When brewing a lager, it is good to do a diacetyl rest after the initial fermentation in order to let the yeast absorb some of the buttery flavors associated with diacetyl (think microwave popcorn).  Diacetyl is a fermentation bi-product naturally produced in your fermentation.  Your yeast dump out a product called alpha-acetolactate which in beer's low pH environment gets transformed into diacetyl (frequently with the help of oxygen).  But the good news is that your yeast will reabsorb the diacetyl given time and temperature.  It takes a little time (sometimes less than a day, sometimes more, but how do you know if you really need the rest and how do you know when it is done.

Well, Brian proposed a very simple test for determining how necessary it is.  Simply take a sample of your beer, microwave it for a just a bit to warm it up and take a sniff.  If you smell buttery notes, you need a rest.  So crank up your temperature 5 or so degrees and let it sit for another day or so.  Repeat the test again to see if it is gone.  If not, give the beer some more time and keep trying until you are satisfied.  Just a simple microwave test to avoid microwave popcorn flavors in your brews.

If you have a brewing tip you would like to share with the club, please send it to markemiley@ earthlink.net.  It may be common sense to you but could save someone else's beer.