Identifying off-flavours in beer: Diacetyl

The goal of any brewer is to not just craft something drinkable, but create something that is truly enjoyable. We spend precious time researching base malts and specialty grains, and investigating which hops will provide just the right bittering balance or pack the perfect punch of flavour. Cleanliness and sanitation are gospel to avoid infection and major flaws, but what else can we keep in mind with our process to ensure our friends aren’t choking down our swill out of politeness? Over the next few months, we’re going to take a look at some of the most important off-flavours and compounds you should know about to help you improve your processes, refine your brewing skills, and make the best homebrew you possibly can.


This is one of the most common off-flavours you will likely run into. It’s one of the easiest to detect, and is surprisingly one of the easiest to avoid. That’s why if there is one off-flavour you should first be familiar with, it’s diacetyl.

Think buttered popcorn. In fact, diacetyl is produced on an industrial scale as a flavour additive for microwave and movie theater popcorn. In beer it can also have a strangely sweet, butterscotch flavour, and in high levels it can give slick feel to the beer.

In rare cases it is desired in the beverage making world. Some Scottish ales and a handful of British beers allow (and even want) a small perceptible level to round out caramel and roasted malt flavours. The key word here is small. Also, if you’ve ever had a buttery chardonnay, or attempted to make one at home, well, you can thank diacetyl for that unique taste.

You might be surprised to learn that diacetyl is, in fact, formed in every beer that you brew and will ever brew. It is a natural by-product of the yeast that is produced during anerobic fermentation.

So if it is always being produced how do you keep it out of your final product? Easy. Just wait.

A lot of homebrewers stress obsessively about hitting their final gravity then rush to get their beer off the yeast as soon as they can because they are worried the yeast will start to die, break down and give off off-flavours. This is the second biggest mistake you can make in brewing, after not cleaning and sanitizing your equipment, of course.

So how long do you wait? The technical answer is, it depends on the strain, health, age, and viability of the yeast. Some can finish cleaning up diacetyl in three days, others five to seven. So the easiest, most fool-proof answer to the question then, is simply wait at least seven days after fermentation has finished. If you are monitoring the gravity, you will be able to know when this kicks in. For those that go off of the activity in the airlock, just start counting the days once you see the bubbling and off-gassing cease.

This wait is the infamous diacetyl rest that you might have heard of before. You might have also heard of a VDK rest, which is just another name for the same thing. Chemists and some professional brewers recognize diacetyl more specifically as an organic vicinal diketone (VDK) compound with the chemical formula of (CH3CO)2. No matter what you call it, you should employ a brief rest post-fermentation to allow the yeast to clean it all up.

A very easy blanket rule you can go by if you don’t have the means to check your gravity every day (we’ll talk about dissolved oxygen and oxidation in another article, so until then, refrain from opening your bucket and carboy every 24 hours please), is to simply leave your beer in the fermentor for a solid two weeks from the date you pitched your yeast. That will give you plenty of time to get through primary fermentation, even if there is a lag, and allow your yeast to clean up diacetyl and other off flavours before autolysis and dead yeast start to become an issue. In all honesty you have about 3-4 weeks before you will have any major off-flavour issues with your yeast so don’t fret.

In addition to time in the fermenter, think about adding some yeast nutrient to your brew specially formulated for beer yeast health and then pitch per the instructions.

As with all your brews, keep your tasting notes and logs and continually refer back to your fermentation notes. Happy brewing!

David Boggs has been homebrewing for over 10 years and currently works as a professional brewer in Edmonton. He holds a certificate in Applied Craft Brewing and is a Recognized BJCP Judge. He is a fan of Belgian ales and anything barrel-aged.

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