A stuck fermentation

Fermentation (wine)

The process of fermentation in wine turns grape juice into an alcoholic beverage. During fermentation, yeast interact with sugars in the juice to create ethanol, commonly known as ethyl alcohol, and carbon dioxide...

occurs in brewing beer or winemaking

Winemaking

Winemaking, or vinification, is the production of wine, starting with selection of the grapes or other produce and ending with bottling the finished wine. Although most wine is made from grapes, it may also be made from other fruit or non-toxic plant material...

 when the yeast

Yeast

Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

 become dormant before the fermentation

Fermentation (food)

Fermentation in food processing typically is the conversion of carbohydrates to alcohols and carbon dioxide or organic acids using yeasts, bacteria, or a combination thereof, under anaerobic conditions. Fermentation in simple terms is the chemical conversion of sugars into ethanol...

 has completed. Unlike an "arrested fermentation" where the winemaker intentionally stops fermentation (such as in the production of fortified wine

Fortified wine

Fortified wine is wine to which a distilled beverage has been added. Fortified wine is distinguished from spirits made from wine in that spirits are produced by means of distillation, while fortified wine is simply wine that has had a spirit added to it...

s), a stuck fermentation is an unintentional and unwanted occurrence that can lead to the wine being spoiled by bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and oxidation. There are several potential causes of a stuck fermentation-the most common are excessive temperatures killing off the yeast or a must

Must

Must is freshly pressed fruit juice that contains the skins, seeds, and stems of the fruit. The solid portion of the must is called pomace; it typically makes up 7%–23% of the total weight of the must. Making must is the first step in winemaking...

 deficient in the nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 food source needed for the yeast to the thrive. Once the fermentation is stuck, it is very difficult to restart due to a chemical compound released by dying yeast cells that inhibit the future growth of yeast cells in the batch. At the winery winemakers take several steps to limit the possibility of a stuck fermentation occurring, such as adding nitrogen to the must in the form diammonium phosphate

Diammonium phosphate

Diammonium phosphate is one of a series of water-soluble ammonium phosphate salts which can be produced when ammonia reacts with phosphoric acid...

 or using cultured yeast with a high temperature and alcohol tolerance. These steps that winemakers may take to prevent a stuck fermentation will each have their own subtle or dramatic effect on the resulting flavors and quality of the wine.

Possible causes

There are several potential instigators of a stuck fermentation. One of the most common found in winemaking is a nitrogen deficient must. Nitrogen is a vital nutrient in the growth and development of yeasts and is usually provided from the wine grapes themselves. Grapes grown in vineyards with soils lacking in nitrogen or grape varieties, such as Chardonnay

Chardonnay

Chardonnay is a green-skinned grape variety used to make white wine. It is originated from the Burgundy wine region of eastern France but is now grown wherever wine is produced, from England to New Zealand...

 and Riesling

Riesling

Riesling is a white grape variety which originated in the Rhine region of Germany. Riesling is an aromatic grape variety displaying flowery, almost perfumed, aromas as well as high acidity. It is used to make dry, semi-sweet, sweet and sparkling white wines. Riesling wines are usually varietally...

, which are naturally prone to have low nitrogen to sugar ratios will be at greater risk for having a stuck fermentation. Another cause rooted in the vineyard is from overripe

Ripeness

In United States law, ripeness refers to the readiness of a case for litigation; "a claim is not ripe for adjudication if it rests upon contingent future events that may not occur as anticipated, or indeed may not occur at all." For example, if a law of ambiguous quality has been enacted but never...

 grapes. Grapes that are overripe will have high levels of sugars that translates into higher alcohol content. Yeast are unable to reproduce in an environment with 16-18% ABV

ABV

ABV is a three-letter acronym that may refer to:* Alcohol by volume, a measure of the alcohol content of alcoholic drinks* Nnamdi Azikiwe International Airport, Abuja, Nigeria, from its IATA airport code...

 but in an environment with multiple stressors the fermentation could get stuck even before the alcohol level reaches that point.

A byproduct of the energy created during fermentation is heat which raises the temperature of the fermenting must as the yeast work. When temperatures near 104°F (40°C), activity slows and yeast start to die. If temperatures stay high and the yeast stop reproducing then the fermentation is at grave risk of getting stuck. Even if the must is cooled back down, fermentation will be very difficult to restart due to a chemical compound released by the dying yeast that serves as an inhibitor to the growth of future yeast cells in the batch. Modern winemaking equipment includes temperature control mechanisms such as stainless steel fermenting tanks with a cooling jacket to help regulate temperatures. Another potential cause is a combination of lack of oxygen and yeast nutrients found in the lipid

Lipid

Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...

s suspended in grape solids. In the absence of oxygen, yeast will seek out the lipids in grape solid as a source of nutrients to sustain its activity. If the wine is lacking grape solids (such as if the wine's been clarified) then the yeast are at risk of starving and causing the fermentation to get stuck. This risk is lower for red wines which are often fermented with their skins

Maceration (wine)

Maceration is the winemaking process where the phenolic materials of the grape— tannins, coloring agents and flavor compounds— are leached from the grape skins, seeds and stems into the must. Maceration is the process by which the red wine receives its red color, since 99% of all grape juice is...

 in open air containers or oak wine barrels which provide plenty of oxygen and grape solids for the yeast to feed on.

Prevention techniques

There are various techniques that a winemaker can employ to minimize the chances of a stuck fermentation happening. The most common is to used a cultured yeast strain with a high alcohol and high temperature tolerance coupled by diligent control of the fermentation temperature. Another technique is to add yeast nutrients, include nitrogen, to the must. The ammonium

Ammonium

The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...

 salt

Salt

In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

diammonium phosphate is a popular inexpensive means of ensuring the must has sufficient nitrogen. Whatever technique the winemaker uses, each has the potential to subtly or drastically affect the resulting flavor and quality of the wine.