This is another simple quality control test I learned about from Chris White and Jamil Zainasheff’s yeast book. This one is even simpler than the forced wort and forced ferment tests, requiring nothing more than some means to pull a sample from your fermentor, two glasses, some foil, and hot water.
Firstly, what is diacetyl?
Without getting into the chemistry, diacetyl is a fermentation by-product which gives a buttery or butterscotch flavour and aroma, and a slippery mouthfeel. If you’re not sure what it’s like, buy yourself a pint of draught Deuchars IPA from any busy, city centre pub, and it will probably be the single most prominent flavour. A low level of diacetyl may be considered appropriate in some beer styles, but high levels never are (sorry Deuchars!). It is produced mainly in the early phases of fermentation, and it can be taken up and reprocessed by the yeast (into flavourless compounds) towards the end of fermentation. The main cause of excess diacetyl is taking the beer off the yeast too early, before the yeast has had time to reprocess it. It can also be exacerbated by low temperatures towards the end of fermentation, which again restricts the yeast’s ability to reprocess it.
The problem is that, whilst diacetyl itself is quite noticeable, the chemical precursor it is produced from (acetolactate) is flavourless and odourless. This means you can bottle or keg a beer with little or no appreciable diacetyl, only to have it develop a noticeable amount during storage and conditioning. And by that stage, there’s really not anything you can do about it… It might disappear if you still have enough yeast in the beer and you give it enough time, but ideally, you want to prevent the problem from occurring in the first place.
You can reduce the amount of diacetyl and acetolactate in your beer by giving it enough time “resting” on the yeast in the fermentor, and you can encourage the yeast to clean it up by raising the temperature towards the end of fermentation – this is known as a “diacetyl rest”. High diacetyl levels in your finished beer usually indicate that you racked it too early, or allowed the temperature to fall too low at the end of fermentation.
To test for both diacetyl and acetolactate, we can use the “diacetyl force test”:
Pull a sample or two from your fermentor (in as sanitary a fashion as possible, obviously) into your glasses – it doesn’t need to be a huge sample, but not too small either. I usually reckon on about 50 – 100ml. Cover with foil. Put one of the samples aside somewhere safe at room temperature. Using a water bath (this can be as simple as a bowl of hot water), raise the temperature of the other sample to between 60 and 70 degrees centigrade and hold it there for 10 to 20 minutes. This heating will force any (flavourless) acetolactate present to change into diacetyl, which we can taste and smell. After heating, cool the sample back down to room temperature (an ice bath is ideal). You can now assess both samples for diacetyl. If you notice it in both, then you have a lot present in the beer and it will certainly need a good diacetyl rest. (This may also indicate a bacterial or wild yeast infection, but don’t panic yet!) If you only notice it in the heated sample, then you have acetolactate present – if you rack the beer now, it is likely to turn into diacetyl over time, so be patient, let it rest some more, and maybe consider raising the temperature. If there is no discernible diacetyl in either (or at least, not more than is appropriate for the style), you can rack and bottle your beer with confidence!
If you really want to minimise the fuss, you can do this test with only one sample – simply check it for diacetyl both before and after heating. It’s not quite as good as being able to compare heated and unheated samples, but it’s certainly better than not checking at all.