A stuck

fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food ...

occurs in brewing beer or

winemaking Winemaking or vinification is the production of wine, starting with the selection of the fruit, its fermentation into alcohol, and the bottling of the finished liquid. The history of wine-making stretches over millennia. The science of wine and ...

when the

yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...

become dormant before the

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

fortified wine Fortified wine is a wine to which a distilled spirit, usually brandy, has been added. In the course of some centuries, winemakers have developed many different styles of fortified wine, including port, sherry, madeira, Marsala, Commanda ...

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

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

and

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

. There are several potential causes of a stuck fermentation; the most common are excessively high temperatures killing off the yeast, or a

must Must (from the Latin ''vinum mustum'', "young wine") is freshly crushed fruit juice (usually grape juice) that contains the skins, seeds, and stems of the fruit. The solid portion of the must is called pomace and typically makes up 7–23% of t ...

deficient in the

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

food source needed for the yeast to 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. Winemakers often take several steps to limit the possibility of a stuck fermentation occurring, such as adding nitrogen to the must in the form of

diammonium phosphate Diammonium phosphate (DAP; IUPAC name diammonium hydrogen phosphate; chemical formula (NH4)2(HPO4) is one of a series of water-soluble ammonium phosphate salts that can be produced when ammonia reacts with phosphoric acid. Solid diammonium phosp ...

or using cultured yeast with a high temperature and alcohol tolerance. These steps will each have their own subtle or dramatic effect on the resulting flavors and quality of the wine. J. Robinson (ed) ''"The Oxford Companion to Wine"'' Third Edition pg 664-665 Oxford University Press 2006

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 This list of grape varieties includes cultivated grapes, whether used for wine, or eating as a table grape, fresh or dried (raisin, currant, sultana). For a complete list of all grape species including those unimportant to agriculture, see Viti ...

, such as

Chardonnay Chardonnay (, , ) is a green-skinned grape variety used in the production of white wine. The variety originated in the Burgundy wine region of eastern French wine, France, but is now grown wherever wine is produced, from English wine, Englan ...

and

Riesling Riesling (, ; ) is a white grape variety that originated in the Rhine region. 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 wh ...

, which are naturally prone to have low nitrogen to

sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...

ratios will be at greater risk for having a stuck fermentation. Another cause rooted in the vineyard is from overripe 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 Alcohol by volume (abbreviated as ABV, abv, or alc/vol) is a standard measure of how much alcohol (ethanol) is contained in a given volume of an alcoholic beverage (expressed as a volume percent). It is defined as the number of millilitres (mL) o ...

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

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 The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...

), 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 Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...

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 in open air containers or

oak An oak is a tree or shrub in the genus ''Quercus'' (; Latin "oak tree") of the beech family, Fagaceae. There are approximately 500 extant species of oaks. The common name "oak" also appears in the names of species in related genera, notably ''L ...

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 use 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 like nitrogen to the must. The

ammonium The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary a ...

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...

, or yeast nutrient, 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.

References

{{reflist Winemaking Brewing Wine terminology