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.
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 judges scoresheet that says he (or she) has detected diacetyl in your beer but you never noticed any.
To understand more about diacetyl and discover ways of preventing it, click here to go to the page about diacetyl.
References: Beer Flavors #1: Diacetyl by George de Piro, Brewmaster C.H. Evans Brewing Company at the Albany Pump Station.