Yeast starter - Airlock or foil?

[ANewrBrewr]

Good looking out "Onlooker" I didn't see your post at first.

Here is a pic of what I use. My LHBS just got these new caps for gallon jars that screw on, but have a hole in the middle for an airlock. As mentioned before I simply don't add the water in the airlock and it works great for my starters!

 

[ANewrBrewr]

Maybe a noob question, but why is it ok to let a starter have contact with air, but in fermenter an airlock is used? Seems like contamination risks are the same ?

Not an expert, but I would assume that a starter has such high volumes of active yeast that if anything makes it to the liquid it would not be strong enough to propagate amongst all the good guys Also while the yeast is actively working it is producing a great deal of CO2 that is forming a constant flow up and out. For the "germs" to get in it would be like trying run through a marathon backwards...too crowded chances are you wouldn't make it very far. Believe it or not some people actually still do open vessel brewing (aside from sours of course).

 

[Pezzonovante11]

To add my .02 to the above: not only is air infection risk, but oxidation as well.

As I understand it, you want oxygen in the beginning while the yeast are propagating. The yeast use it up during this stage. Later, when the yeast finish multiplying, it isn't necessary. Not only that, if there is excessive oxygen present, the product will oxidize and taste "stale." That's why kegged beer pushed out with an air pump tastes bad so quickly vs. CO2.

Back to fermentation for a minute. Has anyone done an experiment comparing airlock vs. open yeast starter? I have to wonder if there isn't plenty of oxygen in the headspace (depending on the amount of headspace present, of course) for the yeast. I mean, we have to be talking about microliters of oxygen that these yeast consume, right? Are we fooling ourselves leaving off the airlock? How much air really transfers? Maybe the stir plate is a matter of increased contact area and not so much increased oxygen in the vessel. I'll bet some really smart people here could attempt to calculate that out. Any volunteers?

Boy, I think I am a true homebrewer now. I really like to mentally masturbate over this stuff :cross:

 

[Caveman3141]

I've made many starters in an old gallon milk jug and just setting the lid loosely over the top. I've never had a problem doing this. Remember, you're just making beer. It can be as complicated as you want but it can also be as simple as you want.

 

[mattd017]

If I had to guess the idea behind a stir plate I'd venture this:
Gas has a lot of liquidity to it. My thought is that as the wort vortexes, that movement transfers to the air above it (CO2), and causes the air to vortex. As it vortexes, it will push the vortexing CO2 up the walls of the flask and out, and will suck more air (from the lightly capped top) down the middle of the flask's opening. The new air, laden with O2, will be sucked down into the vortex, causing air exchange across the surface. Thus, increasing the dissolved O2.
My basis: Take vinegar and oil and pour it into a flask. Let it sit on the stir plate and watch as the two different and completely phobic liquids get drawn down into the vortex and mixed.

This is just from my basic understanding of chemistry/physics. I have no idea if it's true.

 

[Onlooker]

Maybe a noob question, but why is it ok to let a starter have contact with air, but in fermenter an airlock is used? Seems like contamination risks are the same ?

It's not about contamination. With a starter you also want to protect against contamination by having some kind of cover that won't allow dust particles, etc. to get in. That's how bacteria or wild yeast can be transported. Another threat is fruit flies as they carry the very potent acetobacter bacteria that will ruin your starter. That's where the empty air lock can be quite helpful; or the foam stopper as well.

But for a starter you want gas exchange to take place so that oxygen can be made available for the yeast which enables more and healthier yeast growth. We don't want that with our fermenting beer as that can cause oxidation and the resultant off flavors. Whereas we don't care what the starter "beer" tastes like.

 

[Joebrew892]

Sorry I know this thread is ~7 years old, but I'm just getting into brewing and I'm confused by the airlock conundrum. Why can't I use an airlock, but not put any liquid in it, so it in essence becomes a two way valve? Wouldn't this be the same as using foil?

 

[ScrewyBrewer]

Definitely foil. The idea is that nasties, that can contaminate your starter, can't climb. With that said simply covering the top of the flask with sanitized aluminum foil is enough to allow oxygen to enter the spinning wort, without fear of contamination. Plus it's a whole lot easier to do.

 

[lzotarelli]

Here some interesting article... http://braukaiser.com/blog/blog/2013/03/19/access-to-air-and-its-effect-on-yeast-growth-in-starters/