25 Is My Beer Ruined?

“Is my beer ruined?!” This phrase has to be the most frequently asked question by new homebrewers, and usually the answer is, “No.” Depending on the cause, the beer may end up with an odd flavor or aroma, but you will still be able to drink it and chalk it up as another lesson along the road to brewing that perfect beer. Although a lot can potentially go wrong with a batch, most problems arise from just a couple of root causes.

Let’s review our Top Five Priorities for Brewing Great Beer:

1. Sanitation
2. Fermentation temperature control
3. Proper yeast management
4. The boil
5. The recipe

This list is prioritized from highest to lowest, that is to say, if you make a mistake in a higher priority, you can’t fix it by doing a lower priority correctly. Let’s also add a sixth priority to this list—the ingredients. The quality of the ingredients are obviously important to the quality of the beer, but the quality of ingredients will not make much difference if you screw up the brewing process. If you have good ingredients, but a poor recipe, then the beer will not be very good. If you have a good recipe, but you didn’t cook it right (the boil), then it won’t be very good. If you made a good yeast starter, aerated well, and fermented at the proper temperature, but forgot to clean the pickup tube on the fermentor from the previous batch, it won’t be very good. Lastly, if everything else was done right, low quality ingredients such as old malt or high alkalinity water may be a problem. Let’s examine some common fermentation problems and their possible causes.

Common Problems with Fermentation

Problem: I added the yeast two days ago and nothing is happening.

Cause 1: Too cold. Lack of fermentation could be due to several things. The fermentation conditions may be too cold for otherwise healthy yeast to be active. Ale yeast strains vary in their temperature sensitivity, but tend to go dormant below 60°F (16°C). If the yeast was rehydrated in really warm water, for example, 105°F (41°C), and then pitched to a much cooler wort of 65°F (18°C), the large difference in temperature can thermally shock the yeast and cause a longer lag time as they adjust. You can pitch colder yeast and yeast starters into warmer worts, but not the other way around.

Cure: The temperature of the yeast or yeast starter should be within 5°F (3°C) of the wort. If you have already pitched the yeast and are experiencing a long lag time (i.e., the adaptation phase is >48 hours), try warming the fermentor by 5°F (3°C); it may make all the difference.

Cause 2: Poor yeast management. When a batch is not fermenting, the most common problem is with the yeast. If dry yeast has been properly packaged and stored it should be fully viable for up to two years. Check the manufacturing or “best by” date on the package. If the yeast is too old, or has been subjected to poor storage conditions, it may be mostly dead. The viability of liquid yeast can also be gauged by its color; it is creamy white when fresh and darkens to gray or brown as it ages. Old yeast needs to be revitalized before use by growing it to an adequate pitching rate with a starter.

Yeast needs to be treated with care and be given the proper growing conditions. Dry yeast is dehydrated—the cells are parched and in no condition to start work. They need some nice warm water for rehydration, some time to do some stretching, maybe an appetizer, and then they will be ready to tackle a full wort. If the dry yeast is just sprinkled onto the surface of the wort, some of the yeast cells will be up to the challenge, but most won’t. Give your yeast the best chance to make good beer for you.

Cure: Rehydration of yeast in plain water is strongly recommended, because of how osmosis works. Dried yeast cells contain adequate sugar reserves to get them going, so there is no need for sugar in the rehydration process. However, in wort with a high concentration of dissolved sugar, yeast cells cannot draw the water across their cell membranes to hydrate their own nutritional reserves. The water is instead locked up in the wort, hydrating the sugars in solution. Dry pitching does work, but it is not best practice.

Likewise, a liquid yeast culture also needs its breakfast routine. It has been kept in a refrigerator, so needs to be warmed and fed before there will be enough active yeast cells to do the job properly. There are a lot more yeast cells in a dry yeast packet than in a liquid packet. The liquid packet yeast needs to be grown in a starter to produce enough cells to take on the job of a full five-gallon wort. Both liquid and dry yeast cultures will have a lag time from when they are pitched until they start fermenting in earnest. Aeration, the process of dissolving oxygen into the wort, provides the yeast cells with the oxygen they need to faciliate their growth and make enough yeast cells to do the job properly.

Cause 3: Mechanical issues. If the airlock is not bubbling, it may be due to a poor seal between the lid and the bucket, or between the airlock and the lid. Fermentation may be taking place, but the CO₂ is not coming out through the airlock.

Cure: This is not a real problem and it probably won’t affect the batch. Fix the seal or replace the appropriate part.

Problem: I added the yeast yesterday and it bubbled all day, but it’s slowing down or has stopped today.

Cause 1: Poor yeast management. Yeast that is poorly prepared, whether from not being rehydrated, low pitching rate (i.e., no starter used beforehand), or lack of aeration, will often fail to finish the job.

Cure: Pitch new yeast. Don’t be overly concerned about delaying the fermentation; if your cleaning and sanitation were adequate, a day or two spent preparing new yeast will not hurt the beer.

Cause 2: Too cold. Temperature is a major factor for fermentation performance. If the temperature of the room where the fermentor is located cools down, even by only 5°F (3°C) overnight, then yeast activity can be slowed dramatically.

Cure: Always strive to keep the fermentation temperature constant, the yeast will thank you for it.

Cause 3: Too warm. Instead of too cold, the other side of the coin could be that the temperature was too warm, for example, 75°F (24°C), and the yeast got the job done ahead of schedule. This often happens when a lot of yeast is pitched, where the primary fermentation can be complete within 48 hours. This is not necessarily a good thing, as fermentations above 70°F (21°C) tend to produce a lot of esters and phenolics that just don’t taste right. Fermentation happened, but not as well as it might have.

Cure: Always strive to keep the fermentation temperature within the recommended temperature range, the yeast will thank you for it.

Problem: The last batch (did that) but this batch is (doing this).

Cause: It is a different fermentation. Every fermentation is unique, even if you are brewing the same recipe with the same ingredients. If you change a single thing you will have brewed a different beer. Sometimes the difference in a beer is small and sometimes it is profound. If you are brewing identical recipes at the identical temperatures, then a difference in fermentation vigor or length may be due to differences in yeast health, aeration, or other factors. Only if something like odor or taste is drastically different should you worry.

Cure: Be patient. Wait for the fermentation to finish and taste it; the beer may be just fine. Specific off-flavors and their causes are discussed in the next section.

Problem: The airlock is clogged with gunk.

Cause: Vigorous fermentation. Sometimes fermentations are so vigorous that the kräusen is forced into the airlock. Pressure can build up in the fermentor if the airlock gets plugged, and you may end up spraying brown yeast and hop resins on the ceiling.

Cure: The best solution to this problem is to switch to a blowoff hose. Fit a large diameter hose (e.g., 1", or 2.5 cm) into the opening of the bucket or carboy and run it down to a bucket of water.

Problem: White/brown/green stuff is floating/growing/moving.

Cause 1: Normal fermentation. The first time you look inside your fermentor, you will be treated to an amazing sight. There will be whitish yellow-brown foam on top of the wort, containing greenish areas of hops and resins. This is perfectly normal. Even if the kräusen appears slightly slimy, it is probably normal; several yeast strains have a very wet looking kräusen.

Cure: Relax, don’t worry.

Cause 2: Contamination. The beer may be contaminated with bacteria. Bacterial contamination will often generate a gelatinous layer on top of the beer, called a pellicle. Pellicles are generally whitish, or a yellow-brown-ivory color, or at least they should be. (Other colors could be mold.) They can have a powdery or slimy appearance, and can be smooth or textured. Search the Internet for example pictures. Sometimes a contamination will produce ropy gelatinous strands in the beer that resemble rice noodles (but much softer). The beer may smell acidic, phenolic, or buttery, depending on the type of bacteria that is contaminating your beer. Chapter 14, “Brewing Sour Beers,” has more information.

Cure: As a general rule, if you did not intentionally sour the beer with a commercial, or otherwise known, bacterial culture, then you should probably dump it. You could keep it and hope it turns out to be a good sour, but that may take several weeks, months, or a year, and not every sour beer is a good sour beer. Clean and sanitize your equipment thoroughly, and clean and sanitize the room(s) where you prepare and ferment the wort. Old plastic items should generally be replaced with new ones if contamination is a recurring problem.

Cause 3: Mold. The beer may be contaminated with mold. Mold can be blueish, greenish, or black in color. It can also be hairy. Most mold and bacterial contamination in beer is non-hazardous, but there are a couple of types that do pose a health risk. For example, black mold can cause respiratory distress in many people.

Cure: When in doubt, dump it out. Better safe than sorry. Clean and sanitize your equipment thoroughly, and clean and sanitize the room(s) where you prepare and ferment the wort. Sometimes it is easier to replace plastic equipment than to thoroughly clean it. Old plastic items should generally be replaced with new ones if contamination is a recurring problem.

Problem: It smells like vinegar.

Cause: Bacterial contamination. If bacteria have contaminated your beer, chances are the vinegar smell probably is vinegar. Acetobacter (vinegar producing bacteria) and Lactobacillus (lactic acid producing bacteria) are common contaminants in breweries. Sometimes the contaminated beer will smell like cider vinegar, other times like malt vinegar. It will depend on which type of bacteria is living in your wort. Acetobacter may form a pellicle. Acetobacter is an aerobe, so its presence indicates that oxygen is getting to your beer.

Cure: As a general rule, if you did not intentionally sour the beer with a commercial, or otherwise known, bacterial culture, then you should probably dump it. You could keep it and hope it turns out to be a good sour, but that may take several weeks, months, or a year, and not every sour beer is a good sour beer. If you do decide to keep the batch and hope for the best, Brettanomyces can be pitched to clean up the viscosity.

Problem: It smells like extra-butter microwave popcorn.

Cause 1: Diacetyl. Diacetyl is a vicinal diketone that is created by oxidation of fermentation byproducts produced early in fermentation by the yeast. The yeast should clean up the diacetyl toward the end of fermentation, but if the fermentation was weak, hurried, or incomplete, then diacetyl can occur in the beer. These causes are all yeast related, and are often due to cooling temperatures toward the end of fermentation.

Cure: Pitch sufficient amounts of fresh, healthy yeast and conduct a diacetyl rest toward the end of fermentation. Diacetyl is further discussed later on in this chapter, and also in the lager chapter (chapter 11).

Cause 2: Bacterial contamination. Pediococcus is a common souring bacteria that produces a lot of diacetyl early in the fermentation. It can also cause the beer to be cloudy with haze. A pellicle may form.

Cure: As a general rule, if you did not intentionally sour the beer with a commercial, or otherwise known, bacterial culture, then you should probably dump it. You could keep it and hope it turns out to be a good sour, but that may take several weeks, months, or a year, and not every sour beer is a good sour beer. Brettanomyces can be pitched to clean up the diacetyl and viscosity.

Problem: It smells funky, like cloves or a barnyard.

Cause: Wild yeast contamination. There are many different species of yeast in addition to the Saccharomyces species that make up brewer’s yeast. Like Saccharomyces, species of Brettanomyces can readily metabolize wort sugars. The aroma and flavor from a Brettanomyces fermentation is different from that of Saccharomyces, and tends to consist of many phenol compounds. In some beers (e.g., Belgian ales), these phenol compounds can be an integral part of the style and give a wonderful complexity. However, when unintended and uncontrolled, these compounds tend to clash with the base beer that has become contaminated. The spectrum of flavors is quite broad, but the predominant ones are typically described as horse blanket, barnyard, or leather. In some cases, pineapple or tropical fruit esters can be present.

Cure: As a general rule, if you did not intentionally pitch the beer with a commercial, or otherwise known, Brettanomyces culture, then you should probably dump it. A “Brett” beer is not a sour beer, although it can be; instead, it is most often a funky tasting beer. Some people like Brett beers but many do not. You may want to pitch a sour beer bacterial culture to clean up the funk a bit.

Problem: It smells rotten, or like rotten eggs.

Cause 1: Bacterial contamination. Bacterial contamination can also produce sulfury odors. If you are not brewing a lager beer, then this odor is a good sign that you have a bacteria contaminating your beer. Sometimes the odor may be like butyric acid, that is to say, it smells like vomit.

Cause 2: Yeast strain. Many lager yeast strains produce noticeable amounts of hydrogen sulfide (the rotten egg odor) during fermentation. The smell will dissipate after fermentation and will not affect the flavor of the beer.

Cure: A vigorous fermentation should scrub those odors from the beer and the final beer should not taste like it smells during fermentation. Warming the fermentation by a few degrees can invigorate the yeast, producing more CO₂ and help to flush those odors.

Problem: It’s been one or two weeks and it’s still bubbling.

First, check the fermentation temperature; is it in the right range? If so, then take a gravity reading and see if it is between the OG and anticipated FG (a typical FG falls between 1.008 and 1.014). If so, then there is probably nothing wrong—see Cause 1 below. If the specific gravity is less than the anticipated FG, then you may have a bacteria or wild yeast contaminant. Taste the gravity sample for any off-flavors.

Cause 1: Cool temperatures. A beer that has been steadily fermenting (bubbling in the airlock) for a long time (more than a week for ales, more than two weeks for lagers) may not have anything wrong with it. It is often due to the fermentation being a bit too cool, so the yeast is working slower than normal. A slow but normal fermentation should display one bubble in the airlock every few seconds, with an abrupt decline in activity as the beer reaches final gravity.

Cure: This condition is not a problem.

Cause 2: Gusher contamination. Sustained bubbling is often due to “gusher-type” contamination. This type of contamination can occur at any time and is due to the presence of wild yeast or bacteria that eat the normally unfermentable sugars, like dextrins. The result is a beer that keeps bubbling (one or two bubbles per minute) until all of the carbohydrates are fermented, leaving a beer that has no body and very little taste. If it occurs at bottling time, the beer will overcarbonate and will fizz like soda pop, fountaining out of the bottle.

If the beer seems to be bubbling too long, check the gravity with a hydrometer. Use a siphon or turkey baster to withdraw a sample from the fermentor when checking the gravity. If the gravity is still high, above 1.020, then it is probably due to lower than optimum temperature or low yeast pitching rate. If the gravity is below 1.010 and you still see bubbling in the airlock at the rate of a couple per minute, then your beer may be contaminated with wild yeast or bacteria. The beer will not be worth drinking, either due to the total lack of flavor or because unpleasant phenols will be present.

Cure: Improve your sanitation next time.

Problem: The fermentation seems to have stopped but the hydrometer reads high.

Cause 1: Too cold. This situation is commonly referred to as a “stuck fermentation” and it can have a couple of causes. The most common cause is low temperature. As mentioned at the start of this section, a significant drop in temperature can cause the yeast to go dormant and yeast cells will start to settle to the bottom.

Cure: Moving the fermentor to a warmer room and swirling the fermentor to rouse the yeast will often fix the problem. You should see more bubbling in the airlock, and the gravity should be closer to your target after a few days.

Cause 2: Weak or underpitched yeast. The other most common cause is underpitching. Referring back to the previous discussion of yeast preparation above, weak yeast, or low volumes of healthy yeast, will often not be up to the task of fermenting high-gravity wort. This problem is most common with higher gravity beers, where the OG is greater than 1.075.

Cure: Add an actively fermenting yeast starter to the beer. Making yeast starters is described in chapter 7.

Cause 3: Low fermentability. Another common cause is low fermentability of the wort itself, which can be caused by high levels of specialty malts or dextrinous adjuncts. It can also be caused by high mash temperatures; just a few degrees can make a big difference in fermentability.

Cure: Caramel malts and roast malts typically have low fermentability. Estimate your FG by assuming about 50% attenuation from those malts versus 75% attenuation from base and kilned malts. Conducting a forced fermentation (i.e., one dried yeast packet to 1 L wort at warm temperatures) would show you what your maximum attenuation for that wort would be.

Mash temperature can also be the culprit. A mash temperature of 156–160°F (69–71°C) will convert well, but will have a higher proportion of unfermentable dextrin sugars than if the mash had been between 149–155°F (65–68°C). You may want to invest in a higher quality thermometer that has a calibration certificate.

Common Problems After Fermentation

Problem: It won’t carbonate.

Cause: Needs more time. Time, temperature and yeast strain all combine to form a government committee with a charter to determine the range of times when they can expect to be 90% finished with the Carbonation & Residual Attenuation Project. This committee works best without distractions—the meetings should be held in a warm room. If the committee was given enough budget (priming sugar), then it should arrive at a consensus in about two weeks. If the committee don’t get its act together within a month, then it’s time to rattle the cage and shake things up a bit.

Cure: If the temperature is too cool in the room, moving the bottles to a warmer room may do the trick. The yeast may have settled out prematurely, so the bottles need to be swirled to rouse the yeast and get it back into suspension. Beers that have sat in the fermentor for a long time may not have enough viable yeast left to do the job of carbonation. Fresh yeast may need to be added at bottling.

Problem: The bottles are overcarbonated.

Cause 1: Too much sugar. You used too much priming sugar.

Cure: Vent and re-cap all of the bottles. Venting and re-capping may have to be done several times, as it simply lets out gas from the head space, not what is dissolved in the beer. It might be a good idea to uncap and then cover each bottle opening with little squares of aluminum foil, and let them stand for several minutes or hours before re-capping. The important thing is not to use too much sugar the next time.

I recall one story I read on the rec.crafts.brewing forum where a brewer recounted how both he and his partner had each added ¾ cup of priming sugar to the batch, thinking that the other one had not. By venting and re-capping all the remaining bottles after the initial explosions, they thought they had saved the batch. Then a massive storm front swept through and the corresponding drop in barometric pressure caused the rest of the bottles to explode. Be careful!

Cause 2: Bottled too soon. You bottled before fermentation was complete.

Cure: Vent and re-cap all of the bottles. Next time be more aware of the fermentation and make sure it is complete before you bottle. Think about your recipe and make sure you know what its FG should be. Make sure that you pitch enough healthy yeast for a strong fermentation.

Cause 3: Unmixed priming sugar. Sometimes the priming sugar solution doesn’t get mixed very well with the beer and you get uneven carbonation across your bottles.

Cure: You can’t really fix this batch, so be more careful on the next one. Stir or swirl the priming sugar and beer as you rack the next batch.

Cause 4: Wild yeast contamination. A “gusher bug” has gotten into the beer. Gusher bugs (i.e., Brettanomyces and other wild yeast strains) are a real problem, as they will keep on fermenting all the sugars until there is nothing left but fizzy, bitter, alcoholic water. The real danger with overcarbonation is exploding bottles. Bottle grenades can be very dangerous both from flying glass and from glass slivers left in the carpet.

Cure: Refrigerate the bottles and drink them while there is still some flavor left.

Problem: The (finished) beer is hazy or cloudy.

Cause 1: Chill haze. Chill haze is the number one cause of cloudy homebrew. It is partly caused by an insufficient boil (too weak or short), resulting in the haze-active proteins not coagulating.

Cure: The boil should be vigorous and you should see a fair amount of protein flocs floating around in the wort. Chilling the wort quickly down to room temperature (or lower) can help reduce haze by improving the cold break.

Cause 2: Starch. If you made an all-grain beer and had incomplete conversion, or steeped malt for an extract batch that actually should have been mashed, then you probably have residual starches in the beer that will cause cloudiness.

Cure: Check your malts to determine if they need to be steeped or mashed. Verify the mash temperature with a calibrated thermometer and monitor the temperature to make sure that it stays in the right range. Add an infusion of hot water or do a decoction to bring the temperature back up if it is too low. Use an iodine test at the end of the mash to check for residual starch.

Cause 3: Yeast strain. Some yeast strains that have low flocculation, such as German hefeweizen yeast, will cause the beer to be cloudy.

Cure: Give the beer more time to settle, or use a more flocculant yeast strain if you want the beer to clear faster. Low calcium ion levels in the brewing water (<50 ppm) can also lead to flocculation and haze problems. Add calcium sulfate or calcium chloride salts to the brewing water next time.

In all cases, beer cloudiness can be combated by adding fining agents (e.g., isinglass or gelatin) after fermentation. Irish moss can be added towards the end of the boil to enhance clarity. See appendix C for more info.

Common Off-Flavors and Aromas

There are many flavors that contribute to the overall character of a beer. Some of these flavors have been described as malty, fruity, or bitter. However, when it’s time to figure out why a beer tastes bad, we need to be more specific. In this section, we discuss different off-flavors and aromas and what could cause each.

Acetaldehyde
	Acetaldehyde is an intermediate compound in the formation of alcohol that is reduced by the yeast to ethanol toward the end of fermentation. Some yeast strains produce more than others do, but generally the presence of acetaldehyde in beer indicates that the fermentation was stressed or incomplete. The aroma of acetaldehyde can be like green (unripe) apples, cut pumpkin, or the smell of a freshly painted room. The flavor tends to be cidery, or like pumpkin or unripe apple. Residual acetaldehyde can be produced by fermenting a beer too aggressively. This can be caused by overpitching and over-aerating; and also by starting the fermentation too warm and allowing the fermentor to cool as it progresses. A high proportion of simple sugars, such as glucose, fructose, and sucrose, present early in fermentation promote acetaldehyde formation. However, repeatedly feeding the fermentation with new fermentables also promotes acetaldehyde accumulation, so strike a balance. The depletion of the yeast’s own glycogen reserves by leaving a starter on a stir plate for too long (>3 days), storing a yeast starter for more than a few days prior to pitching, or extended fermentations with multiple sugar additions, have all been shown to increase acetaldehyde production and impair its uptake later in fermentation. The yeast must be active and in good health going into the maturation phase of fermentation to effectively clean up the acetaldehyde. To reduce the likelihood of acetaldehyde in your beer: • Do not underpitch or overpitch. • Do not under-aerate or over-aerate. • Do not pitch warm and allow the fermentation temperature to cool during fermentation. • If using simple sugars, add them after the first day (24–36 hours) of fermentation. To clean up acetaldehyde in the fermentor: • Use a diacetyl rest toward the end of fermentation. • Do not rush the fermentation; give it time. • Rouse the yeast to keep it suspended (if necessary). • Use a less flocculant yeast strain (if necessary).

Acetaldehyde

Acetaldehyde is an intermediate compound in the formation of alcohol that is reduced by the yeast to ethanol toward the end of fermentation. Some yeast strains produce more than others do, but generally the presence of acetaldehyde in beer indicates that the fermentation was stressed or incomplete. The aroma of acetaldehyde can be like green (unripe) apples, cut pumpkin, or the smell of a freshly painted room. The flavor tends to be cidery, or like pumpkin or unripe apple.

Residual acetaldehyde can be produced by fermenting a beer too aggressively. This can be caused by overpitching and over-aerating; and also by starting the fermentation too warm and allowing the fermentor to cool as it progresses. A high proportion of simple sugars, such as glucose, fructose, and sucrose, present early in fermentation promote acetaldehyde formation. However, repeatedly feeding the fermentation with new fermentables also promotes acetaldehyde accumulation, so strike a balance. The depletion of the yeast’s own glycogen reserves by leaving a starter on a stir plate for too long (>3 days), storing a yeast starter for more than a few days prior to pitching, or extended fermentations with multiple sugar additions, have all been shown to increase acetaldehyde production and impair its uptake later in fermentation. The yeast must be active and in good health going into the maturation phase of fermentation to effectively clean up the acetaldehyde.

To reduce the likelihood of acetaldehyde in your beer:

• Do not underpitch or overpitch.

• Do not under-aerate or over-aerate.

• Do not pitch warm and allow the fermentation temperature to cool during fermentation.

• If using simple sugars, add them after the first day (24–36 hours) of fermentation.

To clean up acetaldehyde in the fermentor:

• Use a diacetyl rest toward the end of fermentation.

• Do not rush the fermentation; give it time.

• Rouse the yeast to keep it suspended (if necessary).

• Use a less flocculant yeast strain (if necessary).

Alcoholic
	An alcoholic flavor is a sharp, sometimes spicy flavor that can be mild and pleasant or hot and bothersome. When an alcohol taste detracts from a beer’s flavor it can usually be traced to one of two causes. The first problem is often hot fermentation temperatures. At temperatures above 80°F (27°C), the yeast can produce too much of the higher weight fusel alcohols, which have lower taste thresholds than ethanol. These alcohols taste harsh to the tongue, not as bad as cheap tequila, but bad nonetheless. The second cause is generally related to high-gravity worts and a corresponding low pitching rate, which stresses the yeast. Be sure to pitch a healthy, properly fed, and adequately sized yeast starter to properly aerated wort to ensure an ideal fermentation. To reduce the amount of fusel alcohols produced during fermentation: • Do not stress the yeast. • Do not overdo the aeration and yeast nutrient supplements. • Pitch at the intended fermentation temperature; do not pitch warm. • Ferment at the lower end of the recommended temperature range for the yeast. • Do not add large amounts of sucrose or other refined sugars to the wort.

Alcoholic

An alcoholic flavor is a sharp, sometimes spicy flavor that can be mild and pleasant or hot and bothersome. When an alcohol taste detracts from a beer’s flavor it can usually be traced to one of two causes. The first problem is often hot fermentation temperatures. At temperatures above 80°F (27°C), the yeast can produce too much of the higher weight fusel alcohols, which have lower taste thresholds than ethanol. These alcohols taste harsh to the tongue, not as bad as cheap tequila, but bad nonetheless.

The second cause is generally related to high-gravity worts and a corresponding low pitching rate, which stresses the yeast. Be sure to pitch a healthy, properly fed, and adequately sized yeast starter to properly aerated wort to ensure an ideal fermentation.

To reduce the amount of fusel alcohols produced during fermentation:

• Do not stress the yeast.

• Do not overdo the aeration and yeast nutrient supplements.

• Pitch at the intended fermentation temperature; do not pitch warm.

• Ferment at the lower end of the recommended temperature range for the yeast.

• Do not add large amounts of sucrose or other refined sugars to the wort.

Astringency
	Astringency differs from bitterness by having a puckering quality, like sucking on a tea bag. It is dry, somewhat powdery, and is usually due to excess polyphenols (tannins). Astringency is basically haze forming on your tongue; the polyphenols react with proteins in your mouth and coat your tongue. Excess polyphenols can come from over-hopping with low-alpha-acid hops or excessive dry hopping, steeping grains too hot, or oversparging the mash. In other words, high hops, high temperatures, or high pH. Tannin extraction due to high mash pH is a common problem for pale beers brewed with high-alkalinity water. An astringent character in dark beers is more likely due to high temperature than high pH, but pH can still be a cause. Excessively low pH can make dark beers taste acrid, which is not the same as being astringent, but excessive alkalinity can release tannins from dark malts just as easily as from pale malts.

Astringency

Astringency differs from bitterness by having a puckering quality, like sucking on a tea bag. It is dry, somewhat powdery, and is usually due to excess polyphenols (tannins). Astringency is basically haze forming on your tongue; the polyphenols react with proteins in your mouth and coat your tongue. Excess polyphenols can come from over-hopping with low-alpha-acid hops or excessive dry hopping, steeping grains too hot, or oversparging the mash. In other words, high hops, high temperatures, or high pH. Tannin extraction due to high mash pH is a common problem for pale beers brewed with high-alkalinity water. An astringent character in dark beers is more likely due to high temperature than high pH, but pH can still be a cause. Excessively low pH can make dark beers taste acrid, which is not the same as being astringent, but excessive alkalinity can release tannins from dark malts just as easily as from pale malts.

Cidery
	Cidery flavors can have several causes, but is usually due to acetaldehyde, as characterized by a green-apple flavor. Acetaldehyde is a common fermentation byproduct, and different yeasts will produce different levels of it depending on the recipe and temperature. Cidery flavors are often produced by the use of a high proportion of simple sugars, such as glucose and sucrose. Oxidation of acetaldehyde can produce acetic acid, which also contributes to the overall cidery character. See the “Acetaldehyde” subsection above.

Cidery

Cidery flavors can have several causes, but is usually due to acetaldehyde, as characterized by a green-apple flavor. Acetaldehyde is a common fermentation byproduct, and different yeasts will produce different levels of it depending on the recipe and temperature. Cidery flavors are often produced by the use of a high proportion of simple sugars, such as glucose and sucrose. Oxidation of acetaldehyde can produce acetic acid, which also contributes to the overall cidery character. See the “Acetaldehyde” subsection above.

Diacetyl
	Diacetyl is a vicinal diketone that is produced by a purely chemical reaction from precursors that are excreted by the yeast early in fermentation. It is most often described as a butter or butterscotch flavor. Smell a bag of extra-butter microwave popcorn for a good example. Diacetyl is desired to a small degree in many beer styles to round out the flavor, but it can easily become overwhelming. Diacetyl can be the result of the normal fermentation process, or the result of a bacterial infection (e.g., Pediococcus). Ideally, diacetyl precursors are produced early in the fermentation cycle, which are then oxidized into diacetyl during fermentation and subsequently broken down by the yeast toward the end of fermentation. A brew that experiences a long lag time, usually due to weak yeast or insufficient aeration, will produce lots of diacetyl precursors before the main fermentation begins. A diacetyl rest, where the beer temperature is raised by 5°F (3°C) toward the end of fermentation, can invigorate the yeast and help it break down the diacetyl that has formed. A beer that is fermented warm and fast and rushed through the fermentor may leave lots of diacetyl precursors behind in the beer, which will then manifest as diacetyl in packaging. To reduce diacetyl in your beers: • Do not stress the yeast. • Do not rush the fermentation. • Do not overoxygenate your wort, and minimize oxygen exposure after fermentation starts. To clean up diacetyl in the fermentor: • Increase the temperature towards the end of fermentation (i.e., use a diacetyl rest). • Keep the beer on the yeast (i.e., don’t bottle too soon) • Rouse the yeast to keep it suspended (if necessary). • Use a less flocculent yeast strain (if necessary).

Diacetyl

Diacetyl is a vicinal diketone that is produced by a purely chemical reaction from precursors that are excreted by the yeast early in fermentation. It is most often described as a butter or butterscotch flavor. Smell a bag of extra-butter microwave popcorn for a good example. Diacetyl is desired to a small degree in many beer styles to round out the flavor, but it can easily become overwhelming. Diacetyl can be the result of the normal fermentation process, or the result of a bacterial infection (e.g., Pediococcus). Ideally, diacetyl precursors are produced early in the fermentation cycle, which are then oxidized into diacetyl during fermentation and subsequently broken down by the yeast toward the end of fermentation. A brew that experiences a long lag time, usually due to weak yeast or insufficient aeration, will produce lots of diacetyl precursors before the main fermentation begins. A diacetyl rest, where the beer temperature is raised by 5°F (3°C) toward the end of fermentation, can invigorate the yeast and help it break down the diacetyl that has formed. A beer that is fermented warm and fast and rushed through the fermentor may leave lots of diacetyl precursors behind in the beer, which will then manifest as diacetyl in packaging.

To reduce diacetyl in your beers:

• Do not stress the yeast.

• Do not rush the fermentation.

• Do not overoxygenate your wort, and minimize oxygen exposure after fermentation starts.

To clean up diacetyl in the fermentor:

• Increase the temperature towards the end of fermentation (i.e., use a diacetyl rest).

• Keep the beer on the yeast (i.e., don’t bottle too soon)

• Rouse the yeast to keep it suspended (if necessary).

• Use a less flocculent yeast strain (if necessary).

Dimethyl Sulfide / Cooked Vegetable Flavors
	Dimethyl sulfide (DMS) is common in many light lagers and is considered to be part of the character in small amounts, just like diacetyl is in ales. However, DMS is considered an off-flavor in most other styles. In pale beers, DMS can have a creamed corn aroma and flavor, whereas in dark beers the character is more tomatolike. Dimethyl sulfide is produced in the wort during the boil by the chemical reduction of another compound, S-methylmethionine (SMM), which is itself produced during malting. S-methylmethionine is volatile, and more will vaporize the longer the malt is kilned at the end of the malting process. Pale ale malt is kilned longer and hotter than lager malt, which explains why DMS is more prevalent in pale lagers. Dimethyl sulfide is continuously produced in the wort from SMM while it is hot and is usually removed by volatilization during the boil. If the boil is weak (a simmer), the DMS will not be scrubbed from the wort. Longer boils are recommended for beers using Pilsner malt (as opposed to pale ale base malt) to help remove all of the SMM and DMS. Any SMM remaining in the wort can react to produce DMS later in the beer. Although DMS production is promoted by temperatures above 140°F (60°C), it will also be formed at cooler temperatures, so chilling the wort below 140°F after the boil is not the solution—the key is to boil long enough to get rid of the precursor. Dimethyl sulfide is not cleaned up by the yeast during maturation. When caused by bacterial contamination, DMS has a more rancid character, more liked cooked cabbage than creamed corn. Bacterial contamination is usually the result of poor sanitation. Repitching the yeast from an infected batch of beer will perpetuate the problem.

Dimethyl Sulfide / Cooked Vegetable Flavors

Dimethyl sulfide (DMS) is common in many light lagers and is considered to be part of the character in small amounts, just like diacetyl is in ales. However, DMS is considered an off-flavor in most other styles. In pale beers, DMS can have a creamed corn aroma and flavor, whereas in dark beers the character is more tomatolike. Dimethyl sulfide is produced in the wort during the boil by the chemical reduction of another compound, S-methylmethionine (SMM), which is itself produced during malting. S-methylmethionine is volatile, and more will vaporize the longer the malt is kilned at the end of the malting process. Pale ale malt is kilned longer and hotter than lager malt, which explains why DMS is more prevalent in pale lagers.

Dimethyl sulfide is continuously produced in the wort from SMM while it is hot and is usually removed by volatilization during the boil. If the boil is weak (a simmer), the DMS will not be scrubbed from the wort. Longer boils are recommended for beers using Pilsner malt (as opposed to pale ale base malt) to help remove all of the SMM and DMS. Any SMM remaining in the wort can react to produce DMS later in the beer. Although DMS production is promoted by temperatures above 140°F (60°C), it will also be formed at cooler temperatures, so chilling the wort below 140°F after the boil is not the solution—the key is to boil long enough to get rid of the precursor. Dimethyl sulfide is not cleaned up by the yeast during maturation.

When caused by bacterial contamination, DMS has a more rancid character, more liked cooked cabbage than creamed corn. Bacterial contamination is usually the result of poor sanitation. Repitching the yeast from an infected batch of beer will perpetuate the problem.

Estery / Fruity
	Ales are supposed to be slightly fruity, and Belgian and German wheat beers are expected to have a small amount of banana ester character (e.g., isoamyl acetate), but sometimes a beer comes along that could flag down a troop of monkeys. Ethyl acetate, an ester with the aroma of nail polish remover, is also a common problem. Esters are produced by the yeast, and different yeast strains will produce different amounts and types of ester compounds. Most esters in beer are produced from ethanol and only a small percentage are made from fusel alcohol precursors. In general, stressing the yeast produces more esters. Yeast can be stressed by low pitching rates, high fermentation temperatures, low fermentation temperatures, and high-gravity worts; all these conditions will result in more ester formation. Ester formation is complicated, but it involves the molecule acetyl-CoA, which is normally used to convert various nutrients into cellular building blocks, such as fatty acids and sterols. As yeast cells grow and divide, they utilize acetyl-CoA to facilitate metabolism and growth. Ester formation occurs when acetyl-CoA is no longer needed for further cell growth, which happens right after the exponential growth phase. Therefore, ester formation is promoted by high aeration, high levels of nutrients, low pitching rates, and high yeast growth. Ester formation is not promoted by high pitching rates and low aeration. It is also not promoted when there are sufficient sterols and fatty acids for growth supplied in the trub. In other words, a turbid wort supplies nutrients that the yeast would otherwise have to synthesize for themselves using acetyl-CoA; therefore, less acetyl-CoA is needed, and less esters are subsequently formed. Ester formation is inhibited by hydrostatic pressure, which increases the solubility of CO₂ and inhibits yeast metabolism. These kind of pressures are found at the bottom of tall 100-barrel cylindroconical fermentors. This is obviously not a viable method for controlling esters for homebrewers—I mention it because it is one reason that commercial brewers tend to ferment their beers a few degrees warmer than homebrewers. The tall tanks and higher levels of dissolved CO₂ means a warmer fermentation is needed to obtain a similar ester profile to that seen in under normal low pressure conditions. To reduce excessive ester formation in your beer, reduce those factors that promote excessive yeast growth or yeast stress: • Use sufficient aeration. • Ferment cooler but not cold. • Use an appropriate pitching rate. • Do not generate high amounts of FAN in the wort. • Do not separate all the trub from the wort prior to fermentation.

Estery / Fruity

Ales are supposed to be slightly fruity, and Belgian and German wheat beers are expected to have a small amount of banana ester character (e.g., isoamyl acetate), but sometimes a beer comes along that could flag down a troop of monkeys. Ethyl acetate, an ester with the aroma of nail polish remover, is also a common problem. Esters are produced by the yeast, and different yeast strains will produce different amounts and types of ester compounds. Most esters in beer are produced from ethanol and only a small percentage are made from fusel alcohol precursors. In general, stressing the yeast produces more esters. Yeast can be stressed by low pitching rates, high fermentation temperatures, low fermentation temperatures, and high-gravity worts; all these conditions will result in more ester formation.

Ester formation is complicated, but it involves the molecule acetyl-CoA, which is normally used to convert various nutrients into cellular building blocks, such as fatty acids and sterols. As yeast cells grow and divide, they utilize acetyl-CoA to facilitate metabolism and growth. Ester formation occurs when acetyl-CoA is no longer needed for further cell growth, which happens right after the exponential growth phase. Therefore, ester formation is promoted by high aeration, high levels of nutrients, low pitching rates, and high yeast growth. Ester formation is not promoted by high pitching rates and low aeration. It is also not promoted when there are sufficient sterols and fatty acids for growth supplied in the trub. In other words, a turbid wort supplies nutrients that the yeast would otherwise have to synthesize for themselves using acetyl-CoA; therefore, less acetyl-CoA is needed, and less esters are subsequently formed.

Ester formation is inhibited by hydrostatic pressure, which increases the solubility of CO₂ and inhibits yeast metabolism. These kind of pressures are found at the bottom of tall 100-barrel cylindroconical fermentors. This is obviously not a viable method for controlling esters for homebrewers—I mention it because it is one reason that commercial brewers tend to ferment their beers a few degrees warmer than homebrewers. The tall tanks and higher levels of dissolved CO₂ means a warmer fermentation is needed to obtain a similar ester profile to that seen in under normal low pressure conditions.

To reduce excessive ester formation in your beer, reduce those factors that promote excessive yeast growth or yeast stress:

• Use sufficient aeration.

• Ferment cooler but not cold.

• Use an appropriate pitching rate.

• Do not generate high amounts of FAN in the wort.

• Do not separate all the trub from the wort prior to fermentation.

Grassy
	Flavors reminiscent of fresh-cut grass occasionally occur and are most often linked to poorly stored ingredients. Poorly stored malt can pick up moisture and develop musty smells or grassy odors. These odors are often aldehydes, including acetaldehyde. Hops are another source of these “green” flavors. If the hops are poorly stored, or not properly dried prior to storage, polyphenol compounds will become evident in the beer, and these off-flavors can vary from grassy to grainy to astringent.

Grassy

Flavors reminiscent of fresh-cut grass occasionally occur and are most often linked to poorly stored ingredients. Poorly stored malt can pick up moisture and develop musty smells or grassy odors. These odors are often aldehydes, including acetaldehyde. Hops are another source of these “green” flavors. If the hops are poorly stored, or not properly dried prior to storage, polyphenol compounds will become evident in the beer, and these off-flavors can vary from grassy to grainy to astringent.

Husky / Grainy
	Husky and grainy flavors are similar to the grassy flavors described above. Grainy can also include astringency from the grain husks. These flavors are more evident in all-grain beers and are due to poor grain steeping or sparging practices. Follow the same procedures recommended to prevent astringency to correct the problem (see the “Astringency” section above). Highly toasted malts can also contribute grainy flavors. If you are making your own toasted malts, allow them to age at least two weeks after crushing so the harsher aromatic compounds can dissipate. Cold conditioning the beer for a month or two will often help these harsh compounds settle out along with the yeast. Clarifying your beer with gelatin may also help.

Husky / Grainy

Husky and grainy flavors are similar to the grassy flavors described above. Grainy can also include astringency from the grain husks. These flavors are more evident in all-grain beers and are due to poor grain steeping or sparging practices. Follow the same procedures recommended to prevent astringency to correct the problem (see the “Astringency” section above).

Highly toasted malts can also contribute grainy flavors. If you are making your own toasted malts, allow them to age at least two weeks after crushing so the harsher aromatic compounds can dissipate. Cold conditioning the beer for a month or two will often help these harsh compounds settle out along with the yeast. Clarifying your beer with gelatin may also help.

Medicinal
	Chlorophenols result from reactions involving chlorine-based sanitizers (bleach) and phenol compounds. Chlorophenols have very low taste thresholds, and the flavors are often described as medicinal or “Band-Aid like,” or they can be highly phenolic (i.e., spicy like cloves), or smell like hot plastic. The phenols are produced by the yeast, and these then react with chlorine or iodine from leftover sanitizer. Items sanitized with chlorine and iodine-based sanitizers need to drain and dry completely before use to avoid these kinds of off-flavors. Do not exceed recommended concentrations when preparing sanitizing solutions.

Medicinal

Chlorophenols result from reactions involving chlorine-based sanitizers (bleach) and phenol compounds. Chlorophenols have very low taste thresholds, and the flavors are often described as medicinal or “Band-Aid like,” or they can be highly phenolic (i.e., spicy like cloves), or smell like hot plastic. The phenols are produced by the yeast, and these then react with chlorine or iodine from leftover sanitizer. Items sanitized with chlorine and iodine-based sanitizers need to drain and dry completely before use to avoid these kinds of off-flavors. Do not exceed recommended concentrations when preparing sanitizing solutions.

Meaty
	Meaty, brothy, and umami flavors (reminiscent of ham soup) are typically caused by yeast autolysis, in which the yeast cells die and spill their guts into the beer. This is why you will see yeast extract or hydrolyzed yeast as an ingredient in bouillon cubes. Autolysis is caused by starvation of the yeast and is more likely to happen with old yeast packages, poor fermentation conditions, or extended fermentations (i.e., lasting months). Racking the beer away from the yeast cake prior to a long maturation period is the best defense.

Meaty

Meaty, brothy, and umami flavors (reminiscent of ham soup) are typically caused by yeast autolysis, in which the yeast cells die and spill their guts into the beer. This is why you will see yeast extract or hydrolyzed yeast as an ingredient in bouillon cubes. Autolysis is caused by starvation of the yeast and is more likely to happen with old yeast packages, poor fermentation conditions, or extended fermentations (i.e., lasting months). Racking the beer away from the yeast cake prior to a long maturation period is the best defense.

Metallic
	Metallic flavors can be caused by old malt, by adding too much brewing salt for water treatment, or by the presence of iron or manganese from well water. Shiny new aluminum pots will sometimes turn black when boiling water due to chlorine and carbonates in the water, which may cause off-flavors the first time but is not hazardous. Aluminum pots usually won’t cause metallic flavors unless the brewing water is highly alkaline (pH >9), which is rare.

Metallic

Metallic flavors can be caused by old malt, by adding too much brewing salt for water treatment, or by the presence of iron or manganese from well water. Shiny new aluminum pots will sometimes turn black when boiling water due to chlorine and carbonates in the water, which may cause off-flavors the first time but is not hazardous. Aluminum pots usually won’t cause metallic flavors unless the brewing water is highly alkaline (pH >9), which is rare.

Moldy
	Molds are quickly recognized by their smell and taste. Black bread molds and mildew can grow in both wort and beer. Contamination from mold spores is likely if the wort or beer is exposed to musty or damp areas after the boil or early in fermentation.

Oxidized
	Oxidation is the most common problem affecting all beers. Oxygen exposure after fermentation and at packaging is the primary cause. Storage temperature is the second factor. Once carbonated, beer should be stored cold for longest shelf life. The oxidation of fatty acids produces trans-2-nonenal, which has a cardboard taste and the aroma of old paper. See the discussion of oxygen and the wort in chapter 6.

Oxidized

Oxidation is the most common problem affecting all beers. Oxygen exposure after fermentation and at packaging is the primary cause. Storage temperature is the second factor. Once carbonated, beer should be stored cold for longest shelf life. The oxidation of fatty acids produces trans-2-nonenal, which has a cardboard taste and the aroma of old paper. See the discussion of oxygen and the wort in chapter 6.

Soapy
	Soapy flavors can be caused by not rinsing your beer glass very well, but they can also be caused by chemical reactions in the wort. Soapy flavors can result from the breakdown of fatty acids in the trub and by autolysis of the yeast. Soap is, by definition, the salt of a fatty acid; so you are literally tasting soap.

Solventlike
	Solventlike flavors includes alcohol and ester flavors, but are harsher to the tongue. These flavors often result from a combination of high fermentation temperatures and oxidation. The solvents in some plastics, such as rigid PVC, can leach out as a result of high temperatures. Do not use common PVC irrigation tubing for brewing. Make sure your plastics are food-grade!

Skunky
	Skunky or cat-musk aromas in beer are caused by photochemical reactions of isomerized hop compounds. The wavelengths of light that cause the skunky smell are in the blue and ultraviolet range. Brown glass bottles effectively screen out these wavelengths, but green bottles do not. Skunky flavors will result if the beer is left in direct sunlight, or stored under fluorescent lights (as happens in supermarkets). Beers that use pre-isomerized hop extract or very little flavoring hop additions will be fairly immune to damage from ultraviolet light.

Skunky

Skunky or cat-musk aromas in beer are caused by photochemical reactions of isomerized hop compounds. The wavelengths of light that cause the skunky smell are in the blue and ultraviolet range. Brown glass bottles effectively screen out these wavelengths, but green bottles do not. Skunky flavors will result if the beer is left in direct sunlight, or stored under fluorescent lights (as happens in supermarkets). Beers that use pre-isomerized hop extract or very little flavoring hop additions will be fairly immune to damage from ultraviolet light.

Sweaty / Goaty
	I often encounter sweaty or goaty aromas and flavors at restaurants that don’t clean their beer lines regularly. It could be caused by mold, Brettanomyces, or Pediococcus contamination. It is always a sanitation issue.

Yeastlike
	The cause of a yeastlike flavor is pretty easy to understand. If the beer is too young and the yeast has not had time to settle out, it will have a yeastlike taste. Watch your pouring method too, keep the yeast layer on the bottom of the bottle. If the yeast is unhealthy and begins autolyzing, this will release compounds that may be initially described as yeastlike aromas, but may smell soapy or meaty as the beer ages.

Yeastlike

The cause of a yeastlike flavor is pretty easy to understand. If the beer is too young and the yeast has not had time to settle out, it will have a yeastlike taste. Watch your pouring method too, keep the yeast layer on the bottom of the bottle.

If the yeast is unhealthy and begins autolyzing, this will release compounds that may be initially described as yeastlike aromas, but may smell soapy or meaty as the beer ages.