This is a common question that I've seen over and over again, and for good reason. A host of issues can come from packaging too early and even can be unsafe if you are bottle conditioning. I know it is tempting to crash, package, and get to drinking your beer as quickly as possible, especially if it's something you're really excited about (like this Czech premium pale here off the tap). But rushing can turn a good beer bad and reduce the worth of our time spent on brewing efforts. So how can we know that the beers we make are ready to package or chill and drink safely and without one of the most common off flavors?
First and foremost, this post is mostly about the compound known as Diacetyl. But I'll also share how I know the beer is safe to package. Doing what is called a forced ferment test will give you an idea of when the yeast have eaten all the sugars it can use that you provided in the wort production step. Taking a small sample of your beer, after you've pitched your yeast of course, and putting it in a sanitized vessel, on a stir plate for oxygenation (shaking works too if you don't have one), and in a warmer area will accelerate the fermentation to completion so that you can measure and know how fermentable your wort is. Adding extra yeast if you have it will help this even more! Measuring the final gravity of that forced ferment will give you a very good idea of when the bulk fermentation is done so that you can bottle and prime without fear of exploding glass and overcarbonated beer, even if your beer does not ferment as dry as you may have anticipated when developing your recipe or following a published one. The forced ferment test will give you peace of mind and a safer overall homebrew experience, assuming your sanitation and cleanliness are adequate.
Ok, back to diacetyl...or if you're someone who's interested in the chemistry, it is known as 2,3-Butanedione; and it has a flavor reminiscent of buttered popcorn or butterscotch. This compound is a natural byproduct of fermentation that will eventually go away if given the opportunity. It may be difficult for some people to recognize this off flavor in the context of beer. But over time, you should (as long as you are not anosmic to diacetyl) be able to pick it out as you go through the beer-making process and note how it changes throughout. I've also had good luck experiencing this off flavor at local small brewpubs...pew pew.
So now that we know about diacetyl, how do we avoid making the typical homebrewer mistake of packaging too soon and having multiple bottles or a keg of butter beer?
There are some measures you can take to reduce the overall amount of diacetyl produced during fermentation, but there will always be diacetyl produced during fermentation with a normal, non-GMO beer yeast. So for the purpose of this post, I will talk more about monitoring and knowing when it is gone so that you can feel good about chilling and drinking your beer.
We must know the origin of diacetyl and how it works to determine when our beer is totally ready for packaging. I won't get into all the details now; but in the simplest of terms that you need to know, diacetyl will occur when there is an excess of alpha-acetolactate produced by yeast when synthesising one of the key amino acids -- valine. The yeast will excrete the alpha-acetolactate outside of its cell and into the beer, the acetolactate will break down into diacetyl, and the yeast will uptake the diacetyl to ulitmately convert it into a flavor-neutral compound. Here's a simplified visual diagram [1]:
Take note here: this is where homebrewers and even some brewpubs make a mistake when tasting thier beers for packaging. Any remaining alpha-acetolactate in your beer can, and will, reduce into diacetyl over time; so chilling and packaging prematurely will not allow the yeast to reabsorb and remove the diacetyl. Even if you are sensitive and can recognize diacetyl but don't smell it in your recently-terminal primary fermentation, that precursor may still be present in the beer and eventually give you that buttery flavor. And that is where the following process comes from when determining if your beer is ready to package. This is a process that forces the acetolactate to degrade into diacetyl with the use of elevated temperatures and forcing the reaction to occur much more quickly for evaluation. When you know that your beer has done its primary fermentation and has gone through all the fermentable sugars that it can, the following test can be done to monitor and indicate when it is ok for you to chill, package, and enjoy.
Forced Diacetyl Test
Take a small sample, roughly 50-100ml depending on your choice of vessel, and fill it about 3/4 of the way to the top of the container (about 25% headspace).
Heat your sample to 140-160°F [60-70°C] and let it sit for about ten minutes at that temp. A little longer is fine, but don't forget about it for too long!
You can keep it in that temperature window with a few different methods. But for the homebrewer, it is easiest to do a hot water bath. Bring a gallon or so of water to 165-170°F [~75°C] and put your sample container (filled with your sample) into the bath and let it sit for ~20-30 minutes. This will allow your sample to heat up and sit at an appropriate temperature for long enough to develop the diacetyl that you want to detect (if it's there!). Careful to not boil the sample as it can produce a false negative result by allowing the diacetyl to evaporate off.
Remove your sample from the hot bath and let it cool for a moment. You want it to cool down from forced diacetyl temperatures but still be a bit warm to help you detect the diacetyl in the aroma. Around 100°F [~40°C] or body temperature should be great. Once it's cooled, open up your container and stick your nose in the headspace. If you detect some buttery notes, you need to let your beer sit for a few more days and test again. If you don't sense butter, you're good to go! Cold crash your beer and package however you prefer.
It may help to use a neutral beer alongside your sensory to give you a control. Something like a macro lager or a light beer from a trusted brewery in your area will do the trick (not Pilsner Urquell) to give a clean profile to compare with your test sample. You can warm that beer up as well for a more level tasting ground, but that may not be entirely necessary. Anything clean to compare is helpful!
And it's that simple! This is something I use a lot in my own brewing to make sure that I don't have diacetyl in my beers and package as quickly as possible. Of course you can just let your beer sit passively for 3 to 4 weeks and usually be fine, but having an indicator for cellaring/packaging operations makes you much more efficient and gives you peace of mind that your beer won't have one of the most common off flavors in beer. If you've been struggling with homebrew "twang" or you know that diacetyl is an issue for your beers, this is a wonderful and easy method to utilize and be able to consistently make your brews that much better.
Happy Brewing!
Resources:
[1]Nedović, Viktor & Manojlović, Verica & Bugarski, Branko & Willaert, Ronnie. (2011). State of the Art in Immobilized/Encapsulated Cell Technology in Fermentation Processes. 10.1007/978-1-4419-7475-4_6.
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