Charlottesville Area Masters of Real Ale

I was wondering if anyone had any tips/explanation/or has experienced an explosion during their Diacetyl Rest when making a lager?

Here's what happened....
I decided to try out my lager skills (first time with the lager process) and got one of ABS's Oktoberfest packages. Followed the directions and hit the OG right on the nose. They suggested doing the primary fermentation in a glass carboy with blow off tube. I went with that, but I didn't have a real blow off tube so I used a regular rubber stopper with half an airlock stuck in there and then a smaller diameter tube wedged into the airlock. This tube then ran to a bottle to catch any blow off. Everything was looking great in during the primary fermention and was able to get my fridge to hold a steady 42-45 F, which is on the cooler side (should have been more like 45-55F), but I couldn't get it much warmer than that in the fridge. I brewed on Sept 21 and let it do the primary fermentation for 12 Days.

I then attempted the diacetyl rest and removed the beer from the fridge and set it on the floor in my living room and kept the thermostat set at 72 F. It was fine for approximately 12 hrs I believe. I returned to find bits and pieces of hops/beer/foam/hardened black stuff on the ceiling, the floor, the walls, as well as a puddle of about a half gallon of beer on the carpet, not to mention my blow off tube assembly on the ground next to the carboy.

After cleaning things up I was able to reattach the blow off tube assembly and move the carboy to the kitchen. I noticed an intense flow within the carboy occurring which I assumed was the diacetyl rest occurring, but it might have also been very vigorous yeast fermentation activity.

Here's my guess (what are your thoughts?):
1. The lower temperature didn't allow for as complete a primary fermentation as should have occurred, should have let primary fermentation occur for a longer period of time. Because of this, fermentation continued at the warmer room temperature and generated a great deal of foam and pressure, leading to the explosion.
2. The blow off tube was too narrow. I have since fixed this and gotten a wider diameter tube to use for blow off.
3. Room temperature was too high and I should probably invest in one of those refrigerator temperature controllers to have a lower temperature controlled diacetyl rest.

I really should have taken a picture
Has this ever happened to you? Any advice? Thanks!
 

I have had the beer explosion occur before, but not on lagers.  My guess was that it was caused by a number of factors, including:

- The cold temps lagers ferment at allow more CO2 to remain trapped in the beer
- The lager was not probably finished fermenting yet, as you suggest
- Most diacetyl rests involve bumping the temp up by 10 degrees, and your situation almost doubled the temp
- The blow-off tube was probably too small

I would encourage you to get a refridge temp controller.  I got mine (the analog, non-digital one) for about $50 a couple of years ago.  It works great.  This will really help your brewing, as fermentation temp control can really help fine-tune your final beer flavor.  If you want to continue using a blow-off tube, I would get one the size of your carboy neck, and just cram it into there.  That being said, I have not used a blow-off tue in years, and the process seems to work just fine and the beer tastes fine too.  You lose less beer and make less of a mess.

Just my two cents . . .

I think Tom hit all the high points.  I would definitely control the rest a bit more and only let the beer go up around 10-12 degrees.  And I usually try to do the rest when fermentation is about 80% done.  So I don't think you did the rest too early.  Your timing seems good.

Also, although we say it is a "rest", a diaceytl rest isn't very restful.  Typically, we are doing this with batches containing lager yeast and the increased temperature will fire things up and blow out the bad flavors.  This is a good thing, but you need a big blow-off tube to keep your ceiling clean.  I've pasted the inside of my cooler lid with yeast several times.

I have also read and heard that the purpose of the diactyl rest is to ensure the yeast clean up the flavors.  This is not to say that they need to be blown-off and physically removed from the beer, rather that as the yeast consume all of the sugars they will normally eat through, they turn to other fermentation by-products and use them as food.  Thus, they consume the diactyl and acedaldehyde (green apple flavor).  You can feel free to still use a blow off tube, but I have stopped using them as things I have read indicate they may not be necessary and they can cause an awful mess, as you unfortunately found outl