So how do you know if your young beer will need a
diacetyl rest? Do a diacetyl test, that's how. There is a simple test which any homebrewer can do that will answer that question. It requires two glasses , some foil or plastic wrap, some hot water at 140-160°F (60-71°C), and some cold water. The principle is simple. At warmer temperatures, the precursor to diacetyl, alpha-acetolactate (AAL), will oxidize quickly into diacetyl in your young beer. In essence, you will be testing for the presence of excessive levels of AAL in your beer.
How to do a Diacetyl Test:
Put a sample of your young beer into each of two glasses and cover them.
Put one glass in the hot water bath and keep it there for about 10-20 minutes.
After 20 minutes, cool the hot beer to the same
relative temperature as the control sample by placing it in a cold water
bath for a few minutes.
Remove the covers and smell each of your beer samples (don't forget which is which). One of the following things will happen:
Neither sample smells buttery. This is the best-case-scenario
because it means that all of the diacetyl precursor AAL has already been
converted to diacetyl and your beer does not need a diacetyl rest.The heated sample smells buttery, but the control sample does
not. This indicates that there is excessive AAL in your beer and you
should ramp the temp up (if it's a lager) by 5-10° F (1 - 2°C) for a
diacetyl rest, and age it for a few days to allow the AAL to form
diacetyl and the yeast to metabolize the diacetyl. For ales, a few more
days of conditioning is indicated. You need to do the test again to
insure conversion is complete and your beer is stable. Both samples can smell like butter. This might indicate a pedio
infection, in which case you should probably dump the beer. It could
also mean that your yeast cannot metabolize the diacetyl because the
majority of them are petite mutants.
Performing this diacetyl test may seem tedious or
unnecessary, but it may be indicated when you are brewing a beer in
which the yeast often produces excessive amounts of diacetyl, such as a
Kolsch or highly flocculant English yeast strains. Or, if you decide
you want a slight diacetyl note in your beer, which can be pleasant in
some Scottish or English ales, you can perform the test to see if there
is any of the precursor AAL left in the young beer. And finally, you
might want to perform the test if you suspect diacetyl is in your beer
or you have a judge's scoresheet that says he (or she) has detected
diacetyl in your beer but you never noticed any.