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A stuck fermentation occurs in brewing beer or winemaking when the yeast become dormant before the fermentation has completed. Unlike an "arrested fermentation" where the winemaker intentionally stops fermentation (such as in the production of fortified wines), a stuck fermentation is an unintentional and unwanted occurrence that can lead to the wine being spoiled by bacteria and oxidation. There are several potential causes of a stuck fermentation-the most common are excessive temperatures killing off the yeast or a must deficient in the nitrogen 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. 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 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.
A chronic problem in winemaking is "stuck fermentation," when yeast that should be busily converting grape sugar into alcohol and carbon dioxide prematurely shuts down, leaving the remaining sugar to instead be consumed by bacteria that can spoil the wine. A team of researchers including UC Davis yeast geneticist Linda Bisson has discovered a biochemical communication system behind this problem. Working through a prion -- an abnormally shaped protein that can reproduce itself -- the system enables bacteria in fermenting wine to switch yeast from sugar to other food sources without altering the yeast's DNA. "The discovery of this process really gives us a clue to how stuck fermentations can be avoided," said Bisson, a professor in the Department of Viticulture and Enology. "Our goal now is to find yeast strains that essentially ignore the signal initiated by the bacteria and do not form the prion, but instead power on through the fermentation." She suggests that the discovery of this biochemical mechanism, reported Aug. 28 in the journal Cell, may also have implications for better understanding metabolic diseases, such as Type 2 diabetes, in humans. Bacteria, yeast and fermentation Biologists have known for years that an ancient biological circuit, based in the membranes of yeast cells, blocks yeast from using other carbon sources when the sugar glucose is present. This circuit, known as "glucose repression," is especially strong in the yeast species Saccharomyces cerevisiae, enabling people to use that yeast for practical fermentation processes in winemaking, brewing and bread making, because it causes such efficient processing of sugar. Prions play key role In this study, the researchers found that the glucose repression circuit is sometimes interrupted when bacteria jump-start the replication of the prions in membranes of yeast cells. The interference of the prions causes the yeast to process carbon sources other than glucose and become less effective in metabolizing sugar, dramatically slowing down the fermentation until it, in effect, becomes "stuck." "This type of prion-based inheritance is useful to organisms when they need to adapt to environmental conditions but not necessarily permanently," Bisson said. "In this case, the heritable changes triggered by the prions enable the yeast to also change back to their initial mode of operation if environmental conditions should change again." The researchers demonstrated in this study that the process leading to a stuck fermentation benefits both the bacteria and the yeast. As sugar metabolism slows down, conditions in the fermenting wine become more conducive to bacterial growth, and the yeast benefit by gaining the ability to metabolize not only glucose but also other carbon sources as well -- maintaining and extending their lifespan. Solutions for winemakers Now that this communication mechanism between the bacteria and yeast is more clearly understood, winemakers should be better able to avoid stuck fermentations. "Winemakers may want to alter the levels of sulfur dioxide used when pressing or crushing the grapes, in order to knock out bacteria that can trigger the processes that we now know can lead to a stuck fermentation," Bisson said. "They also can be careful about blending grapes from vineyards known to have certain bacterial strains or they could add yeast strains that have the ability to overpower these vineyard bacteria."
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 and Riesling, 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 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 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 lipids 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 wine barrels which provide plenty of oxygen and grape solids for the yeast to feed on.
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 salt diammonium phosphate, 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.