Yesterday I brewed a 6-gallon batch of 1.051 OG hefeweizen. Added White Labs yeast nutrient during boil and aerated vigorously. No time for a starter, so I pitched two packs of WLP300. Jamil and others suggest starting fermentation at 62F. I cooled to 64 and pitched the yeast, and then put my 7 gallon PET carboy into a fermentation cooler bag with some frozen ice bottles. I am tracking the fermentation with a Tilt. While I expected the temperature to rapidly drop to 62, it slowly increased to 66. I added some more ice bottles. No change. I though that was odd so I taped a probe to the side of the fermenter, which read 64. I also have a stick-on thermometer lower down on the carboy, which read 57, but I don't trust the stick-ons as much, and its sitting right next to the ice. I let it go overnight. In the morning the Tilt was still reading 66. Gravity still sitting at 1.051. I dropped a temperature probe in the top, and got a reading of 64. The sticker on the bottom was now showing low 50s and the yeast were all piled on the bottom. Something was clearly not right here. I felt the sides of the carboy and sure enough the bottom felt very cold and the top warm. I swirled it all up really well and boom the Tilt immediately dropped to 55F. So apparently, over the first 8 hours or so of my fermentation, the wort stratified into warm top and cold bottom, and my Tilt measurements were accurate but the bottom half of the beer where the yeast were was getting super chilled. I'm sure this isn't a problem for people using fridge/freezer chambers, but could be a big problem for people with swamp coolers/cooler bags/etc. Anyway, I've never experienced this before. The tall, skinny shape of the PET carboy probably contributed. I am warming up my carboy now and hope the fermentation will get going quickly.
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