Preventing Yeast Off-flavors: Tips from the Pros

Brewer:  Russ Seideman
Brewery:  Seidemann Brewing Co.
Years of experience:  Seven months
Education:  BS in chemical engineering, University of Wisconsin, Madison; MBA in management, Northwestern University’s Kellogg Graduate School, Evanston, Ill.
House Beers: Rio Salado Munich Export and Rio Salado Amber Altbier

In general, the more common off-flavors associated with yeast are phenolic and some of the sour flavors (Lactobacillus and acetaldehyde). Phenolics, associated with a plastic or Band-Aid smell, are often caused by wild yeast.

To prevent wild yeast, sanitize (well). If you aerate your wort very well, your yeast will get to work faster and stronger. When you areate, you give yeast the nutrition they need. The healthy yeast then crowd out bacteria.

Old yeast and insufficient aeration can cause excessive esters and soapy off-flavors through yeast autolysis. Autolysis, releases fatty acid. Yeast do this somewhat as a normal function of their metabolism. If you’ve aerated well you avoid excessive autolysis decay, because the yeast have enough oxygen to consume.

If you pitch enough yeast, aerate enough, and limit lag time (time between pitching and the start of fermentation) to no more than six hours, you decrease your chance of getting off-flavors. Any lag beyond 48 hours is not good. Lag time ties in with cleanliness. During lag time bacteria can spread before the yeast have a chance to kick in. If you’ve got a good, vigorous yeast going, it will crowd out the bacteria. During the lag phase wort is very vulnerable.

A yeasty taste is due to autolysized yeast, which comes from leaving your beer on the yeast too long. If you leave your beer on the yeast for a month at a temperature above 80° F, autolysis is likely to occur. If yeast cells run out of sugar, they will begin to decompose.

Pitching too much yeast could also be a factor, although it is not often a problem with homebrewers, who pitch too little yeast more often than too much. But it can happen.

Diacetyl is another off-flavor in which yeast play an important role. Diacetyl is associated with buttery and butterscotch flavors. Yeast form some diacetyl. Some yeast form more than others depending on the strain and amount. If you brew correctly, later on in the process yeast will reabsorb diacetyl.

The off-flavor can be caused if you try to force your fermentation and bottle too soon. Some yeast are slower at reabsorbing the diacetyl. There are others that don’t create as much of it. Diacetyl is permitted in small quantities in certain styles but never in lagers.

Performing a diacetyl rest helps. After primary fermentation let the beer increase in temperature five to 10 degrees for a couple of days. This encourages the yeast to reabsorb diacetyl at a faster rate. That’s done commercially as well as in homebrewing. But be careful. If the rest is too long, you get autolysis.

If you raise the temperature for a fairly short period of time right after fermentation has ended — one to two days depending on the yeast strain and length of fermentation — most diacetyl can be reabsorbed.

Another yeast-related off-flavor can occur when repitching. Taste the beer made with the original yeast to get an idea of whether there is any sourness or any other off-flavor. Sourness indicates that bacteria is stuck in your beer. If the beer tastes bad, don’t repitch the yeast from that batch. Nothing will propagate bad yeast like bad yeast.

Diacetyl is also associated with certain bacteria. Hydrogen sulfide (rotten egg smell) is very dependent on the strain of yeast. Certain strains, especially lager yeast strains, tend to produce hydrogen sulfide during fermentation. You may detect this smell during fermentation, but as long as beer stays in contact with yeast long enough and you’re fermenting at the right temperature, the hydrogen sulfide will be reabsorbed and go away.

High ester levels are not always an off-flavor but usually are with lagers. Esters, which taste and smell like banana or other fruit, have a lot to do with fermentation temperatures that are too high, in the mid-70° F range. You want temperatures that are 50° to 55° F for lagers and 60° to 65° F for ales, depending on the yeast.

Pitching rate also plays a role. Esters are more likely when the pitching rate is too low. A lot of times people who pitch low amounts of yeast tend to compensate by cranking up the temperature. Solvent-like fusel alcohols depend on the yeast strain and fermentation that is too high, anything above 75° F.

The Tips

• Aerate your wort well to avoid autolysis, which can lead to excessive esters and soapy flavors.

• Limit lag time to avoid bacteria. A vigorous fermentation will crowd out bacteria, which can lead to sour and phenolic off-flavors.

• Make sure the yeast you use for repitching is fresh. Taste the batch it was used in initially to test it.

• Perform a diacetyl rest that is long enough to reabsorb the diacetyl but not so long that autolysis occurs