomyces yeasts influence aroma and flavor profiles, creating diverse intermediates.
– Impact of ambient yeasts includes potential for off-flavors, aromas, and stuck fermentation.
The role of yeast in winemaking is the most important element that distinguishes wine from fruit juice. In the absence of oxygen, yeast converts the sugars of the fruit into alcohol and carbon dioxide through the process of fermentation. The more sugars in the grapes, the higher the potential alcohol level of the wine if the yeast are allowed to carry out fermentation to dryness. Sometimes winemakers will stop fermentation early in order to leave some residual sugars and sweetness in the wine such as with dessert wines. This can be achieved by dropping fermentation temperatures to the point where the yeast are inactive, sterile filtering the wine to remove the yeast or fortification with brandy or neutral spirits to kill off the yeast cells. If fermentation is unintentionally stopped, such as when the yeasts become exhausted of available nutrients and the wine has not yet reached dryness, this is considered a stuck fermentation.
The most common yeast associated with winemaking is Saccharomyces cerevisiae which has been favored due to its predictable and vigorous fermentation capabilities, tolerance of relatively high levels of alcohol and sulfur dioxide as well as its ability to thrive in normal wine pH between 2.8 and 4. Despite its widespread use which often includes deliberate inoculation from cultured stock, S. cerevisiae is rarely the only yeast species involved in a fermentation. Grapes brought in from harvest are usually teeming with a variety of "wild yeast" from the Kloeckera and Candida genera. These yeasts often begin the fermentation process almost as soon as the grapes are picked when the weight of the clusters in the harvest bins begin to crush the grapes, releasing the sugar-rich must. While additions of sulfur dioxide (often added at the crusher) may limit some of the wild yeast activities, these yeasts will usually die out once the alcohol level reaches about 15% due to the toxicity of alcohol on the yeast cells physiology while the more alcohol tolerant Saccharomyces species take over. In addition to S. cerevisiae, Saccharomyces bayanus is a species of yeast that can tolerate alcohol levels of 17–20% and is often used in fortified wine production such as ports and varieties such as Zinfandel and Syrah harvested at high Brix sugar levels. Another common yeast involved in wine production is Brettanomyces whose presence in a wine may be viewed by different winemakers as either a wine fault or in limited quantities as an added note of complexity.