How hard/easy is it to infect a yeast starter?

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StarCityBrewMaster

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When we ferment a batch of beer we use an airlock so that the outside air/contaminants can't get into the wort and infect it while allowing co2 to escape.

Yet, from my understanding, yeast starters are supposed to be made using aluminum foil as a top so that air (oxygen) can enter the starter to help with yeasts reproduction.

It just seems to me that this would be a fairly easy way to infect a batch of beer if in fact the starter becomes infected itself.

Am I way off?

I have my 2nd ever yeast starter going right now, hints my concern! Brewing as I type :)

Warrior, Amarillo & Simcoe is what's on the menu!
 
Another reason for my concern is that my yeast starter has a tiny off smell. It's nothing overwhelming or terrible it just doesn't smell as good as a fermenting batch of beer.

With that being said I have been using notty and safale dry yeast for that past 8 months and I am now using WL001 California Ale yeast for the first time which I understand could be why.
 
If it was so easy for beer to get infected from the air, you'd still have that chance with an airlocked fermenter: as there's quite a bit of headspace above the unfermented wort. Keep in mind that we were making drinkable beer long before the discovery of yeast. No brewer keeps complete sterile environments because there is no need for that. The main thing that's needed to reduce off flavors from unwanted bacteria, is to start off with a larger and healthier yeast colony. Naturally fermented beers take a lot longer to start their ferment because they are waiting for the cells to build up.
 
Another reason for my concern is that my yeast starter has a tiny off smell. It's nothing overwhelming or terrible it just doesn't smell as good as a fermenting batch of beer.

With that being said I have been using notty and safale dry yeast for that past 8 months and I am now using WL001 California Ale yeast for the first time which I understand could be why.

Yeast starters have a different smell then beer because there's no hops in them. Most my starters smell like yeasty sourdough bread. If your starter smells like that, or bannana, or even a sulfur smell....there's still no cause for worry.

I just think instead of getting worked up about your starter, you should RDWHAHB!
 
I always use foil over my starters and will sometimes cover my carboy opening with foil instead of an airlock for the first few hours of fermentation if it is going to be cooling a few degrees to prevent suck back of liquid from the airlock.
No problems so far.
I think as long as stuff can't fall down into the opening you are ok.
 
It is harder to infect a starter because you are pitching such a relatively large amount of yeast. For example, a smack pack is on the low side for 5 gal. but it is an overwhelming amount of yeast for 1-2 qts. If other beasties are in there, 100 billion yeast will simply crowd them out. But you can put an airlock on a starter if you aerate well if you want.
 
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