pitching on old yeast cake

i have had a kolsch yeast cake at 40F for about 3 weeks. i just racked the beer out of the carboy, replaced the airlock, and put the carboy back in the fridge. can i put fresh wort on top of this yeast cake or has it been too long. also, should i bring the cake up to ferm. temp? i have never done this before, just curious.

i tried to find a previous thread on this, but didn't

If your going to keep the cake in the fridge make sure it has a thin layer of beer still on it. Even then I wouldn't wait more than a day or so. I've re-used yeast cakes before (kolsch works great because you can use the cake to make an oktoberfest!!), but I rack the beer off the cake the same day I'm brewing. I also would bring the yeast back up to room temp before you rack your new beer. Treat the cake the same way you would treat any yeast.....

Also, a whole yeast cake is a pretty big overpitch for most beers; mrmalty's pitch rate calculators include one for figuring out what fraction of the cake to use.

thanks, i will pony up the $7 for a new yeast. i make an oktoberfest with the kolsch yeast that is awesome. very versatile yeast.

I did this with a Vienna lager last winter. I ordered 2 kits, but only 1 yeast pack arrived. Racked the first batch off the cake into a secondary and let it sit, covered while I brewed up the second batch. When the wort from the second batch was sufficiently cooled, I poured it onto the cake. Maybe not the best practice, but I was not able to taste a significant difference between the two batches. Probably not award-winning beer, but it worked just fine for me.