The Homebrewer's First Aid Kit: Solving 9 Common Brew Day Problems

Welcome to your go-to guide for troubleshooting common beer kit + homebrewing problems

Every brewer, novice or seasoned, has stared down a silent airlock, a weird smell, or a beer that tastes off. Before you even think about dumping that batch, breathe. Most problems can be fixed, or at least prevented next time.

 

This page works like a first-aid kit. Each card runs in a straight line: the problem, quick triage, the science, how to diagnose, how to fix, prevention. 

 

Jump to a problem Airlock is not bubbling Fermentation is stuck Beer tastes sour Is my beer infected Boil-over Decoding off-flavors Finished beer is too sweet Missed target gravity Bottled beer is flat

Prep beats panic. Treat water, check seals, verify temperature, then pitch healthy yeast.

Fermentation worries

My airlock is not bubbling

Fermentation

The problem

Silence in the airlock after 24 to 48 hours. No bubbles, rising doubt.

Quick triage

• Check the lid and grommet seal. Press the lid, watch airlock fluid move.
• Shine a light for krausen. If you see foam, fermentation is on.
• Take gravity. If OG has dropped, you are fermenting, period.

The science

Yeast eats sugars and produces ethanol and CO₂. Gas escapes through the easiest path. A leak lets CO₂ bypass the airlock, so bubbles are not a reliable indicator.

Diagnose

• Seal test by pressing the lid. Movement equals pressure.
• Krausen check with a flashlight.
• Hydrometer or refractometer reading versus OG.

Fix

1. Reseat lid and airlock. Use keg lube on O-rings if needed.
2. Correct temperature, 18 to 22°C, 64 to 72°F for typical ales.
3. If no krausen and no gravity drop after 72 hours, re-pitch fresh yeast.

Prevention

Verify airtight seals before pitch. For liquid or harvested yeast, make a starter to confirm vitality.

My fermentation is stuck

Fermentation

The problem

Fermentation started well, then stalled early. Gravity is stable above the expected FG.

Quick triage

• Warm it gently by 1 to 2°C, 2 to 3°F.
• Gently swirl to rouse yeast without oxygen.
• Recheck gravity over 48 hours.

The science

Yeast can flocculate early, stall from cold shock, or lack nutrients and FAN. High-gravity wort increases stress.

Diagnose

Identical gravity readings across 2 to 3 days, still above expected FG.

Fix

1. Rouse yeast by swirling the fermenter.
2. Increase temperature gradually, for example 19 to 21°C, 66 to 70°F.
3. Pitch an active starter of a robust strain, such as US-05. Consider nutrient addition for low-FAN worts.

Prevention

Pitch enough healthy cells, oxygenate wort pre-pitch, control temperature. Use nutrients for adjunct-heavy grists.

Taste, smell, and appearance problems

My homebrew tastes sour

Flavor

The problem

Sharp acidity like vinegar or yogurt that was not intended by the recipe.

Quick triage

• Taste again after chilling. Cold reduces perceived acidity.
• Check for visible film or pellicle.
• If mildly tart but clean, you may keep it cold and drink soon.

The science

Bacteria or wild yeast consume sugars and produce lactic or acetic acid. Great in a deliberate sour, not in an IPA.

Diagnose

Taste is primary. Visual signs may include a film or pellicle on the surface, ropey texture, or a persistent haze.

Fix

True infections are not reversible. If mildly tart and clean, you may drink it. Safety note. Do not bottle an infected beer that is still sweet. Bottle bombs are dangerous.

Prevention

Absolute sanitation post-boil. Replace scratched plastic. Control oxygen exposure.

Is my beer infected, visual signs

Flavor

The problem

Unexpected growth on the surface; textures or colors that do not look like normal krausen.

What you might see

• Pellicle. Chalky, wrinkled, bubble-like film. Often Brett or bacteria.
• Mould. Fuzzy, colored spots. Always bad.
• Ropey strands. Thick, viscous, often Pediococcus.

Fix

• Pellicle or ropey texture. Consider it infected. Decide to drink if clean enough, otherwise dump.
• Mould. Dump the batch. Health risk is not worth it.

Prevention

Meticulous sanitation and minimal headspace. Oxygen on warm beer encourages growth of the wrong microbes.

Clarity follows process. Notes and measurements turn chaos into control.

Brew day and process problems

The dreaded boil-over

Process

The problem

Foam surges over the kettle at the boil threshold and makes a hot, sticky mess.

Quick triage

• Lower heat immediately.
• Stir to collapse foam.
• Mist the surface with cold water from a clean spray bottle.

The science

Hot break proteins trap steam and CO₂. The matrix swells and lifts rapidly, which is why the surge happens fast.

Diagnose

Watch the surface as it nears a boil. Tight, glossy foam rising quickly needs action.

Fix

1. Reduce heat at once.
2. Stir to break the foam.
3. Use a brief water mist if needed.

Safety. Hot wort is slippery and scalding. Turn off burners before cleanup. Avoid stepping on wet floors.

Prevention

• Use a kettle at least twice your batch size.
• Consider Fermcap-S in the kettle as directed.
• Watch closely at the boil approach.

Decoding off-flavors

Flavor

The problem

Beer looks fine, tastes wrong. Butter, cardboard, cabbage, or other out-of-place notes dominate.

Diagnose

Use the quick reference to isolate the flavor, then map it to cause and fix.

Quick reference

Off-flavors quick reference

Flavor or aroma	Tastes or smells like	Likely cause and solution
Acetaldehyde	Green apple, latex paint	Cause Yeast pulled off too early. Fix Let it finish, then give a warm conditioning phase.
Diacetyl	Movie popcorn, butterscotch	Cause Stressed yeast. Fix Maintain temperature, then raise 2 to 3°C, 3 to 5°F for two days.
DMS	Cooked corn, canned cabbage	Cause Incomplete boil off of precursors. Fix Vigorous rolling boil 60 minutes or more with the lid partly off. Chill fast.
Oxidation	Wet cardboard, stale paper	Cause Oxygen on finished beer. Fix Not reversible. Prevent with gentle transfers and CO₂ purges.
Medicinal or phenolic	Clove, bandage, smoky	Cause Wild yeast, sanitizer carryover, or yeast choice. Fix Sanitize properly, rinse as directed, and choose style-appropriate yeast.

Perception matters. Dry hopping increases aroma which can make beer taste less sweet, however it does not add IBUs. Fix root causes first.

Prevention

Pitch healthy yeast, control fermentation temperature, boil vigorously, avoid splashing finished beer.

My finished beer is too sweet

Flavor

The problem

Beer tastes cloying and heavy, like unfermented wort or soda, with little snap on the finish.

Quick triage

• Confirm FG against target. Recheck after 48 hours.
• Chill a sample; colder beer can read cleaner to assess baseline.
• If still in primary and high, see stuck fermentation steps.

The science

Residual sweetness equals unfermented sugars. Causes include stalled yeast or a mash that produced many unfermentable dextrins due to high temperature.

Diagnose

Check FG. A mash above 69°C, 156°F, increases dextrin formation and body.

Fix

If high mash temperature is the cause, you cannot remove dextrins. You can improve balance with hop aroma, or blend with a drier beer. As a last resort consider a small hop tea addition for perceived bitterness.

Prevention

Control mash temperature. For a drier profile mash at 64 to 66°C, 147 to 151°F. For fuller body mash at 68 to 69°C, 154 to 156°F.

I missed my target gravity

Process

The problem

OG reads significantly above or below the recipe target after mash or post-chill.

Quick triage

• Verify thermometer accuracy and volume marks.
• Stir thoroughly before sampling to avoid stratification.
• Use temperature-corrected readings.

The science

Gravity tracks dissolved sugars. Misses come from extraction efficiency or incorrect volumes.

Diagnose

Measure pre-boil gravity and post-chill OG. Compare to recipe targets.

Fix if gravity is low

• Pre-boil. Extend boil to concentrate.
• Post-boil. Add Dry Malt Extract. Rule of thumb. 100 g DME raises a 20 L batch about 0.002 points. Example for 23 L. About 115 to 120 g moves OG by ~0.002.

Fix if gravity is high

• Dilute with clean, sanitized water to target OG. Simple ratio. New volume equals current gravity points divided by target points times current volume.

Prevention

Ensure a proper crush, monitor mash temperature and pH, measure volumes carefully.

My bottled beer is flat

Packaging

The problem

Bottles open with no hiss, head collapses instantly, mouthfeel is thin.

Quick triage

• Condition warm at about 21°C, 70°F, for three weeks.
• Move warmer for one week if cold stored by mistake.
• Gently invert once to rouse yeast. Do not shake.

The science

Bottle conditioning is a mini-fermentation. Priming sugar plus residual yeast equals CO₂ that dissolves under pressure.

Diagnose

No hiss, no head, thin mouthfeel.

Fix

1. Wait longer at the right temperature.
2. Verify caps are sealing correctly.

Prevention

Confirm fermentation is complete before bottling. Use a priming sugar calculator. Aim for the right CO₂ volumes by style.

Hydrometer Thermometer Refractometer Star San Fermcap-S Keg lube

Conclusion

Brewing is a craft of small variables. Notes, measurements, and calm decisions turn problems into progress. Keep this guide handy. Fix what you can, learn from what you cannot, and brew again.

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