Is it typical for lag time to increase for re-used yeast on subsequent batches?

[SpeedYellow]

I've been reusing a wyeast lager strain and found that after 3-4 reuses, the lag time on my last couple batches has been much greater. On the first few uses, I saw fermentation in 12-16 hours, but now it's not starting until around 48 hours. Obviously unacceptable.

My process has been to ferment the beer, do a d-rest, then cool to 32F a few days, then rack the beer, collect 1/3rd of the yeast, and store in the fridge for a few days until the next batch is brewed, then pitch the unwashed yeast directly from the fridge into 46F wort. Worked great the first few times, but no more.

Any idea what gives?

Edit: Note also that the fermentation VIGOR has not dropped. I measure CO2 output daily and see that at peak fermentation, output is still strong and the fermentation lasts 6-9 days once it starts.

 

[PDX_T]

Yes. The yeast are oxygen starved. Are you aerating your wort?

 

[SpeedYellow]

Yes, I aerate the wort, and my aeration regime hasn't changed at all, ever. I use an aquarium pump and stone for 20 minutes. This time I let it go for 30 minutes and didn't seem to help. Visually it seems to be aerating just fine.

 

[BigFloyd]

Since it's a lager, I'd be inclined to harvest 1/2 the cake instead of just 1/3. That may help shorten your lag time.

Also, you may want to bump the crash temp up to around 35-36*F. That's still plenty cold to crash the yeast out, yet not so cold that you have to worry about the formation of ice crystals within the slurry.

 

[progmac]

i don't think 48 hours lag time for a beer fermented at 50 degrees is necessarily unacceptable. ultimately, the proof is in the pudding.