Fermentation vacuum question

[slyinski]

I just acquired a glass front fridge that I'm using for fermentation temp control. The first few days as it was fermenting i didn't notice anything out of the ordinary. When fermentation slowed down (not creating a lot of CO2) i noticed every time the fridge kicked on to cool down there would be a vacuum created in the carboy. I know the whole pressure difference in warm to cool wort thing but my question is with the negative pressure do i have to worry about wild yeast or other spoilers be sucked into the fermenter? My fridge is set up with a 6.5 gal carboy with a S style airlock and a thermowell in the stopper. i have the temp controller probe in thermowell and set up to turn fridge on/off when beer falls out of temp range. Any help would be much appreciated.

[EagleDude]

No worries ... you are fine without any wild yeast concerns. The liquid in the S shaped or regular airlock is mainly for preventing transfer of Oxygen. The down then up design keeps bacteria, which can't fly, from making it into the beer. The main concern would be if large amounts of oxygen gets sucked into the fermentor which could aid in stailing and oxydative characters in the beer ... also probably not too much of a concern with a few degrees in temp changes.

[Lewybrewing]

A big concern? No. But I would look at setting up a blog off tube to eliminate any potential issues in the future. I for one wouldn't want mid 60 degree water being suck into my fermentations.

Lewy

[slyinski]

Ty for responses. My two main concerns were wild yeast/bacteria and oxygen. There is no real amount of liquid that goes in. I use a S shaped airlock with Star San as the liquid.
Correct me if I'm wrong but I believe CO2 is heavier then O2 so if oxygen is coming in then it would stay on top then be pushed back out when temps stabilize and positive pressure is restored. Never coming in contact with the beer. Still makes me a bit nervous to see outside air going in. If anyone has suggestions on an alternative way to set up temp probe I'm all ears. Thanks again. Cheers.

[Bad Goat Brewing]

......Correct me if I'm wrong but I believe CO2 is heavier then O2 so if oxygen is coming in then it would stay on top then be pushed back out when temps stabilize and positive pressure is restored. Never coming in contact with the beer. Still makes me a bit nervous to see outside air going in.........

The Co2 does mix with the O2 going in. I think most people think it's not that big of a deal. There was a long talk about his about a few months back.

viewtopic.php?f=&t=31864

[Bad Goat Brewing]

. If anyone has suggestions on an alternative way to set up temp probe I'm all ears. Thanks again. Cheers.

II've switched to using a wide piece of painters tape to the probe to the fermenter. Using thermowells I had much larger temp swings in the fermenter. Now i just have a mechanic thermometer in my thermowell, and it reads with in a 1/2" degree of the probe taped to the side of the fermenter.

[Afterlab]

The easiest thing you can do is to put a cheap, clean and sanitized plastic shopping bag over the airlock and top of the carboy. The second easiest thing you can do is to ensure the enclosed environment you're fermenting in isn't a location that wild yeast and bacteria would like to harbor in.

Now that I think about it, you should really be avoiding this problem all together by removing your carboy out of the fridge as fermentation appears to be slowing down. Warm that carboy up to room temp or warmer after the krausen falls and the vacuum won't be a problem.