Wyeast 2565 Kolsch

I made in the last 5 weeks 3 - 5 gallon batches of a Cream Ale.  I used Wyeast 2565 Kolsch.  The 1st 5 gallon batch I put into a 6 gallon glass carboy.  Pre boil gravity 1.047 and post boil gravity 1.057.  Later that night the airlock blew off the carboy.  I then used a blow off tube.  The second batch I used a 6 1/2 gallon carboy and had no problems.  Pre boil gravity 1.047 and post boil gravity 1.054.  The third batch I used a 6 1/2 gallon carboy and the airlock blew off again.  Pre boil gravity 1.049 and post boil gravity 1.059.  The second batch had the lowest post boil gravity.
Looks like the higher post boil gravity has an affect on the fermentation.  I did use a yeast starter for all 3 batches and the same starter size.  The dates on the yeast packs where also the same.  The grain bills where the same.

I guess I am trying to figure out the combination why sometimes the airlock would blow off the carboy.  I make other beer styles and I never encountered this problem.

Cheers!!

Greetings,

My initial thoughts are that there may have been a slight temperature variance in the wort and/or the ambient room temperature.  I don't believe the couple points you mentioned in the pre-boil and post-boil readings would made that big of a impact.  Plus, if the starter size and the yeast viability were the same, temperature is the only final variance.  All that said, I have had the same conundrum  with no real answers.  So, I have gotten the habit of always using a blow-off tube, no matter what.  Then, after a few days, I replace it with my airlock.

I would be interested in learning if there were any differences in the taste, appearance, aroma, etc. of the three batches.  Keep us posted!!

KellerBrauer, good point about the temperature.  The temperature of the wort is something I do not document.  I pitch the yeast at temps that are within their fermentation temperatures.  So for me that varies on each brew.  Also the ambient room temp varies for each brew.

Will keep you posted on the taste, appearance, etc of the three batches.

Cheers!!