Sticking

“Sticking” (or a “stuck fermentation”) refers to when an alcoholic fermentation stops prematurely before all the fermentable sugars have been converted to alcohol. The yeast goes dormant or dies, which can happen for a variety of reasons, such as a nutrient deficiency or a temperature shock, which leaves residual sugar in the wine that the winemaker didn’t intend to be there. Sticking can sometimes be noticed by a winemaker as a sudden halt in bubbling or a change in the wine’s appearance.

In a normal fermentation, yeast consumes grape sugars, converting them into alcohol, carbon dioxide, and heat until either the sugars are depleted or the alcohol level becomes toxic to the yeast (typically around 15-16% ABV for most strains). The quality and sugar content of the grapes play a crucial role in the fermentation process. Must with high sugar levels often ferments slowly, resulting in sluggish yeast. Using the wrong yeast strain or improper fermentation conditions can also cause problems.

The life of yeast cells depends on proper nutrients and environmental conditions. Sometimes, a temperature fluctuation or nutrient deficiency can be enough to cause sticking, which interrupts the process unexpectedly inside the fermenter and can be difficult to diagnose without careful monitoring. This often leaves the wine with an unbalanced sweetness. For example, a stuck fermentation might result in a wine with only 10% alcohol instead of the intended 13%.

Is Sticking a Fault?

Sticking is generally considered a serious winemaking fault, though the severity depends on the context. At some point, the winemaker will decide whether to attempt to restart the fermentation process or accept the wine as is.

Why it’s problematic:

• Creates an unintended sugar-alcohol balance that can make the wine taste cloying or unbalanced.
• Leaves the wine microbiologically unstable—residual sugar can fuel unwanted bacterial growth, spoilage organisms, or refermentation in the bottle.
• Can lead to off-flavors or an undesirable smell from stressed or dying yeast
• Disrupts production schedules and ties up tank space.
• May result in financial losses if the wine cannot be salvaged or must be discarded due to irreparable spoilage.
• Signs of stuck fermentation include a lack of bubbling, unexpected sweetness, or a change in aroma.

Exception: In some cases, winemakers intentionally halt fermentation early to retain sweetness (dessert wines), but this process is controlled and planned, and not a “stuck” fermentation.

How Winemakers Use or Prevent It

Winemakers don’t typically “use” stuck fermentations—they work hard to prevent them. However, here’s how they manage both prevention and remediation:

Prevention:
To avoid sluggish or stuck fermentation, winemakers select a robust yeast strain that is tolerant of alcohol and suited for the specific wine style. Monitoring for sluggish or stuck fermentation is essential, and taking action early can prevent more serious issues.

Remediation:
If a sluggish or stuck fermentation occurs, winemakers choose an appropriate method for restarting, as the right method can make the difference between success and failure. The process of restarting often involves preparing a yeast starter in a separate, sanitized container (sometimes called a Mother Restart Tank) to acclimate the yeast before adding it to the main batch. Racking the wine to a clean vessel helps remove sediment and introduce air, which is vital for yeast health during the early stages of restarting fermentation.

Adding fresh grape juice provides additional nutrients to support yeast activity. It is important to ensure the wine is free of unwanted sediment before restarting, as this can impact the outcome of the fermentation process. After racking, the wine should be poured carefully to avoid disturbing any remaining sediment.

Prevention Strategies

Yeast health and nutrition: Ensure adequate yeast nutrients (nitrogen, vitamins, minerals) are available. Nutrient deficiencies are a leading cause of stuck fermentations. Many winemakers add yeast nutrients at the start and mid-fermentation.

Proper yeast selection: Choose robust yeast strains appropriate for the sugar level, alcohol tolerance needed, and fermentation conditions. Choosing the wrong yeast can lead to fermentation problems or undesirable flavors.

Temperature control: Maintain optimal fermentation temperatures (typically 60-85°F depending on wine style). Cold temperatures can cause fermentation to slow or even stop. White wine is especially sensitive to temperature fluctuations, so extra care is needed. Avoid sudden temperature shock during yeast inoculation to protect yeast health and prevent stuck fermentations. Too cold and yeast goes dormant; too hot and yeast can die.

Adequate yeast population: Use a sufficient amount of healthy yeast cells to create a strong fermentation from the start.

Avoid toxins: Limit exposure to pesticide residues, high sulfur dioxide levels, or other compounds that can inhibit yeast development.

Monitor sugar levels: Regularly test Brix/sugar levels to catch problems early.

Remediation if Sluggish or Stuck Fermentation Occurs

Restart with fresh yeast: Use a method for restarting fermentation that involves selecting a robust yeast strain (such as EC-1118 or DV10), preparing a starter culture in a separate, sanitized container, and ensuring the yeast is healthy before inoculation.

Gradual acclimation: Gradually mix the stuck wine with fresh grape juice and the actively fermenting starter to help the yeast acclimate to the wine’s environment and sugar levels.

Rack and aerate: Rack the wine to a clean vessel to remove sediment and introduce air, which supports yeast health before restarting the fermentation process.

Temperature management: When rehydrating yeast, avoid temperature shock by carefully matching the temperatures of the yeast slurry and the wine to prevent yeast stress and ensure a successful restart.

Careful pouring: After racking, wine should be poured gently to avoid disturbing the sediment to maintain clarity.

Blending: In severe cases, blend the stuck wine with another wine to balance the residual sugar.

Sterile filtration and stabilization: Removes the yeast completely and stabilizes the wine to prevent refermentation, accepting the higher residual sugar as a style choice.

For example, after a sluggish or stuck fermentation, using this method with a strong yeast strain and gradual acclimation in a separate container has successfully restarted fermentation and produced a clean, finished wine.

Conclusion

Sticking represents one of the more challenging problems in winemaking, transforming what should be a controlled biological process into an unpredictable crisis. While it’s decidedly a fault rather than a technique, understanding its causes has led to significant advances in fermentation management. Modern winemakers armed with proper yeast nutrition, temperature control, and monitoring tools can largely avoid this issue, but when it does occur, patience and careful intervention can often save the vintage.

The key lesson is that prevention through attentive cellar practices is far easier than cure, making stuck fermentation a problem that underscores the importance of supporting yeast health throughout the entire fermentation process.