Mastering Lager Fermentation: When Lagering Goes Wrong (Pt. 2)

In Part 1, we walked the lager process forwards — from mash to glass, showing how every decision shapes the next one. If you haven't read it yet, it's worth starting there. The map makes the troubleshooting make more sense.

This article is the other half.

If your lager has a specific problem right now, something you can taste but can't quite name, or something you know exactly and just can't fix, this is where you find out where it came from and what you can actually do about it.

(Note: Some problems have fixes and some only have prevention. We'll be honest about which is which.)

Before we get into specific faults, there's one diagnostic question worth asking first, because getting it wrong sends you in completely the wrong direction.

Is this fault biological or process-derived?

Biological faults are produced by yeast metabolism and require living, active yeast to clean up. Diacetyl. Acetaldehyde. H₂S. If the yeast is gone, cold, or dead — that window is closed. You cannot condition your way out of a biological fault once the yeast have left the building.

Process faults come from ingredients, equipment, or technique, not from yeast. DiMethyl Sulfide (DMS) is the clearest example. It comes from the boil and no amount of fermentation management fixes it. Prevention is the only way.

First and foremost, you should know which one you have before you do anything else. Everything below is organised around that distinction.

“An ounce of prevention is worth a pound of cure” ~ Benjamin Franklin

(You should note that I didn't plug our yeast nutrient there, but I could have…)

Troubleshooting Lager Issues

Troubleshooting Issue #1: Sensory Profiles and Causes of Off-Flavours

1. Acetaldehyde: Green Apple, Solvent, Fresh-Cut Pumpkins

Type Of Problem	Cause	Fix
Biological	Either a lack of energy (NADH — made from O2) and/or insufficient zinc enzyme function.	Living, warm yeast with adequate zinc and high NADH levels from proper wort aeration.

Questions to Ask:

• Did you aerate enough? 

• Provide enough zinc?

• Crash before cleanup was complete? 

• Pressure-ferment without nutritional compensation? 

If yeast are still active, then you should warm the beer up and give it time. If they're gone — this is very hard to fix. Prevention is the answer.

2. Diacetyl: Butter or Butterscotch

Type of Problem	Cause	Fix
Biological, same family.	Insufficient FAN, low oxygen, permature crashing, or pressure fermentation without a diacetyl rest.	While yeast are active, you should raise the temperature. For a rapid fix, aim for 18-20ºC for 24-48 hours. If the beer is already cold and the yeast are gone, you have a much harder problem.

3. DMS: Cooked corn or vegetables

Type of Problem	Cause	Fix
Process fault	Improper vapour management or insufficient boiling time or vigor.	None. Focus on prevention for your next batch.

Questions to Ask:

• Did you boil vigorously for at least 90 minutes with good vapour escape? 

• Did you cool the wort quickly?
  

4. H₂S: Rotten Egg

Type of Problem	Cause	Fix
Biological	Low FAN — specifically the Sulfate Reduction Pathway running short of FAN.	If yeast are active and warm, they will reabsorb it. Allow for off-gassing. Step crashing gives yeast more time to scavenge it. If the beer is sealed cold with sulfur still present, it will stay.

Questions to Ask:

• Are we using any adjunct or other low FAN sources in the grist?

• Are we supplementing properly?

• What is the FAN content needed by my yeast? (Check out our Yeast Nutrition Guide for more information)
  
  

5. SO₂: Struck Match

Type of Problem	Cause	Fix
	The other end of the FAN spectrum — too much FAN relative to oxygen, backing up the SRP. Also check hydrosulfonate breakdown if the beer has been warmed or transferred recently.	Lower FAN content (less or shorter step mashes) OR increase adjunct content.

Questions to Ask:

• What is the FAN content needed by my yeast? (Check out our Yeast Nutrition Guide for more information)
  
  

6. Onion, garlic, or cabbage = MeSH or DMDS.

Type of Problem	Cause	Fix
	Oxidation products of methionine and H2S that develop over time. Usually autolysis, post-fermentation oxidation, or yeast left in contact too long past FG.	None. Focus on prevention for your next batch.

Questions to Ask:

• Did you remove your yeast promptly? 

• Is your cold-side oxygen management tight?
  

Troubleshooting Issue #2: Causes and Fixes for Cloudy Lager 

This one is not simple, as it can have multiple causes.

1. Your yeast won’t flocculate out.
   Fix: Check your step crash protocol. Lager yeast FLO gene expression is weaker and more variable than ale yeast. Try more time at intermediate temperatures, and check calcium & magnesium levels.

2. Protein-polyphenol haze: Crashing too fast forms microgels which won’t settle easily. 
   Fix: Check Irish moss usage — both over- and under-dosing can cause problems. Low alpha hops generally add more polyphenol content per IBU, making benefits from step crashing greater.

3. Oxidation-driven: If the beer clears in the tank, but hazes after packaging as oxygen is accelerating protein-polyphenol bonding in the package.
   Fix: None. Focus on prevention for your next batch. 

Troubleshooting Issue #3: Thin Body with Harsh Carbonation

Possible Causes:

a) Thin: over-fining stripping body proteins, high adjunct levels without dextrin compensation, insufficient lagering time

b) Harsh carbonation: coarse CO₂ from insufficient lagering, warm serving temperature, or carbonation added too aggressively.
Fix: Reduce finishing usage, and add more body via more protein. Keep the product cold and let it slowly carb.

Troubleshooting Issue #4: Lager Tastes Great While Fresh, But Stale Within Weeks 

Cause: Oxidation. The beer was clean to start, packaging damaged it.
Fix: 

• Check dissolved oxygen at packaging. 

• Check for temperature abuse in the cold chain releasing hydroxysulfonates. 

• Consider whether your recipe produces enough reductones to buffer against oxidation. Decoction and melanoidin-rich malts help here.

Bonus: Ten Principles of Great Lagers

The whole series, reduced and simmered to what actually matters.

1. Yeast is the fourth ingredient. Treat them accordingly.
   
   

2. Nutrition is not optional. FAN, oxygen, zinc, magnesium, phospholipids — these underpin everything downstream. A depleted fermentation cannot be rescued by lagering time alone.
   
   

3. There are no solutions, only trade-offs. Every technique solves one problem and creates another. Know what you're paying before you commit.
   
   

4. Biological before physical. Let yeast finish their cleanup before you crash. Cold kills the cleanup, not the problem.
   
   

5. Step, don't shock. Gradual temperature changes give yeast and haze particles the time they need to behave correctly.
   
   

6. Remove your yeast at final gravity. Every day of contact after FG is a degradation risk. Not potential risk. Actual.
   
   

7. The cleanest wort wins. Boil intensity, aeration, and cooling are the foundation. Fix these before you optimize anything else.
   
   

8. Sub-recognition off-flavours hurt you even when no one can name them. Your drinker's brain treats ambiguous sensory signals as threats. Clean means intentional, not neutral.
   
   

9. Match your method to your technology. S.D.G. principles in a 60-foot CCV without accounting for hydrostatic pressure will cause you problems.
   
   

10. Taste your beer. Don't just time it. The beer tells you when it's done. Perform daily tasting, forced haze testing, and turbidity tracking. The calendar is not a quality tool.

Want More Advanced Brewing Guides? Tell Us! 

We have lots of things we can teach you, the brewer, about. Let us know in the comments if you want to see any of the following.  We love to make resources that meet you where you are!

• Sauergut: An old acidification technique making a quiet comeback in craft. More to it than most people realise!

• Fermentation Geometry: Tank shape is not neutral. A horizontal lagering vessel and a 60-foot CCV are not doing the same thing at the same temperature. We touched on hydrostatic pressure. This goes deeper.

• Krausening: What is it actually doing biochemically? Why does reintroducing active wort help? This one deserves its own video.

• Colloidal Haze Stability: Why does some haze reform? What makes a permanent haze permanent? Different problems from sedimentation, but equally fascinating.

• Building a QC System: How to stop being blind to your own beer. Palate training, panel structure, and what to actually measure and when.

• Impact of Water on Lagers: One vital area we haven't covered yet, but it absolutely deserves its own deep dive.

Have a topic in mind that’s not on the list? Leave a comment! We pay attention.

 

Looking to Continue on Some Lagering Deep Dives?

The full Advanced Lager Techniques series is on our YouTube channel. Every topic covered in this post has a dedicated video going considerably deeper. Check out the video on this exact subject as part of that series below!

 If you prefer a quick read or skim, you can check out our Knowledge Base — home of over 80 quick, to-the-point entries, including Best Practices for Lager Fermentations, Troubleshooting Haze (Unexpectedly Hazy Beer), and more.