Wine, in its simplest form, is fermented grape juice. But the process that transforms those humble grapes into the complex and varied wines we enjoy is a fascinating interplay of biology, chemistry, and art. Central to this transformation is fermentation—a process that has captivated winemakers and scientists alike for centuries. In this blog post, we’ll explore the science behind wine fermentation, delve into the role of yeast in this magical process, and share insights from expert winemakers on how they harness fermentation to create wines that are truly extraordinary.
1. What is fermentation?
The basics: fermentation is a biochemical process in which sugars, primarily glucose and fructose, are converted into alcohol (ethanol) and carbon dioxide by the action of yeast. In winemaking, this process is fundamental, as it transforms grape juice into wine, imparting alcohol and contributing to the wine’s flavor, texture, and aroma.
The equation:
Glucose
Why it matters: while this equation might seem straightforward, the reality is far more complex. The conditions under which fermentation occurs—temperature, yeast strain, and nutrient availability—can drastically influence the wine’s final profile. Understanding fermentation is key to both winemakers and enthusiasts seeking to grasp what makes each wine unique.
2. The role of yeast: the unsung heroes of fermentation
Saccharomyces cerevisiae: the most common yeast used in winemaking is saccharomyces cerevisiae, a species favored for its efficiency in converting sugar to alcohol and its ability to withstand higher alcohol concentrations. This yeast is responsible for the majority of fermentations in both red and white wine production.
Natural vs. Commercial yeast:
Natural yeast: also known as “Wild” or “Native” yeast, these microorganisms are naturally present on the grape skins and in the winery environment. Wines fermented with natural yeast can develop complex, unique flavors due to the variety of yeast strains involved. However, the process can be unpredictable, leading to variations in the wine’s final taste.
Commercial yeast: these are specially selected strains of saccharomyces cerevisiae that are cultured and sold to winemakers. Commercial yeasts offer more control and predictability, allowing winemakers to steer the fermentation process towards specific flavor profiles and aromas.
Expert insight: “Using wild yeast is like letting the vineyard speak for itself,” says paul draper, the legendary winemaker at ridge vineyards. “It brings out the unique character of the terroir, but it requires a deep understanding of the process, as it’s less predictable than using commercial yeast.”
Why it’s fascinating: yeast doesn’t just convert sugar to alcohol—it also produces a host of secondary compounds that contribute to the wine’s flavor and aroma. These include esters (which can give fruity aromas), higher alcohols, and other byproducts that influence mouthfeel and complexity. This is why two wines made from the same grapes, but with different yeast strains, can taste markedly different.
3. Fermentation types: primary and secondary
Primary fermentation:
What it is: this is the initial fermentation process where yeast converts the majority of the grape sugars into alcohol. It typically lasts between one and three weeks, depending on the temperature and other conditions.
Temperature control: the temperature during primary fermentation is crucial. Cooler fermentations (10-15°c/50-59°f) are often used for white wines to preserve delicate aromas, while warmer fermentations (20-30°c/68-86°f) are preferred for red wines to extract more color and tannins.
Insights: winemakers often monitor the fermentation closely, adjusting temperature or aerating the must to ensure that the yeast remains healthy and active. According to a study published in the journal of agricultural and food chemistry, precise temperature control during fermentation can significantly enhance the aromatic profile of wines, particularly in varieties like sauvignon blanc and riesling .
Secondary fermentation (malolactic fermentation):
What it is: often referred to as “Mlf,” this is a secondary process where lactic acid bacteria convert malic acid (which is tart, like green apples) into softer lactic acid (which is more like milk). This fermentation is common in red wines and some white wines, like chardonnay.
Impact on flavor: malolactic fermentation softens the acidity and can add creamy, buttery notes to the wine, particularly in whites. In reds, it contributes to a smoother mouthfeel and can enhance the wine’s complexity.
Expert insight: “Malolactic fermentation is a winemaker’s secret weapon,” says peter gago, chief winemaker at penfolds. “It can completely transform the mouthfeel and balance of a wine, turning something sharp and green into something rounded and full-bodied.”
4. Fermentation vessels: beyond the barrel
Stainless steel tanks:
Why they’re used: stainless steel is inert, meaning it doesn’t impart any flavors to the wine. This makes it ideal for preserving the pure fruit characteristics of white wines and some reds. It also allows for precise temperature control, which is crucial during fermentation.
Flavor impact: wines fermented in stainless steel tanks are often described as clean, crisp, and fruit-forward, without the influence of oak or other external flavors.
Oak barrels:
Why they’re used: oak barrels not only allow for slow oxygenation, which can soften tannins and enhance complexity, but they also impart their own flavors, such as vanilla, spice, and toast.
Flavor impact: the type of oak (american, french, or hungarian) and the level of toasting can significantly affect the wine’s profile. Wines fermented and aged in oak tend to be richer, with added layers of complexity.
Expert insight: “The choice of fermentation vessel is as crucial as the choice of yeast or grapes,” notes jean-claudeberrouet, former winemaker at château pétrus. “It’s about matching the vessel to the desired wine style. Stainless steel for purity, oak for complexity.”
Concrete tanks:
Why they’re used: concrete tanks offer the benefits of both stainless steel and oak. They provide a stable temperature and micro-oxygenation without imparting any flavors, allowing the wine to develop texture and complexity while retaining its fruit purity.
Flavor impact: wines fermented in concrete often have a distinct minerality and a fuller mouthfeel, making them popular for certain varietals like syrah and grenache.
5. Fermentation challenges: when things go wrong
Stuck fermentation:
What it is: a stuck fermentation occurs when the yeast becomes inactive before all the sugars have been converted to alcohol, leaving the wine sweet and prone to spoilage.
Causes: this can be caused by several factors, including low temperatures, nutrient deficiencies, or too much alcohol (which can inhibit yeast activity).
Solutions: winemakers can attempt to restart fermentation by warming the must, adding yeast nutrients, or inoculating with a more robust yeast strain.
Volatile acidity (va):
What it is: va is the presence of acetic acid (vinegar) and ethyl acetate (nail polish remover) in wine, often resulting from spoilage bacteria during or after fermentation.
Impact on wine: high levels of va can make wine undrinkable, though small amounts are sometimes considered acceptable in certain styles, adding to the complexity.
Prevention: proper hygiene, temperature control, and the use of sulfur dioxide (so2) can help prevent va from developing during fermentation.
6. The magic of fermentation: beyond science
Fermentation and terroir:
Connection: the yeast strains present in a vineyard and winery are part of the terroir—the unique environment that shapes a wine’s character. Wild fermentations, in particular, can express the terroir more clearly, as the native yeast reflects the vineyard’s specific conditions.
Expert insight: “Fermentation is where the magic happens,” says randallgrahm, founder of bonny doon vineyard. “It’s where the winemaker’s hand, the vineyard’s voice, and the yeast’s whisper come together to create something truly unique.”
The art of timing:
Timing is everything: the decision of when to stop fermentation (for sweet wines) or to blend different batches can significantly influence the wine’s final style. This requires a blend of scientific knowledge and intuitive artistry.
Expert insight: “Knowing when to intervene and when to let nature take its course is what separates good winemakers from great ones,” says michelrolland, a world-renowned wine consultant. “It’s about understanding the process deeply and respecting the wine’s natural evolution.”
Conclusion: the beauty and complexity of wine fermentation
Fermentation is where the alchemy of winemaking truly takes place. It’s a process rooted in science, yet imbued with art and tradition. The role of yeast, the choice of vessel, and the management of fermentation conditions all contribute to the final character of the wine, making each bottle a unique expression of its origin and the winemaker’s vision.