This is a practical guide to troubleshooting beer fermentations with kveik and other Nordic and Baltic farmhouse yeasts. Whether you brew traditional farmhouse ale or modern beer with farmhouse yeast, this guide is for you. I’ll also give tips for fermenting farmhouse ale with baker’s yeast.

The evolution has made Nordic and Baltic farmhouse yeasts very robust and competitive against bacteria. Easy and trouble-free fermentations are one important reason to use them. But fermentation, or any other kind of life, is never totally free of troubles. Especially if you are reusing yeast and keeping farmhouse yeast cultures, knowing the troubles helps you to keep the yeast clean.

Since 1998, I have brewed over 500 batches of beer and have seen many kinds of fermentation problems with brewer’s yeast, farmhouse yeast, and baker’s yeast. Standard brewer’s troubleshooting guides have offered little help when I have fermented with Nordic and Baltic farmhouse yeast outside the box of modern brewing. Instead, I have learned to solve farmhouse fermentation problems by troubleshooting my own beer and talking with traditional brewers. This practical problem-solving manual is part of my Guide to Kveik and Farmhouse Yeast. The other parts are Introduction, Fermentation, Reusing and Maintaining Yeast, and Farmhouse Yeast Descriptions.

Disclaimer: I have brewed my share of bad-tasting and unintentionally sour beer when brewing farmhouse beer and testing dozens of yeasts. I have been frustrated many times but also become a good problem solver. The analysis below is based on my home-scale observations and deductions, backed up by the opinions of beer experts and microbiologists. While I think that the analysis is practical and pretty conclusive, it is far from scientific: I haven’t done or ordered laboratory analyses to identify the bacteria or alien yeast.

Outline

When fermentation goes off-road, identifying and fixing the problem can be frustrating, especially if the problem persists over several batches. I have been there, and I want to make the troubleshooting as easy as I can for you. I’m not going to list all possible fermentation flaws found in the literature. Instead, I’m going to talk about troubles that I have personally identified either from my brews or farmhouse ales on my travels. I’ll concentrate on problems that appear soon after brewing, either during fermentation or when drinking fresh ales. That’s already a pretty long list that probably covers most problems with fresh ales fermented with farmhouse yeast or baker’s yeast.

I have written these instructions with three goals in mind. First, I advise you on how to save the problematic batch or help you decide if it cannot be saved. Second, I guide how to avoid similar problems in the future. Third, I give recommendations to those who reuse and maintain farmhouse yeast cultures. I use only simple home-scale methods. Gravity and pH readings are very helpful but aren’t absolutely necessary. I classify the most typical farmhouse fermentation problems into five categories:

• Slow start
• Slow, stuck, or incomplete fermentation
• Undesirable flavors
• Contamination by alien yeast
• Contamination by lactic acid bacteria 
• Contamination by gluconobacter (a member of the acetic acid bacteria family)

In my experience, these are the most typical fermentation problems with farmhouse yeasts and baker’s yeast. I’m skipping problems with contaminated draft dispensing systems, which can cause somewhat similar symptoms but are problems in cleaning practices. 

Identifying Fermentation Problem

Fixing a fermentation problem begins by identifying the cause. You may not be able to be certain, but you must at least make a guess. 

First of all, a very slow start of fermentation can explain nearly all kinds of fermentation problems. Even if fermentation eventually becomes vigorous, it can become slow or stuck later on, undesirable flavors are more probable, and the wort is more vulnerable to alien yeast or bacteria. The beer isn’t necessarily ruined, but the slow start is always a symptom of weak yeast or inadequate fermentation conditions. In my opinion, a healthy farmhouse fermentation above 25 °C (77 °F) should start within eight hours. 

If fermentation starts well but then slows down unexpectedly, the usual suspect is overly flocculant yeast or weak yeast. Also, unpleasant flavors are likely caused by weak yeast. However, also some souring bacteria can slow fermentation down and create bad flavors. For this reason, I advise you to first rule out souring bacteria, especially if you are reusing yeast.

If souring bacteria has attacked your brew, you probably won’t instantly detect lactic or acetic acid flavor. Instead, early bacterial infections make beer first dryer than you would expect by the gravity readings of fermenting beer. When the acidity continues to grow, you may begin to sense slight tartness, as if someone had poured cider or fruit juice into your brew. If you clearly sense lactic or acetic acid, you either have very sharp senses, or the contamination has progressed far, and the acid concentrations have grown significantly. A pH meter is an extremely valuable tool in analyzing contaminations, but the first sign of bacterial infection may not be extremely low pH. Instead, you may observe that pH is in the range of 4.0–4.2 when you expect it to be above 4.3. This is where experience helps a lot: a record of previous brews helps you to decide what is beyond normal. 

If, in addition to signs of tartness or sourness, fermentation is crawling almost to a halt, your problem might be a souring bug called gluconobacter that can excrete compounds that slow down the usual alcoholic fermentation by yeast. This bacteria can also make the wort very turbid, even more turbid than active yeast. 

With fast and hot farmhouse fermentations, bacterial contaminations can occur fast, even within two days of pitching yeast. Different bacteria behave differently and require different actions. For this reason, it would be best to also identify or guess what bacteria it is. We’ll continue this analysis in the next section, Identifying Souring Bacteria.

If you cannot detect any signs of tartness or sourness, the problem is probably just your yeast, but I advise you to rule out the possibility of an alien yeast. By alien yeast, I mean either wild yeast or traces of yeast that you used earlier (so-called cross-contamination). The most common sign of yeast intruder is phenolic flavors when you expect none. If you notice the nasty taste of plastic or the smell of a burnt electricity circuit, the source is likely wild yeast. However, unhealthy fermentation with phenolic yeast (such as Lithuanian, weizen, or Belgian) can also cause that. Alien yeast can also cause very dry beer, sometimes with gushing when serving.

Also, healthy fermentation can release unpleasant aromas which are nothing to worry about. The typical ones are sulfur (either rotten egg or fired match), diacetyl (butter), and acetaldehyde (green apple). Kveiks and other farmhouse yeasts create less of these aromas, but I wouldn’t be concerned if the smell of early fermentation is slightly disturbing. These aromas become a problem only if they end up in the finished beer. 

I hope that now you have an idea of the likely causes. As you read and analyze further, you’ll find more evidence that either supports your initial assumptions or conflicts with them. Questioning the initial thoughts is part of the game.

Identifying Souring Bacteria

This section is for those who suspect a bacterial problem. I’ll guide you on how to identify the bacteria more precisely and figure out where it came from. 

A wide variety of bacteria create acids. Yet, in my experience, fresh farmhouse ales have only two major branches of troublemakers: lactic acid bacteria (LAB) and gluconobacter in the acetic acid bacteria family. At first, the symptoms of these bacteria may be similar: unexpectedly dry beer, perhaps with a slight acidity and a moderate drop in pH. Then, the paths of these bacteria diverge and require different actions from a brewer. 

If you fermented with baker’s yeast and sense tartness, likely the problem is LAB from the yeast. Baker’s yeast is known to contain small amounts of LAB. In my experience, active yeast is fairly effective in keeping the LAB at bay, but once fermentation slows, the ale becomes more vulnerable to this bacteria. A minor LAB infection isn’t usually aggressive, and in unintentionally sour ales, the lactic acid taste usually grows slowly. If LAB is allowed to ferment further, you begin to sense acidic sharpness similar to sour ales fermented with this bacteria (such as Berliner Weisse or Gose) or sour milk products (yogurt, sour milk, or sour cream). Often, the tartness or sourness is clean without other unpleasant flavors. Some members in the lactic acid bacteria family can create off-putting aromas, such as butter, but in my experience, these bacteria are rare in fresh ales.

Hop bitterness (at least 10 IBUs) is a known defense against LAB. Traditional brewers using Nordic and Baltic farmhouse yeasts almost invariably brew with hops. Sahti brewers using baker’s yeast rarely add enough hops to inhibit bacteria. Instead, they use cold storage to fight the bacteria, and traditional sahti often goes sour if stored warm. If fresh beer has firm bitterness and still shows signs of acidity, I’m inclined to suspect other bacteria, though some members of the lactic acid bacteria family can tolerate hop bitterness.

In my experience, Nordic and Baltic farmhouse yeasts are fairly good at dominating LAB. One of my brewing failures revealed the amazing power of kveik: my aim was to brew an unhopped raw sour ale with Sigmund’s kveik and LAB cultured from live yogurt. I made separate starters from both cultures on the brew day and started fermentation at 38 °C (100 °F). Five hours wasn’t enough for LAB to become vital, and kveik completely overruled the ale. After a day, the ale had fermented completely, and I could hardly notice any signs of sourness. Since then, I have brewed an excellent sour ale with the same kveik and yogurt but with a more carefully prepared LAB starter.

Instead of LAB, I have noticed that gluconobacter is a more likely contaminant when fresh farmhouse ales are fermented very warm with farmhouse yeasts. Gluconobacter is known to infect also modern and industrial beer, but this bacteria is rarely mentioned in today’s brewing guidebooks. It is far more common in farmhouse ales for obvious reasons. Gluconobacter thrives in high-gravity worts and grows best at around 30 °C (86 °F), which isn’t the usual brew today, except in some farmhouse breweries. I have observed gluconobacter only in strong raw ales with gravity of at least 1.075 (18.2 °Plato). It doesn’t seem to thrive at all in low or moderate gravity ales. I’m not sure if the wort boil makes a difference, but I have never noticed this bacteria in a boiled ale. 

Usually, gluconobacter creates only mild acidity, but it can excrete compounds that prevent yeast from fermenting. That’s why this sucker is so problematic. At warm high-gravity worts, it can grow fast and begin to slow the fermentation even when the gravity is halfway. Another sign of early gluconobacter infection is high turbidity: the beer can look even more turbid than your usual fermenting beer. Gluconobacter tolerates hop bitterness. It tolerates alcohol, too, but seems to slow down after 6–7 % ABV. 

Here’s an example of how gluconobacter takes over fermentation: in 2022, I brewed sahti and split the wort into two fermenters with Stalljen and Hovden kveiks that I had dried myself. Both batches started fast at 30 °C (86 °F) despite a very low pitch (3 g for 20 liters). Both batches got contaminated by gluconobacter at slightly different speeds. Luckily, I measured gravity and pH, and I got very good data on a gluconobacter attack. I have listed the measurements in the table below. In my opinion, this is a very representative case of how this bacteria gets hold of strong farmhouse raw ale. First, the pH starts to go lower than expected, but beer doesn’t become extremely acidic. Then, the bacteria begin to slow alcoholic fermentation. I tried to get the gravity down, but both batches got stuck at a gravity of around 1.045 and a pH of around 4.0.

Hours from pitching	Stalljen SG	Hovden SG	Stalljen pH	Hovden pH
0	1.103	1.103	5.20	5.20
32	1.058	1.060	4.48	4.39
55	1.052	1.056	4.27	4.17
81	1.047	1.050	4.15	4.05

Gluconobacter creates gluconic and acetic acids, but the beer infected hardly tastes like vinegar. Instead, the tart flavor reminds me of cider and red berries, such as strawberry, lingonberry, and cranberry. In small quantities, the tartness can be very pleasant. I have tasted these notes faintly in some very traditional Norwegian kornøls with gorgeous complexity. These farmhouse ales aren’t sour beers, but I’m sure gluconobacter played a side note. I wish I could dial in this tartness, but this bacteria is difficult to control. If you aren’t a Norwegian farmhouse brewer, I suggest avoiding this bacteria completely, and I’ll give advice for that in the section Fixing Problems with Gluconobacter below.

Where the Alien Yeast or Bacteria Came From?

You need to ask this question when an alien yeast or bacteria contaminates your beer. Without knowing the contaminant source, the problem is more likely to occur again in future batches. 

The equipment is the usual suspect in modern brewing. Inspecting chillers, fermenters, and hoses first is smart. Narrowing down the contaminant source is much easier when you are confident about the cleanliness of your gear. In my home brewery, I have attributed a few bacteria or wild yeast contaminations to unclean equipment, but the last such case was more than ten years ago. In all such cases, the contamination grew slowly within weeks, and I noticed the problem only after bottling or kegging. 

If you are pretty sure that your equipment is not the contamination source, the next suspects are your yeast and the environment where you are brewing. The purity of yeast culture should always be under suspicion when you are reusing or maintaining yeast. When your yeast is known to contain bacteria (as some farmhouse yeasts do), that sounds like the likely contamination source. However, if you cannot brew clean farmhouse beer with traditional best practices, either the bacteria within yeast has become too powerful, or bacteria from other sources is ruining your beer. 

When I suspect that a yeast stock that I’m keeping might be contaminated, I usually brew a small test batch from malt extract and inspect the beer by taste and measurements (gravity and pH). I can brew a small batch in a more controlled manner and see better how the yeast performs. If yeast passes this fermentation test, it’s a good indication, but not a full proof, that the yeast is clean. Commercial laboratories offer contamination screenings that are also available for homebrewers. I haven’t yet done this hardcore option.

I used to think that contamination from the environment could cause only slow infections that become apparent weeks after brewing. A series of contaminations during the summer of 2024 proved me wrong. I brewed four batches, of which three were sour only in 2–3 days. These batches were strong sahti-like raw farmhouse ales. The first two sour batches were fermented with two different farmhouse yeasts that I had maintained for years. At this point, I suspected a yeast contamination. I cleaned cold-side brewing gear meticulously to rule out the possibility of contaminated equipment. I fermented the next two batches side-by-side with the same commercial single-strain kveik. One went sour, and the other didn’t. Both batches were high-gravity raw ales fermented under the same conditions and with the same yeast. I’m sure the bacteria load was similar on both batches. The difference was that the clean batch got a slightly bigger pitch of yeast and started faster. The yeast overpowered the bacteria in the clean batch. 

The three contaminated batches had similar symptoms: slight cidery acidity, pH around 4.0, high turbidity, and stuck fermentation within two days from brewing. The symptoms indicated a gluconobacter infection. Also, the wort and fermentation conditions favored this bacteria: raw ale with gravity in the range of 1.090–1.100 and fermentation at 25–30 °C (77–86 °F). Hop bitterness didn’t slow the bacteria.

I became convinced that gluconobacter ruined these three beers, but where did it come from? Infections from the equipment or yeast seemed highly unlikely. Two other possible contamination sources are the no-boil process and the environment. The mash for these beers rested above 75 °C (167 °F) for at least 45 minutes, and I can’t believe that gluconobacter could survive this mash. I have brewed more than a hundred clean, no-boil ales, and I firmly believe that high enough mash temperature sanitizes the wort. 

This reasoning led me to believe that the souring bacteria came from the surroundings. I brewed these beers outside on the porch of our house during late summer. Apples were becoming ripe. Gluconobacter can cause rot in apples, and we had some rotten apples in our garden. At first, it was hard to believe that bacteria carried by airflow from the garden could cause so fast infection. On the other hand, the conditions were perfect for gluconobacter: the garden was a buffet for the bacteria, and then they found my sugar-rich wort at the optimal growth temperature. Three dogs running in the garden while brewing didn’t help. I suspect that gluconobacter gets a foothold fast or not at all. Its superpower is kicking yeast in the groin. Based on pH and gravity measurements from the infected batches, the bacteria probably lowered pH and slowed the yeast before obvious symptoms occurred two days after brewing. 

Unfortunately, I cannot blame rotten apples for all gluconobacter contaminations. The ruined sahti described in the section Identifying Souring Bacteria above was brewed inside in April 2022. Springtime was emerging, but the ground was still partly covered with snow. Sahti fermented with home-dried Stalljen and Hovden kveiks (one wort split into two fermentations) went sour and got stuck two days after brewing, with the typical symptoms of gluconobacter. Again, the fermentation temperature of 30 °C (86 °F) was optimal for gluconobacter, but I am still unsure where it came from. Then I suspected that both kveiks were contaminated, but after gaining more experience, I’m inclined to think that the bacteria came from the brewery’s surroundings. Even during colder seasons, our old wooden house is far from sanitary brewery conditions. Horse and dog gear are everywhere, and three big dogs raise dust into the air. Sometimes I wonder how I can brew a single clean beer in this house.

Fixing Slow Start

Even if the slow onset of fermentation produces good beer, you should aim for a faster start next time. It increases the chances for the perfect beer, and with imperfect beer, it makes troubleshooting much easier. Bear in mind, however, that an extremely fast start can lead to problems, too, if yeast begins to ferment immediately without growing first more yeast cells.

Remedies for the future: A slow start is often caused by loss of yeast vitality. This can happen, for example, when yeast slurry is stored for months. Dried farmhouse yeasts are more robust, but they can lose vitality, too, especially if stored warm (for the viability of dried kveik, see excellent Sui Generis Brewing blog post). A small loss of vitality can be compensated with a higher pitch, but if you suspect significant vitality loss, it would be best to grow more yeast with a smaller test batch. A less vital yeast needs a longer time to become active.

Advice for maintaining yeast: I wouldn’t harvest from a slow-start batch if that can be avoided. When home-maintaining farmhouse yeast cultures, sometimes a weak yeast is all we have. Then, I would reculture the yeast with a small amount of low-gravity and nutrient-boosted wort. I would start making full batches only when test batches give vital yeast. 

Fixing Slow Fermentation Without Sourness

Without seeing or tasting your problem brew, the likely cause of slow fermentation is highly flocculant yeast or weak yeast. A drop in fermentation temperature can cause this kind of problem as well.

Fix for this batch: I already discussed these problems in the fermentation part of my farmhouse yeast guide. If the problem is just highly flocculant yeast, stirring or shaking the yeast back to work should be enough. Increasing the fermentation temperature might also help. If the fermentation doesn’t get better in a day, I advise adding new yeast to the fermenter. Get the fermentation going vigorously in a small amount of starter wort before you add the yeast. Giving the new yeast some extra nutrients may help.

Remedies for the future: If stirring/shaking fixes the problem well, be proactive next time with the same yeast: start stirring/shaking early, even before problems occur. If the beer fermented okay but was finished at a higher gravity than you wanted, you can try to aerate more next time. 

Advice for maintaining yeast: If stirring/shaking helps and gives you a good beer, your yeast is probably in good health, but it just flocculates very early. You can either continue maintaining the yeast this way or try to occasionally harvest for slower flocculation: harvest later or harvest occasionally from the bottom instead of the top.

Fixing Undesirable Flavors Without Sourness

I’ll narrow down the off-flavors to the most typical for farmhouse yeasts: solvent, hot alcohol, and overboard fruitiness. The usual cause is weak yeast or a failure to treat your yeast right. 

Fix for this batch: Solvent flavor, alcohol sharpness, and excessive fruitiness are certainly symptoms of imperfect fermentation, but the beer won’t necessarily be lost. Conditioning time will reduce these undesirable flavors. You may get good beer, though you might need to wait longer than intended. When brewing high-gravity beers, small amounts of these undesirable flavors are acceptable when the beer is fresh. Time will probably smooth out and fix the flavor. Excessive solvent or sharp alcohol won’t fade easily, but enough time can still render the beer palatable, though I wouldn’t expect a masterpiece. 

Remedies for the future: I recommend reviewing yeast handling practices: pitch rate, fermentation temperature, nutrients, and aeration (see my fermentation guide). A low pitch rate can cause these kinds of flavor problems. 

Advice for maintaining yeast: I wouldn’t harvest yeast from this batch. The mother yeast culture is probably fine. If you have improved yeast handling practices, and the next batches with the same yeast remain problematic, you have a reason to suspect that the yeast has lost its vitality. You could try to grow more yeast and test. If that doesn’t help, you may have to let go of this yeast.

Fixing Problems With Lactic Acid Bacteria

I see two different paths, one where LAB infections are to be expected and the other where the bacterial attack comes as a surprise. The first case includes mainly baker’s yeast, which is prone to lactic acid bacteria but makes good non-sour beer in experienced hands. Many sahti brewers have gained experience the hard way. I’ll guide you the easier way. The other case includes farmhouse yeasts, where LAB is less common.

When yeast is known to contain traces of LAB, moving the beer cold soon after fermentation and storing the beer cold is a well-known and important antidote against souring. Even when brewers know this, they sometimes fail to cellar the ale fast enough, especially if fermentation ends faster than expected. When yeast finishes, LAB has changes to take over, and the ale can begin to go sour before cellaring. Another typical problem is storing the beer too warm or just letting the beer get warmer during travel. I know sahti brewers who consider 5 °C (41 °F) cold enough to keep sahti from going sour. I have noticed that temperatures above 10 °C (50 °F) allow the sourness of sahti to grow, though very slowly. At room temperature, sahti can go sour in less than a day. 

With a Nordic or Baltic farmhouse yeast, I wouldn’t expect a very fast LAB infection. A slight lactic sourness after weeks of storage is more likely. Cold storage is a precaution for this kind of infection.

Fix for this batch: you cannot make beer unsour, but I wouldn’t pour this kind of batch down the drain right away. Nordic and Baltic aren’t brewing sour beer intentionally. Yet, slight tartness can fit into the flavor profiles of some traditional farmhouse ales. Some versions of sahti, koduõlu, or Norwegian kornøl can be very enjoyable, complex, and also traditional with a slight tartness. If the beer is obviously sour, you cannot call it traditional Nordic or Baltic farmhouse ale. Instead, you might be able to make a proper sour ale following the recipe Sour Frustration Ale below.

Remedies for the future: When you are fermenting with baker’s yeast, knowing the fermentation and cellaring time requires practice. Sahti brewers generally want to avoid extremely fast fermentation, which is difficult to control. Slower fermentation at lower temperatures gives more control. You can also try to inhibit LAB with hop bitterness (at least 10 IBUs).

Advice for maintaining yeast: you cannot reuse yeast from a sour-gone batch. It is difficult to be sure if the bacteria was already in the yeast culture. If you want to maintain the yeast culture used for this batch, I would either do a small brewing test to see if the culture is okay or revert to an earlier culture known for producing good beer.

Fixing Problems with Gluconobacter 

The fix depends on how severe the gluconobacter attack is. Late infections may just give a tart side note that you might actually like. Early attack of this bacteria is difficult because, in addition to mild sourness, the fermentation might get stuck.

Fix for this batch: minor gluconobacter infection can even add positive flavors and complexity. You may be able to drink it as such if the infection is minor: the palate isn’t too heavy, the flavors aren’t unpleasant, and the appearance isn’t highly turbid. When the fermentation becomes stuck or slow at 50 % of apparent attenuation, getting the fermentation going again is hard, even with an active yeast. Blending the infected beer with other healthy, actively fermenting beer is a better way to push the gravity down. I would use at least 20 % of the healthy beer. When you get the fermentation further enough, you might be able to brew good sour beer with the Sour Frustration Ale recipe below.

Remedies for the future: After the major gluconobacter attack in late 2024 and reading more about this critter, I have found an interesting antidote: gluconobacter does not grow above 37 °C (99 °F), and fermenting above this temperature creates an unfavorable environment for this bacteria. So far, I have successfully brewed two sahtis with this method, but the summer of 2025 will be the final test when I’m brewing sahti during the apple season. I have fermented plenty of high-gravity beers with Sigmund’s kveik at 37–40 °C (99–104 °F), and the beers have never been infected. In summer 2024, one of the infected beers was fermented with Sigmund’s kveik at 30 °C (86 °F). I’m pretty sure that temperature made a big difference.

Advice for maintaining yeast: you cannot reuse yeast from an infected batch. If you want to maintain this yeast culture, I recommend either checking the yeast with a small test batch or reverting to the earlier harvest of the culture known for producing good beer. One option is to grow more yeast from the stock culture above 37 °C (99 °F). If it ferments clean, it’s a good indication that the yeast can produce clean beer with it in the future.

Sour Frustration Ale Recipe

I came up with this recipe out of pure frustration when three batches of beer went sour during the Summer of 2024. I had promised to brew strong raw ale for the Historical Beer Festival in Italy, but I kept getting sour beer. In the end, I was able to send one small non-sour batch of sahti.

So, I had three batches (altogether 70 liters) of mildly sour raw ale. The fermentation was stuck at around 6–7 %, with plenty of residual sugars left. The flavor wasn’t bad, but the beer felt heavy and unbalanced. I thought that with the right kind of adjustments, these could become a proper sour beer. I cellared the first ale for later use. For the second batch, I brewed a small amount of raw sour ale with kveik and LAB to blend it with the main sour batch. After a few months of aging, this blend became a reasonably good sour beer with a complex acidity profile. 

When the third batch went sour, I was too tired to brew another batch for blending. Instead, I brewed two liters of beer from DME and kveik and poured that into the infected ale while still fermenting. Then, I poured in a half-liter of fermented cucumber brine (salty and sour liquid with live LAB) and two lemons (I squeezed the juice and threw in halved lemons). It was an impulsive, quick-and-dirty solution. After one month in a bottle, this third batch was passable, and after six months, it was one of my best sour beers. The ale tastes sweet and sour with a perfumed fruit garden and a touch of sea salt. The acidity was mellow but also very complex and balanced. My conclusion is that when the soured ale doesn’t taste nasty, it can be a base for good sour beer. Usually, it benefits from blending, aging, and the addition of acidic fruits or berries. 

This kind of beer is certainly experimental. It involves plenty of monitoring and figuring out what works. You also take a risk of contaminating your equipment. I understand if you choose to pour the beer down the drain. The chances of getting good beer are less than with your average beer. On the other hand, if you get a good result from the experimentation, you’ll have a big smile, and you will learn a lot about beer. 

Note 1: Harmful bacteria cannot grow in beer that has at least 1-2 % AVB and a pH below 4.5 (see Escarpment Labs Blog on low-alcohol ales). So, accidentally soured beer isn’t dangerous to drink as long as yeast produces some alcohol before contamination and the pH has dropped below 4.5. Pathogenic bacteria can grow in beer wort, but that would require contamination before alcoholic fermentation. 

Note 2: Gluconobacter can make the ale highly turbid. High turbidity gives an unpleasant mouthfeel and also feels odd in the stomach. You probably don’t want to drink more than small samples of highly turbid ale. Conditioning (especially at cold) reduces turbidity, but it can take months. The gluconobacter haze can be very persistent. So, highly turbid ale is fairly useless when fresh, but time can cure it if you have the patience.

Recipe: Sour Frustration Ale

Main ingredient:
Unintentionally sour beer

Optional ingredients of your choice:
Other beer (can be sour but need not be) for blending
Acidic fruits or berries, such as lemons

Instructions:
If your unintentionally sour beer fermentation is stuck at a fairly high gravity, add actively fermenting beer to bring the gravity down. Let the blend ferment for as long as it needs to be finished, which may take even weeks. Even when the accidentally soured ale isn’t stuck, consider blending it with other beer to make your beer more balanced and complex. 

Consider also adding acidic fruits or berries. They won’t fix a bad beer, but they can give it richer acidity and flavor. Lemons are an easy choice because they are firmly acidic and aromatic. Other acidic fruits or berries, such as lingonberries, cranberries, or rhubarb (I know, it’s a vegetable) are fine too. I would add mildly acidic fruits or berries only if the base beer is firmly acidic. You can also add brine of fermented cucumbers or other vegetables to add acidity and a touch of salt. Use this option only if you have fermented the vegetables yourself or know exactly what’s in the brine. I recommend adding all these flavorings to the fermenter and letting them infuse for at least a week. 

Consider conditioning the beer in bulk before bottling or kegging. Conditioning at cooler temperatures would be better, but shorter warm conditioning can also improve the beer. Especially if gluconobacter made your ale turbid, I advise you to condition the ale to drop some of the haze before packaging. 

Packaging this kind of ale requires care. If you bottle-condition after weeks or months of conditioning, you might need to add a small dosage of yeast at bottling. If the beer didn’t ferment dry before packaging, you may have refermentation in the package. Monitor the bottles or kegs occasionally if pressure build-up seems possible. 

Acknowledgments

Thanks to Lars Marius Garshol and Xavier Tortosa for the discussions on the farmhouse yeasts and souring bacteria.