One of the best parts of being a homebrewer is sharing your beer. However, if you’ve noticed your friends don’t quite finish their pints, or give a generic, “It’s good!” when you ask for feedback, you may be running into one of these very common mistakes. Here are four reasons your homebrew is subpar, and what you can do about it.

It tastes like butter.

Diacetyl, a chemical compound that has a similar flavor and mouthfeel to butter, is a unique off flavor. If present, it can continue to develop in your beer over time, especially when pulled off a home kegging system. The precursor of diacetyl (Ahoy! Science words ahead) is acetolactate, which is flavorless in beer. But when beer containing that precursor gets hot, like in the trunk of a hot car on the way to a competition or your friend’s house, it will convert into buttery diacetyl through oxidation.

One way to avoid this is to perform a simple test. The science-y name is a vicinal diketone (VDK) test, but don’t be intimidated! All you have to do is fill two glasses with homebrew and immediately cover each tightly with foil. Set one covered glass aside, and place the other in a water bath that warms the beer to about 150 to 160 degrees Fahrenheit. Hold it at that temperature for at least 20 minutes.

Next, cool the heated sample to room temperature (~68 degrees Fahrenheit) with an ice water bath. One glass at a time, remove the foil and take short sniffs to inspect for a buttery quality. If the warmed glass has a stronger buttery aroma, all of the acetolactate has not yet been converted into diacetyl and reabsorbed by the yeast. You want all of the acetolactate to convert before bottling, so to make sure this happens, raise the fermentation temperature and rest it there for about two days, then test again.

If diacetyl is present at the same level or has increased since the last test, you’re likely getting it from contamination as opposed to acetolactate conversion. In that case, it’s time to break out your cleaning and sanitizing gear and start a new batch of beer.

It smells like corn.

When beer smells like corn (and hasn’t been brewed with it), the problem is dimethyl sulfide (DMS). DMS forms from the precursor S-methylmethionine (SMM), a compound in barley that is mostly driven off during the malting process. Only the very lightest malts — those that spend the least amount of time being kilned — still contain SMM by the time a brewer is using them. Pilsner malt is almost always the culprit when it comes to DMS. It takes a few simple steps to prevent a pint of corn aroma:

  • Boil hard. DMS is a volatile compound that is easily blown off with a vigorous boil. A 90-minute boil with well-modified malts will kick that corn stench to the curb. If you’re experimenting with less modified malts, like floor-malted pilsner or an especially large or high-gravity batch, opt for a 120-minute boil. It is imperative that the kettle is uncovered during boiling and cooling. If a lid is left on, or if condensation drips back into the wort, DMS won’t escape.
  • Chill fast. The best way to keep DMS levels low after a vigorous boil is to cool quickly. When wort is warm, but not boiling, SMM is still converting to DMS but not being released into the air. The quicker your wort is cooled to 70 degrees and below, the less SMM will turn into DMS.
  • Ferment vigorously. The last chance to push DMS out of your beer is during fermentation. A very active fermentation can blow off DMS just like the boil. Be sure you’re pitching the correct amount of healthy yeast, and that your beer has a blow off tube or airlock so DMS can escape with CO2 and other byproducts.

It’s sweet and syrupy.

Pastry stouts aside, syrupy-thick, sweet beer is not usually a brewer’s intention. Beer with a too-full body is most often a result of under-attenuation, meaning yeast were unable to convert the correct amount of fermentable sugars into alcohol and carbon dioxide. There are a few reasons yeast stop or slow the fermentation process.

One reason is that yeast is not healthy. When fermenting more than one or two gallons of wort, the best practice is to create a yeast starter. This is essentially a mini-fermentation with a small batch of wort and a pack of dry or liquid yeast. This activates the yeast cells, and they multiply, so by the time the mixture is pitched into wort, the little critters are healthy and ready to go. This starter calculator by Jamil Zainasheff is the go-to tool for homebrewers. Use it to check the viability of your yeast and pitch more packs if needed.

If yeast were viable and the beer is still under attenuated, there are more steps to troubleshoot. First, be sure both the wort and the yeast packets or starter are between 65 and 70 degrees Fahrenheit when pitched. A drastic temperature difference of more than 3 degrees can shock and even kill yeast, leaving them unable to ferment.

Finally, check your mash temperature. A low mash rest between 146 and 148 degrees Fahrenheit, before raising the temperature to a more standard 152 to 154 degrees, gives certain enzymes a chance to thoroughly break down starches. This will make the finished beer drier. If you have a hard time controlling mash temperature, aim for a lower temperature. If the mash becomes too hot, enzymes will denature and stop working, leaving your wort full of sugars the yeast won’t be able to ferment.

It burns!

A scent of alcohol that’s burning or hot is a sign of fusel alcohols and/or overabundant ester production. In low levels, both fusels and esters are important and enjoyable parts of beer aroma and flavor — they provide floral, fruity notes and a comforting warming sensation that give beer complexity — but even the booziest barleywines shouldn’t burn going down.

Homebrew that’s too boozy is almost always the result of fermentation that’s too hot. Certain yeast strains can take the heat, especially Belgian and saison strains, but even these should ferment at temperatures that rise slowly from the initial chilling temperature (65 to 70 degrees Fahrenheit) by increasing just a few degrees a day. This allows the yeast to consume the bulk of the fermentable sugars at a temperature where production of esters and fusels is low. By the time fermentation is up in the high 70s or 80s, there are just enough fermentable sugars left to create a pleasant amount of these compounds.

If you’re having a fusel problem, aging could be the answer. Fusels can slowly be converted into esters as long as active yeast is present. (Most high-alcohol homebrew is best after aging for this reason.) Esters can’t be converted into another compound, so once an ester is created in beer, it’s there to stay.