Every homebrewer has made most of these. The brewers who get good aren't the ones who never make mistakes — they're the ones who learn to recognise the symptoms, understand the causes, and recover when something has gone wrong. This article is the troubleshooting guide we wish we'd had on batch one.
Each mistake below follows the same structure: the symptoms you'll see, the underlying cause, how to prevent it next time, and what (if anything) you can do to save the current batch. Most brewing problems are recoverable. The ones that aren't are still useful information for the next brew.
For broader context on starting brewing, see our homebrewing for beginners guide. For the actual brew-day walkthrough, see how to brew from a fresh wort kit.
The ten most common mistakes
Inadequate sanitation
The single most common cause of failed homebrew. Wild yeasts and bacteria are everywhere — on your skin, in the air, on every surface in your kitchen, in the air drying on freshly washed equipment. If they get into the wort before your brewing yeast establishes dominance, they'll happily eat the same sugars and produce off-flavours that no amount of conditioning will remove.
Symptoms
Sour or vinegary flavour. A thin grey or white film forming late in fermentation (a healthy white foam — krausen — in the first few days is normal; persistent or oily-looking films later are not). Wet cardboard taste. Beer that smells "off" in a way clean fermentation never does. Worst case: a pellicle, which is a wrinkled, leathery-looking layer on the surface caused by certain wild yeasts.
Cause
Something that touched the wort or yeast after the kit was opened wasn't properly sanitised. This usually isn't your hands or a major surface — it's a small thing. The inside of the fermenter lid where you forgot to swirl sanitiser. The hydrometer stem that didn't quite get dunked. The scissors used to open the yeast packet. The cloth used to wipe the kit pouch.
Prevention
Treat sanitation as the entire game, not a step in it. Mix up your sanitiser solution before you do anything else on brew day. Every surface, every implement, every part of every implement that will touch your wort or yeast must spend at least 30 seconds in contact with the sanitiser. Keep your bucket of solution next to you all day. If you drop anything, dunk it before it touches the wort again.
The fix when it's already happened
Often there isn't one. But before you tip the batch, taste a small sample — many beers that look "infected" actually taste fine and finish out as drinkable beer. Some contamination produces flavours that mellow with conditioning. Give it the full 14 days of fermentation, then 2 weeks of bottle conditioning, and re-evaluate then. If the finished beer is sour, your batch became a Berliner Weisse against your will — you can drink it or try again.
Letting fermentation temperature run hot
The Australian-specific mistake. Most of the country sits above ideal fermentation temperatures for most of the year, and even mild summer evenings can push a fermenter into the danger zone.
Symptoms
Harsh, solvent-like flavour (often described as "fusel" or "hot"). A boozy nose-burn even at moderate ABV. Banana, bubblegum, or clove-like notes (these are esters, produced when yeast ferments warm; they're style-appropriate in some Belgian and German beers but unwelcome in an Australian Pale Ale). In severe cases, an acetone or nail-polish character that no amount of conditioning will remove.
Cause
Wort temperature above 22°C during the most active phase of fermentation (days 1–4). Note that the wort temperature is usually 2–4°C higher than the surrounding air during active fermentation — the yeast itself generates heat — so an "ambient 22°C" room can produce 26°C wort.
Prevention
Put the fermenter somewhere that holds 18–20°C reliably for the full two weeks. In an Australian winter, an interior cupboard or a tile-floored bathroom often works. In summer, you'll need active cooling: a damp t-shirt over the fermenter sitting in a tub of water (3–5°C of evaporative cooling), or ideally a second-hand bar fridge with an aftermarket temperature controller. We cover this in detail in our equipment essentials guide.
The fix when it's already happened
If the beer is still actively fermenting and you've caught the temperature problem in time, gently lower the surrounding temperature by 1–2°C at a time (don't shock the yeast). If fermentation has already finished, additional conditioning time can mellow some off-flavours, but harsh fusel notes are usually permanent. Bottle the beer anyway — sometimes they fade with a few months in the bottle, and you've learned the lesson for next batch.
Under-pitching yeast
Less obvious than the temperature and sanitation mistakes, but produces real problems. The amount of yeast you add (the "pitch") needs to match the volume and gravity of the wort. Pitching too little means the yeast cells you do have are under stress, multiplying rapidly to compensate, and producing more off-flavour by-products in the process.
Symptoms
Slow start to fermentation (airlock not bubbling within 36 hours). Stalled fermentation (gravity not reaching expected final value). Estery or solvent flavours similar to the hot-fermentation problem above. Sulphur or rotten-egg aromas from stressed yeast.
Cause
Most commonly: using a single packet of dry yeast for a batch that needed two, or using old yeast where most cells are no longer viable. Many homebrew kits supply a single 11g sachet which is fine for an average-gravity ale at 23 litres, but inadequate for higher-gravity beers (above 1.060 OG) or larger batches. Liquid yeast packs lose viability over time — a 6-month-old smack-pack has roughly half the cells of a fresh one.
Prevention
Use fresh yeast (check the date on the packet). For standard ales at 23 litres, one 11g packet of dry yeast is correct. For higher-gravity beers or larger batches, pitch two packets or rehydrate the yeast first to maximise viable cell count. If you're using liquid yeast, build a starter the day before brew day to multiply the cell count.
The fix when it's already happened
If fermentation is slow but not stalled, give it time and keep the temperature stable — the yeast will catch up. If fermentation is genuinely stalled (gravity unchanged for 5+ days, well above expected final gravity), pitch a fresh packet of dry yeast directly into the fermenter, swirl gently to disperse, and seal. Most stalled fermentations restart within 24 hours.
Pitching yeast onto wort that's too hot
Yeast is alive. Above about 35°C, individual yeast cells start dying. Above 40°C, you kill most of them. Pitching yeast onto wort that hasn't cooled properly can kill the pitch before fermentation begins.
Symptoms
No fermentation activity. Airlock not bubbling 48 hours after pitching. Wort still smells like sweet wort, not yeast.
Cause
Wort wasn't cooled to fermentation temperature before yeast was added. Common in extract or all-grain brewing where the wort is hot off the boil and needs to be chilled. Less common with fresh wort kits, which arrive at refrigerated or room temperature.
Prevention
Check wort temperature with your stick-on thermometer before pitching. Target the range printed on the yeast packet (usually 18–22°C for ales). If the wort is hotter, wait. A wort chiller speeds this up considerably for extract and all-grain brewers; an ice bath in the sink works for smaller batches.
The fix when it's already happened
Pitch a fresh packet of yeast once the wort has cooled to the correct temperature. The dead yeast cells you killed first time around won't cause problems; they'll just settle out as part of the trub.
Bottling before fermentation is finished
The eager-beginner mistake. The airlock stops bubbling around day 7 or 8 and you assume fermentation is done. It isn't. The yeast is still working, and bottling early traps live sugars and active yeast in a sealed bottle, producing dangerously high pressure.
Symptoms
Over-carbonation. Gushers (beer that foams out of the bottle the moment you open it). In extreme cases, exploded bottles — literal glass shrapnel — either spontaneously while the beer is conditioning, or when opened.
Cause
Bottling at a higher gravity than the beer's true final gravity. Without a hydrometer reading, you're guessing — and visible activity (airlock bubbles) is an unreliable indicator. Fermentation can appear stopped while still producing several gravity points of additional drop.
Prevention
Always confirm fermentation is complete with two hydrometer readings 48 hours apart. If the numbers match, fermentation is finished and bottling is safe. If they don't match, leave it longer and check again. For most ales, this means bottling somewhere between day 12 and day 16.
The fix when it's already happened
If you've bottled and you suspect early bottling, move the bottles somewhere cold (ideally a fridge) to slow the residual fermentation. Open one bottle every few days into a glass to release pressure and check for over-carbonation. If you're worried about explosions, transfer the bottles to a sturdy plastic crate with a lid — if one bursts, the rest stay contained. Don't store potential bottle bombs near where people walk barefoot.
Bottle bombs are genuinely dangerous — the pressure inside a glass beer bottle can exceed 100 PSI before the bottle gives way, and the resulting glass shards have caused serious injuries. If you have any doubt about whether your beer was fully fermented before bottling, store the bottles somewhere contained until you're sure they're safe.
Over-priming when bottling
The less-talked-about sibling to bottling early. Even if fermentation was complete, adding too much priming sugar produces over-carbonated beer and risks the same bottle-bomb consequences.
Symptoms
Gushers when opened. Excessive foam. Beer that's painfully fizzy in the mouth. The same explosion risk as bottling early.
Cause
Adding more priming sugar than the batch volume requires. The rule of thumb is roughly 130–150g of dextrose for a 20-litre batch — but the right amount depends on the beer style (a hefeweizen wants more carbonation than a stout) and the temperature of the beer at packaging (warmer beer has less dissolved CO2 and needs more priming sugar).
Prevention
Use an online priming sugar calculator. Inputs are: batch volume, beer temperature at packaging, and target CO2 volumes for your style. Don't just dump a standard amount in; calculate it for your specific batch. For first-time brewers, 130g of dextrose for a 20L batch is a safe default that produces modestly-carbonated beer suitable for most styles.
The fix when it's already happened
Same as mistake #5: refrigerate to slow the residual fermentation, open bottles cautiously, transfer to a contained storage spot if you're worried about bursts. Severely over-carbonated beer can sometimes be salvaged by carefully un-capping each bottle, pouring it into a glass to let it foam down, then re-capping with fresh caps and a calculated lower priming sugar dose — though most brewers just drink them as-is and learn for the next batch.
Not taking gravity readings
The hydrometer is the cheapest piece of essential brewing equipment and the one most often skipped. Without it, you're guessing about everything that matters: when fermentation is done, what your ABV is, whether anything's gone wrong.
Symptoms
The brewer can't tell whether the fermentation is finished. The brewer can't tell what the ABV is. The brewer can't diagnose problems — "stalled fermentation" is invisible without numbers. The brewer can't compare one batch to the next or learn from previous brews.
Cause
Either not buying a hydrometer (~$15) or buying one and not using it. Some beginners skip it because they think it adds complexity; in fact it removes complexity by giving you actual data instead of guesses.
Prevention
Spend the $15. Take three readings per batch: original gravity (OG) just before pitching yeast, then two readings 48 hours apart late in fermentation to confirm completion. Write all three down in a notebook or spreadsheet. Over time, you'll build a record of what worked and what didn't.
The fix when it's already happened
Buy a hydrometer for your next batch. For the current batch, you can still bottle — just be extra cautious. Wait the full 14 days at fermentation temperature, then let the fermenter sit somewhere cool for another 3–4 days before bottling. The extra time gives any lingering fermentation more chance to complete, reducing the bottle-bomb risk.
Opening the fermenter to check on it
You'll be tempted. The fermenter is sealed, you can't see what's happening, and the urge to lift the lid for a peek is universal. Resist it. Every time the seal is broken, oxygen enters the fermenter and creates an opportunity for contamination.
Symptoms
Often nothing visible at first — one peek probably won't ruin a batch. Repeated lid-lifting over the two weeks of fermentation increases your risk of: oxidation (cardboard or paper flavours), wild yeast contamination, off-flavours from airborne bacteria settling on the wort.
Cause
Curiosity. Anxiety about whether fermentation is going well. The (understandable) desire to see the krausen that everyone talks about. Worry that the airlock has stopped bubbling.
Prevention
Understand that everything you need to know is visible through the airlock and the stick-on thermometer. If the airlock is bubbling, fermentation is active. If the temperature is in range, the beer is fine. The only legitimate reason to open the fermenter during fermentation is to dry-hop or to take a gravity sample — both should be done with sanitised tools and minimal exposure time.
The fix when it's already happened
If you've opened the lid once or twice briefly, you're probably fine — finish fermentation, bottle as normal, taste the result. If you've been opening the fermenter daily, the beer may have picked up oxidative or contamination off-flavours. Bottle it anyway and see what you get; the worst case is a learning experience for next time.
Oxygen exposure after fermentation
Before fermentation begins, oxygen is your friend — the yeast needs it to build healthy cell walls. After fermentation is finished, oxygen becomes the enemy. It produces stale, papery, cardboard-like flavours that get worse over time.
Symptoms
Beer that tastes fresh and clean when you first crack a bottle, but progressively staler over the weeks that follow. A "wet cardboard" or "sherry-like" character that develops as the beer sits. Loss of hop aroma in hop-forward styles like Pale Ales and IPAs (oxidation breaks down the delicate hop compounds first).
Cause
Splashing or aerating the beer during transfer to the bottling vessel or keg. Headspace at the top of bottles that's pure air rather than CO2. Letting the beer sit in an open container during bottling. Repeatedly opening the bottling tap without re-closing it.
Prevention
Treat finished beer gently. When transferring from fermenter to bottling bucket or keg, use a siphon with the output tube placed at the bottom of the receiving vessel so the beer flows in below the surface (not splashed onto the surface). Fill bottles using a bottling wand — the wand fills from the bottom up, displacing air without splashing. Don't transfer back and forth more than necessary. If kegging, purge the keg with CO2 before filling.
The fix when it's already happened
You can't reverse oxidation once it's set in. The damage compounds over time, so the practical advice is: drink the beer quickly before the oxidation gets worse. Cold storage slows the process. Hop-forward styles age the worst — if you've oxidised an IPA, drink it within a month. Maltier styles like stouts and porters tolerate oxidation better.
Storing finished beer in the wrong conditions
You've made the beer. Now where does it live? Temperature and light during storage affect quality just as much as during fermentation. Beer that ages well in cool darkness becomes undrinkable in a warm, sunlit garage.
Symptoms
"Skunky" or sulphurous flavour, especially in hoppy beers (this is the same compound produced when beer is exposed to UV light — it's the reason brown bottles exist). Stale, oxidised flavours developing prematurely. Carbonation issues — beer that's gushy because it warmed up too much, or flat because it never quite carbonated properly in cold conditions.
Cause
Storing finished beer in a warm room (above 22°C), in direct or indirect sunlight, or in clear or green bottles exposed to light. Storing it too cold during the bottle-conditioning phase (below 15°C, the yeast doesn't carbonate properly). Storing it in a fridge that gets opened constantly, causing temperature swings.
Prevention
Two-phase storage. Phase 1 (carbonation): after bottling, store at room temperature (18–22°C) for two weeks. This is when the priming sugar carbonates the beer. Phase 2 (long-term): after carbonation, move bottles somewhere cool and dark for storage. A cool cupboard, a beer fridge, or a garage corner away from sunlight all work. Brown bottles protect against UV; clear and green bottles do not. Keep them away from windows.
The fix when it's already happened
Light-struck (skunky) beer can't be unskunked. Move it out of the light, but the existing damage is permanent. Stale beer follows the oxygen-exposure rule — drink it faster, store the rest cold. Beer that didn't carbonate properly because it was too cold can sometimes be rescued by moving it somewhere warmer for another 1–2 weeks and checking again.
A note on perspective
Reading a list of ten things that can go wrong sounds intimidating. It shouldn't. Each mistake above is genuinely common, and yet most homebrewers produce drinkable beer on batch one anyway. The reason is that brewing is forgiving — the yeast wants to make beer, and most of these mistakes have to combine to create truly bad beer. One mistake is usually survivable. Two might give you a flawed but drinkable batch. Three is when things start going sideways.
Brewing is forgiving. The yeast wants to make beer.
The brewers who get good are the ones who treat each batch as an opportunity to learn one thing. Took a gravity reading? You'll know whether the next batch fermented better. Controlled the temperature this time? You'll be able to taste the difference. Each lesson compounds. By batch five, most beginners are making beer they're genuinely proud of.
If you'd like to skip some of these mistakes entirely, fresh wort kits take a few of the variables out of your hands — you don't have to worry about wort cooling temperature, hop additions, or boil management because the brewery has done that part. But the mistakes that matter most — sanitation, fermentation temperature, patience — happen after the wort is in the fermenter, and they're entirely on you regardless of method. Pick your starting point, control those three, and you'll be drinking your own beer in about a month.
For a complete brew-day walkthrough, see our how to brew guide. For the broader equipment picture that prevents many of these mistakes, see our equipment essentials guide.