Do stir plates make starters sour?

I just want you to know this is not a panicked thread where I am debating wether or not to keep my yeast. From what I have read sour smells dont mean the starter is bad.

That being said.....i made a starter yesterday afternoon with WLP001. I have used this yeast many times and have made a starter every time I have used it. The starters usually have a strong yeasty/bready smell (which I kinda like). I just gave my starter a sniff and it smells sour. Best way I can describe it is like sour dough yeast. The only difference from my other starter and this one is on this one I used a stir plate. I have never used a stir plate before so I was wondering if this is normal. The only thing I can think of is that the beer is more oxidized because of the stir plate. Could that cause a sour smell?

They can often taste/smell a little "sour" from the constant aeration. It is different than an actual sour, but is quite similar. I threw out my first starter from a stirplate because of this same issue. Now I know this is quite normal.

I've noticed this somewhat sour smell as well and I also noticed that it varies from none at all to easily detected. It's not the same as full on sour as with an acetobator contamination can be. Early on, I tossed a starter because of this smell, but soon learned that it is not all that unsual. I also think that a starter made at higher temperatures will exhibit more of this phenomenon.

Made one last night; stirred throughout the night and smelled fine when pitched a few hours ago.

I guess im just wondering whats going on chemically to create the sour smell and taste. I would think it has something to do with the oxidation but ive never heard anyone associate sourness with oxidation before. Usually people describe a bland or wet cardboard taste.

Just to add on.....i stepped my starter up this morning. I made 2L of starter wort and it is fermenting away on my stir plate. It still has the sour smell and I cant figure out why. I decanted all but a small amount of liquid off of the original stater. Also i only made it two hours ago. So I guess that kinda throws my oxidation theory out the window. Does the sourness usually carry over right away? Im confused.

Here's a good piece of information to consider:

Most starters are made with ale yeasts, and you want a higher fermentation temperature relative to consumption temperaturs when making yeast starters. This usually is around 70-80 degrees F. Keep in mind that while these aren't ideal fermentation temperature ranges for DRINKING the product, they do make for some awesome yeast sexing conditions. The sour, off, stinky flavors is a direct result of slightly-higher-than-normal fermtemps, so I wouldn't be concerned.

If it smells like rotten meat, though, your yeast may be dead and you should toss it!

JonBoy47 said:

Just to add on.....i stepped my starter up this morning. I made 2L of starter wort and it is fermenting away on my stir plate. It still has the sour smell and I cant figure out why. I decanted all but a small amount of liquid off of the original stater. Also i only made it two hours ago. So I guess that kinda throws my oxidation theory out the window. Does the sourness usually carry over right away? Im confused.

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You want as much oxygen going into your starter as possible. That's what the stir plate is for!

If your water comes from a clean, filtered source, your wort has been boiled and cooled, your yeast isn't dead, and you see typical starter effects (krausen, bubbles, lightening of wort color), then just let it sit! Don't mess with it!

I recall a Basic Brewing episode where they brewed base malts without any hops, and all they got was sour beer. Would that have anything to do with it?

dantheman13 said:

I recall a Basic Brewing episode where they brewed base malts without any hops, and all they got was sour beer. Would that have anything to do with it?

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Not necessarily with the starter-- you're not supposed to hop starters.

But, hops contain acids that kill bacteria. That's why they were used when regular drinking water may have otherwise harmed you. Beer was made so that you could hydrate by drinking something that wasn't septic. So the water was boiled (to kill the bacteria inherently in the water), and the wort was hopped (to prevent bacteria growth).

It makes sense that hop-less beer might sour. Wort is a wonderful breeding ground for bacteria + fungi and without hop oils, they'd have a smorgasbord.

JoePro said:

Not necessarily with the starter-- you're not supposed to hop starters.

But, hops contain acids that kill bacteria. That's why they were used when regular drinking water may have otherwise harmed you. Beer was made so that you could hydrate by drinking something that wasn't septic. So the water was boiled (to kill the bacteria inherently in the water), and the wort was hopped (to prevent bacteria growth).

It makes sense that hop-less beer might sour. Wort is a wonderful breeding ground for bacteria + fungi and without hop oils, they'd have a smorgasbord.

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Yeah, I was mistaken on my recollection of the Basic Brewing experiment. I went back and looked it up, and it turns out that the beers without hops that were fermented with dry yeast were sour, whereas the exact same beer fermented with liquid yeast was not sour. Here is the reference:

http://www.basicbrewing.com/index.php?page=august-8-2008---base-malt-experiment-iii

JoePro said:

Here's a good piece of information to consider:

Most starters are made with ale yeasts, and you want a higher fermentation temperature relative to consumption temperaturs when making yeast starters. This usually is around 70-80 degrees F. Keep in mind that while these aren't ideal fermentation temperature ranges for DRINKING the product, they do make for some awesome yeast sexing conditions. The sour, off, stinky flavors is a direct result of slightly-higher-than-normal fermtemps, so I wouldn't be concerned.

If it smells like rotten meat, though, your yeast may be dead and you should toss it!

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Not quite sure what rotten meat smells like but im pretty sure its not that. Smells more like a yeasty sour milk. And you are probably right about the temps. The starters have been fermenting at about 76 to 78 degrees.

Well another addition to the saga....this morning I turned the stir plate off and checked on the starter. It has now developed a yeasty vinegar smell. The vinegar smell is not very strong but it is definately present. I have read several posts on vinegar smells and it seems the opinions are mixed as to whether the starter is any good or not.

After more reading on vinegar smells it seems the consensus is the presence of acetobacter. Now my understanding is that aceto requires oxygen to convert ethanol to vinegar. Is this an issue for an actual batch of beer? If the yeast quickly consume the available oxygen in the wort and begin producing co2 then the aceto should not be able to ruin your batch of beer as it dosnt have the oxygen needed to produce vinegar. Any thoughts?

JonBoy47 said:

After more reading on vinegar smells it seems the consensus is the presence of acetobacter. Now my understanding is that aceto requires oxygen to convert ethanol to vinegar. Is this an issue for an actual batch of beer? If the yeast quickly consume the available oxygen in the wort and begin producing co2 then the aceto should not be able to ruin your batch of beer as it dosnt have the oxygen needed to produce vinegar. Any thoughts?

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The problem is, IMO that the acetobacter may get a leg up on the yeast out the gate and do their damage early on while oxygen is available. I think we can detect relatively small amounts of vinegar in the beer fairly easily.

I would taste the starter as well as giving it the sniff test. The vinegar taste should be very overt if it is an acetobactor contamination.

I'm guessing at all of this, so YMMV a lot.

Catt22 said:

I would taste the starter as well as giving it the sniff test. The vinegar taste should be very overt if it is an acetobactor contamination.

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I will taste the starter this weekend after it has been in the fridge for a day or two. Even if it is infected im not going to throw it away. I cant waste $6 worth of yeast! Ill have to find something to pitch it into!!!