I pitched American Lager Yeast at about 70F. Now I can see some Krausen on the top of the Wort/Beer. I am moving it to a colder room now (about 50F) but should I have pitched the yeast at a cooler temp to start with?
I cool my lager wort overnight before pitching, but it takes several days for the yeast to get going. And I am taking a calculated risk of infection. I'm not sure there's a hard and fast rule on this. With no starter, I think a higher pitching temp like yours may be appropriate to make sure things get underway. Still, now that things are underway, get that temp down into the low 50's immediately or you'll get some funky beer in the end.
Warm pitch followed by temperature reduction before fermentation starts. Timing can be an issue. This is the method I use, mainly because it is easier. I pitch warm and set the fermenter in a cool place, so the temperature can drop slowly (24-48 hours).
Cold pitch huge starters. Starters are grown warm and cooled. Temperature shock is a possibility.
No, you have to do the diacetyl rest while the beer is on the yeast cake, and it should be done just before the fermentation finishes, when you're about 75% of the way to your expected fg.
The reason it has to be on the yeast cake is that it's a function of the yeast to eat the diacetyl, in effect cleaning up after themselves. If you remove the beer from the yeast cake, you are keeping the yeasts from doing their job.
Well, it's better and faster to do a diactyl rest while you have the beer on the yeast cake, but it's not necessary. The suspended yeast can get you there.
I've had a couple beers where I racked to the secondary and then kegged without ever sampling, only to find upon the first pour that I had a big diacetyl problem. I mean BIG diacetyl problem, too, like my mouth felt like it was dipped in friggin' movie theatre popcorn butter kind of problem. For both, I pulled the keg out of the keezer and just let it hang out at room temperature for a couple of days. That cleared the diacetyl up just fine.
I may have been lucky, and it could have just been the yeast (California V in both cases), but it did work.
Like TexLaw said. Since I pitch my lagers cold, I rarely even get diacetyl. I taste it and see if I can detect any diacetyl. (Although in John Palmer's book, he gives a great way to check by boiling some of the beer.) I've only really needed the diacetyl rest one time. Still, you won't hurt your beer by doing one.
I've understood that the best time to do a diacetyl rest is just as the primary fermentation is wrapping up and immediately after it ends. The yeast are still nice and awake, but they are looking for something other than sugars to eat. There's that nice diacetyl just sitting there, waiting for them.
If you use dry yeast or make a starter with liquid yeast, you'll get better results by pitching at ferment temps (50-55F). I get super clean results that way, no esters and no diacytel and I don't have to worry about timing a diacytel rest. When I used to pitch warm, esters were a problem and inappropriate for the style.