Yeast is a vital microorganism in wine making. From ancient fermentation to modern advancements, yeast influences taste, texture and terroir in artisan and industrial wine production.
A microscopic fungus, yeast works to transform grape juice into a complex, nuanced wine. The most common variety is regular brewer's or baker's yeast, the remarkable Saccharomyces cerevisiae.
Neolithic brewing is so popular, S. cerevisiae is considered the first domesticated microorganism. This robust species is also found in wild strains in nature, transforming honey and fruit sugars into alcoholic drinks.
The connection between yeast and wine making goes back thousands of years. Earliest evidence of wine fermentation is c. 6000-5000 BCE in today's Georgia, and Iran. West Iran is part of ancient Mesopotamia.
For millennia, winemaking is a process of mystery. Ancient civilizations know crushed grapes left on their own will eventually become intoxicating, but people have little concept of the microorganisms driving the process.
A few ancient philosophers put forth the idea of tiny unseen creatures all around us, but they're laughed out of the Agora. In the 19th century the work of Louis Pasteur uncovers the action of yeast in fermentation.
He proves fermentation isn't a spontaneous chemical process but driven by single-celled organisms, converting sugars to alcohol and carbon dioxide. This discovery dramatically improves understanding and quality of wine.
How Yeast Works
In winemaking, yeast consumes the sugars in grape juice, primarily glucose and fructose. Through anaerobic respiration, also called fermentation, it converts fruit into ethanol (alcohol) and carbon dioxide.
This simple process is incredibly complex. It's cascade of biochemical reactions, each contributing to the final character of the wine.
Sugar Consumption: Yeast cells uptake sugars from the grape juice.
Fermentation: Inside the yeast cell, enzymes break down the sugar molecules.
Alcohol Production: The broken-down sugars are converted into ethanol, and carbon dioxide, which is released to form a froth of bubbles.
Flavor Compound Creation: Besides alcohol and CO2, yeast produces compounds like esters, aldehydes, and higher alcohols. These byproducts are responsible for much of the complex aromas and flavors of wine.
The fruity notes in a young Beaujolais, spicy undertones of a Syrah or the buttery richness of Chardonnay and other nuances are created by yeast.
Temperature control and oxygen levels during fermentation are important to maintain. Fermentation temperatures above 30°C can cause undesirable tastes.
Cooler fermentations of 15-20°C promote more refined flavors. By manipulating conditions, winemakers can guide the natural behavior of yeast to achieve a desired wine style.
A Vineyard's Microbial Ecosystem
While commercially available Saccharomyces cerevisiae is the yeast most commonly used for winemaking, the skins of grapes are home to a diversity of wild yeast species and other microorganisms.
Native Yeast
Kloeckera apiculata, Metschnikowia pulcherrima, Candida stellata, Torulaspora delbrueckii, Brettanomyces, Hanseniaspora and other non-Saccharomyces species are often found on grape skins.
These yeasts contribute to initial stages of fermentation with unique aromas and flavor profiles. Wild yeasts often cannot complete fermentation on their own and die, or more often go dormant, as alcohol levels rise.
In nature wild yeast is usually accompanied by its good friend Acetobacter, the acetic acid bacterium. Acetobacter consumes the alcohol made by yeast and produces acetic acid, turning wine to vinegar.
For yeast this is a boon. It enjoys acidic environments and can continue munching sugar without the ill effects of too much booze.
In wine making, this is a catastrophe. Thus, vintners often start a batch with wild yeast, and introduce the more alcohol-tolerant S. cerevisiae for higher alcohol levels.
Yeast goes dormant at 8% - 20% alcohol depending on species. It becomes part of the lees or leftover sediment of the wine and may be used in further brewing. Almost half the lees is viable yeast.
Cultivating Native Yeast
Some winemakers opt for wild or native yeast found on their own grapes. This approach is considered an expression of terroir, allowing the specific ecosystem of the vineyard to impart its unique character to the wine.
Foot Trodding: An age-old method involves manually crushing grapes to release their juice and begin fermentation using native yeast from the grape skins.
The "Pied de Cuve": Another method involves creating a small starter culture by fermenting a batch of grapes a few days before harvest. This active culture of native yeast is then added to the larger batch of grapes for fermentation.
Culturing wild yeast is a riskier endeavor, requiring a deep understanding of the microbial ecology of the vineyard and careful monitoring. Wine makers in Burgundy adopt this technique.
It expresses distinct flavors of the region's soil and climate. The fermentation process using wild yeast may be less predictable, but the results can be wines of exceptional complexity and character.
Spontaneous fermentation is a hands-off approach, leaving the natural processes to their business. This often results in compelling wines with a story of origin.
Wine from the Sonoma Coast may have a briny, coastal character due to local yeast interactions. A Bordeaux wine often showcases its robust and sophisticated profiles thanks to varying native microorganisms.
Terroir: Yeast and the Sense of Place
The influence of yeast on the terroir of wine is a topic of increasing study. While soil, climate, and grape variety are well-established elements of terroir, the microbial life on grape skins adds another layer to the equation.
Regional Differences
Different regions have unique types of wild yeast on grapes, influencing the style and character of wines from those areas. A Zinfandel from Sonoma can be different from a Zinfandel of the Amador County area, even if both are grown in similar conditions.
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Vintage Variability
The microbial composition on grapes can vary from year to year, influenced by factors like rainfall, temperature, and humidity. Wines from the same vineyard can taste different from vintage to vintage.
Sense of Place
By using native yeast and other local microbes, winemakers capture the specific microbial signature of the vineyards. Wines are uniquely expressive of their time and place.
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