Common Fermentation Problems

Question:

What are some common problems with fermentation that I need to avoid?

Answer:

Problems with fermentation may include the following: underpitching; overpitching; improper temperature; rapid temperature variations; light-struck; excessive fermentation time, stuck fermentation

Pitching is the act of adding the active yeast starter to the chilled and aerated beer wort.

Underpitching will result in excessive contamination. The wort is in its most vulnerable state after cooling and prior to high-kraeusen blowout. Sufficient yeast must be pitched to bring about high-kraeusen blowout within 24 hours, and ideally less than 12 hours after cooling. This time between pitching and high-kraeusen is called “lag time”, and the smaller the lag time the better. Underpitching will give any contamination a chance to establish itself in the beer, resulting in DMS, diacetyl, cooked vegetable, and similar off-flavors. Another result may be a delayed reaction in the bottled beer, resulting in overcarbonation from a seemingly normally-carbonated beer left for two or three months.

Overpitching will generally result in “yeast bite”, a noticeable yeasty taste in the beer. Re-using all of the slurry from the bottom of a primary fermenter in a new batch will usually overpitch the batch. Overpitching results in rapid consumption of the limited amounts of dissolved oxygen, simple sugars, and yeast nutrients. This will result in yeast autolysis, or self-digestion, and will result in yeasty, sulfur-like flavors.

The optimal amount of yeast is about 0.4-0.6 ounce of pasty, thick yeast sediment per gallon of wort, or about 2.0 to 3.0 ounces per gallon. For dry yeasts this translates to no less than 12 grams of yeast, activated before pitching by dissolving in 90 degree F sanitized water (not wort or sugar water) for about 20 minutes. For cultures, a starter of at least 16 oz of wort (for five gallons) should be prepared and allowed to go into high kraeusen before pitching into the wort. Using less than this amount (such as directly from a bagged culture package) will underpitch the batch and result in an excessively long lag time.

Improper temperature will result in a totally different beer from what is intended. Too high a fermentation temperature for lager yeasts will, in most cases, result in fruity-estery tastes that are uncharacteristic of lagers(unless you are making a steam beer with a heat-tolerant lager yeast). Tocold a temperature with an ale yeast can cause it to stop working and flocculate out prematurely, resulting in a stuck fermentation, with accompanying diacetyl, acetaldehyde, and excessively sweet tastes.

Rapid temperature fluctuations will often result in production of DMS and other sulfur-like compounds, particularly with certain lager strains. It can also disrupt the yeast life cycle, resulting in premature flocculation and mutant yeast growth. Temperature changes should be limited to no more than six degrees (Fahrenheit) per 24 hour period once the yeast has established itself in the wort.

Light can destroy a fermenting beer in the same manner as it can destroy bottled beer, by causing a breakdown of hop oils to undesirable sulfur compounds. This will result in a skunky taste and aroma. Keep glass carboys away from strong sources of light at all times.

Leaving the beer in contact with the yeast sediment and trub for any extended length of time is undesirable. Such trub can contribute many off flavors such as medicinal, sulfury, diacetyl, and astringency. Yeast autolysis (decay) can also provide food for any stray bacteria present in the beer and permit it to grow. While racking to a secondary is generally not necessary for most ales due to the short fermentation time, it is a good idea to carefully rack to a clean secondary any beers destined to be secondary-fermented or cold-lagered.

Stuck fermentations are generally a result of either premature flocculation of the yeast before it has a chance to properly attenuate the wort, or, in the case of high-gravity beers, a lack of alcohol tolerance in the yeast strain which causes it to die out and stop working before the fermentables in the wort have been consumed. “Rousing” the yeast, which can be done by gently shaking the sealed carboy, may sometimes result in the fermentation resuming sufficiently to complete the attenuation. Rousing must not result in additional oxygen being dissolved in the beer at this stage. Another method is to add more fresh yeast, in the form of a starter culture. This is the best method of completing the fermentation of a high-alcohol beer started with a normal yeast strain.

Often times a fermentation is considered “stuck” when, in reality, all the fermentables have been consumed. A beer made with a high dextrine content may only attenuate to around 40% – 45% of the starting gravity, with the high end gravity a result of the unfermentable dextrine content of the beer. No amount of rousing or repitching will result in a lower final gravity of such a beer.