Slow or Stuck Fermentations
Although “slow” or “stuck” fermentations are rare they can occur. The symptoms of abnormal performance are typically fermentation rates that are too slow (or too rapid) and those that indicate altered patterns of yeast growth or physicochemical behavior. For example, symptoms may include a long lag phase accompanied by a very slow fermentation rate, followed by no fermentation activity at all. In other cases, after a normal lag phase active fermentation may simply stop before all fermentable carbohydrates are consumed.
The are many causes of sluggish or stuck fermentations. The majority of slow fermentations probably arise because of pitching rate, yeast viability, or level of aeration. If slow or stuck fermentations are a continuing problem, wort nutrient deficiencies may be the cause. Under normal conditions, nutrients for yeast growth are usually sufficient. However, if the yeast has been handled poorly, thereby increasing its requirement for a particular nutrient, the wort may not be able to supply the necessary nutrients.
The mode of treatment for sluggish or stuck fermentations really depends on the cause. If the yeast is contaminated it requires careful investigation of possible sources and corrective action. Yeast deterioration can be eliminated by using a freshly propagated yeast batch. If wort nutrients are a cause they can be increased by adding yeast extract or yeast nutrients. Increasing trub carryover may have a beneficial effect on fermentation.
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