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Alcoholic fermentation is stuck or sluggish


A stuck alcoholic fermentation is a condition where there is no yeast activity, whether it never started or has ceased, and yeast can no longer convert sugar into alcohol. A sluggish fermentation occurs when yeast is struggling to ferment, and it could potentially stop fermenting altogether and become stuck. For alcoholic fermentation to become and remain active, yeast requires a favorable environment until all the sugar has been converted into alcohol. As there are several factors or causes that can impact yeast activity, a stuck or sluggish alcoholic fermentation is not an uncommon occurrence, even in commercial winemaking, but fortunately, in most cases, fermentation can be successfully restarted.

If any corrective actions fail to start or restart fermentation, or in addition to these, re-inoculate the must or wine with fresh yeast, preferably a strong fermenting strain. To improve chances of success, prepare a yeast starter or, alternatively, use a 5% volume of actively fermenting wine as inoculum, supplement with more yeast nutrients, and ferment progressively. And if you counterfine wine with bentonite before the start of fermentation, first rack the wine and supplement with a complex nutrient, such as Fermaid K or Superfood, to replace micronutrients as bentonite will cause yeast to sediment, strip essential vitamins and therefore hamper fermentation.

Assuming that you have determined the desired style of wine, assessed the environmental conditions, and selected an appropriate yeast strain accordingly, possible causes of stuck alcoholic fermentation and corrective actions include:


Possible Causes

Corrective Actions, if any

a) Fermentation temperature is too low or too high

b) Sugar concentration is too high

c) Alcohol content is too high

d) Poorly prepared yeast starter

e) Lack of oxygen available to the yeast

f) Lack of nutrients

g) pH is too low

h) Volatile acidity (VA) is too high

i) Free SO2 content is too high

j) Elevated amount of LAB present in the grapes or juice

k) Elevated amount of mold in the grapes

l) Low yeast count

a) Adjust temperature

b) Conduct a progressive fermentation

c) Re-inoculate with a strong fermenting yeast strain

d) Re-inoculate with fresh yeast

e) Aerate must/wine by stirring or racking

f) Prepare a fresh inoculum and add yeast nutrients

g) Deacidify and re-inoculate with fresh yeast

h) Reduce VA and re-inoculate with fresh yeast

i) Aerate must/wine by stirring or racking

j) Treat must/wine with lysozyme

k) Add yeast nutrients and treat with lysozyme

l) Re-inoculate with fresh yeast



Fermentation temperature is too low or too high

Generally, white wines are fermented cool and red wines are fermented relatively hot. Most yeast strains have a fairly wide fermentation temperature range but may not ferment outside the range or may become stuck or sluggish at the low and high ends. If your fermentation area’s temperature is too low or too high, increase or decrease temperature accordingly to get the must or wine within the recommended range. Stir the must or wine thoroughly to distribute temperature evenly and to get the yeast back into suspension. Fermentation should (re)start within 24–36 hours.

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Sugar concentration is too high
Yeast feeds on sugar, but if there is too much sugar in must or wine, the yeast might actually become overwhelmed and be inhibited. If the sugar concentration is high, (where the potential alcohol is beyond the yeast’s alcohol tolerance threshold), the recommended method to (re)start fermentation is to conduct a progressive fermentation by incrementing the must volume. Inoculate a small volume of must – 2.5% of the total volume diluted with an equal volume of water – at 18°–20° C (64°–68° F) with a strong fermenting yeast strain, such as Lalvin EC-1118, RED STAR Premier Cuvée, White Labs Cabernet Red, or Wyeast Zinfandel, at a rate of 25 g/hL (1 g per 4 L or gallon), and add an equal amount of complex nutrients. Reserve the remaining volume of must in a sealed container and place it in a fridge or cold cellar. Once yeast activity starts and fermentation is vigorous, double the must volume by adding an equal amount of reserved must that is warmed up to within a few degrees of the temperature of the yeast starter. Fermentation will slow down for a short time but should become vigorous once again. Double the must volume repeatedly in this manner until the whole batch is fermenting. If fermentation seems to become vigorous shortly after each volume addition of must, you may combine the entire volume at approximately the halfway point. If you are adding sugar to must to increase the alcohol content, avoid over-chaptalizing to prevent a stuck fermentation; use the progressive fermentation method and chaptalize in stages.

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Alcohol content is too high

Yeast can also become inhibited at high alcohol, typically at around 15–16% alc./vol. If yeast activity has ceased and the alcohol content has exceeded the recommended level for the selected yeast type, re-inoculate the wine with a stronger, alcohol-tolerant yeast strain, such as those recommended above for high sugar musts, and add yeast nutrients. First prepare a yeast starter by inoculating a 5% volume of must. When this volume of must is fermenting vigorously, add it to the remaining must; the whole batch should start refermenting.

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Poorly prepared yeast starter
Yeast that has not been correctly rehydrated according to manufacturer’s instructions, e.g., too low of a temperature or too short of a rehydration time, or a yeast starter that has not been adequately prepared are common causes of stuck fermentation, particularly with novices who may tend to rush through the process. Rehydration and inoculation are simple and trouble free procedures, and should never be the cause of a stuck fermentation, all other parameters being favorable. If you suspect that your inoculum was not prepared properly, prepare some fresh yeast culture and re-inoculate.

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Lack of oxygen available to the yeast
Oxygen (air) negatively impacts the quality of wine; however, during active fermentation the fermenting must is protected against oxidation by the yeast as the yeast will quickly scavenge the oxygen. At the onset of active fermentation the yeast will benefit from a small amount of oxygen. If yeast is deprived of air, it could be inhibited and not complete the fermentation. At the beginning of active fermentation, stir or rack the must and leave some headspace to provide oxygen to the yeast. If fermentation is sluggish, try stirring the must vigorously, by racking or by pumping over in conjunction with other corrective actions as required to stimulate fermentation.

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Lack of nutrients
Although not required, fermentation will always benefit from the addition of nutrients. If you suspect that your must is nutrient-deficient, add yeast nutrients pre-fermentation. Likewise, if your fermentation is sluggish or stuck because of a lack of nutrients, prepare a fresh inoculum and add yeast nutrients. Also, read the manufacturer’s notes; some yeast strains specifically require nutrients during rehydration or during fermentation to ferment properly.

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pH is too low or volatile acidity (VA) is too high
Yeast can also become inhibited in juice or wine with a pH that is too low. First confirm that the problem is not due to high volatile acidity (VA). Measure the TA and pH of the must or wine and deacidify until they are within the desired ranges. Then, re-inoculate with fresh yeast. If fermentation is stuck and the problem is determined that VA is too high, there may not be much hope depending on the extent of the problem. Try to reduce VA , and then re-inoculate with fresh yeast.

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Free SO2 content is too high
A high free SO2 concentration is another factor that can inhibit yeast. If fermentation is stuck and free SO2 is high, stir, aerate or pump over the must to try and reduce the free SO2 concentration as much as possible.

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Elevated amount of LAB present in the grapes or juice
A high concentration of lactic acid bacteria (LAB), good or bad, can start feeding on sugar and produce excessive volatile acidity (VA), which will inhibit yeast. If LAB concentration is high, treat the must or wine with lysozyme at a rate of 250–300 mg/L and re-inoculate with fresh yeast to restart a sluggish or stuck fermentation.

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Elevated amount of mold in the grapes or low yeast count
Moldy grapes can also deprive yeast of much needed nutrients. Add yeast nutrients at the maximum recommended rate when first inoculating the must or to restart a sluggish or stuck fermentation if grapes are moldy. The must or wine will also benefit from a lysozyme treatment. The best way to prevent this problem is to do a severe triage (pre-selection), discarding any moldy bunches before crushing.

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