The Great Scientific Siege: Can yeast defeat bacteria?

Hi, guys. I'm Caleb. First time here.

I've got a lovely starter which in the past has demonstrated some lovely banana aromas, so I was thinking of isolating the yeast in order to see how a fermented beverage would turn out using it. Now, I'd rather not have the sourness from the Lactobacilli and such, so I was wondering if I could get some advice on allowing the yeast (S. exiguus? C. milleri?) to survive and dominate the starter, while also somehow reducing the amount of bacteria to contribute either only a mild sourness, or simply kill them off entirely.

I suppose a cheating, easy-way-out way to do it would be to find a non-toxic, anti-bacterial-but-not-anti-fungal thing to put in it, but I doubt such a product exists. If it does, please let me know. :)

I know it'll be a tricky task, but I think a siege approach will work if continued for a long time.

So, from what I've read, cold temperatures promote wild yeast growth, or rather discourage bacteria growth. So what I've done is pour a small amount of starter (I have plenty of it left if you want me to do it a different way) into a new container, add lots of water and a big tablespoon of raw sugar to it. I put it back in the fridge with the lid on overnight, then this morning it occured to me that it might prefer the more complex sugars in flour, so I dumped a few tablespoons of cheap white flour (10% protein) into it, shook it up and put it back in the fridge.

 Is there anything else you recommend I do? Any tips of any kind?

 

Muchly appreciated. I look forward to reading your replies.

 

Caleb

 

 

 

8 comments

 What are you going to ferment with it?

"He is no fool who gives what he cannot keep to gain what he cannot lose." Jim Elliot

 

I'm not sure, possibly a braggot from wheat or barley malt, maybe one of each to test which I like better, or maybe just two wheat braggots, one with sourdough starter and one with a recommended beer/mead yeast.

Unless you would recommend a certain wine that ferments favourably with sourdough cultures...?

 Ok lots of ideas to ponder with this idea.  The easy way to isolate the yeast from the bacteria would be to let the bacteria kill itself off.  The bacteria will keep making acid until it reaches a level that is fatal to itself.  In the wine industry we don't like lactobacillus so we keep the pH of the wines under 3.75 for the most part.  Now that probably isn't what you want to do but the information my be useful to you.  I make wine without adding yeast because I use the yeast that is on the skins of the grapes.  There are also many other micro organisms that are in the grape must that will add flavor and complexity to the wine.  Now at work we kill all this stuff off and inoculate with a know yeast strain so we get a proven product so it will be lacking some of the add flavors and complexity when done this way.

I have been thinking lately that when Louis Pasteur isolated yeast he really took the flavor of foods backwards because he ignored the other micro organisms that add some much to the end product.  We see this very clearly in yeasted bread vs sourdough bread.  I think we are now seeing trends in winemaking along these same lines but not to the degree that we see in bread.  Now beer is something I know very little about but I know it was make long before Louis Pasteur so I am interested in how beer was made before him.  I'm thinking that it should prove to be very interesting.

There are lactic fermented drinks if you want to use your starter that way but I have never messed around with them.

"He is no fool who gives what he cannot keep to gain what he cannot lose." Jim Elliot

 

Thank you very much LeadDog for your thoughtful response. :)

The main thing I'm worried about is having a beverage so sour that I can't taste the other complexities, as I have often found when tasting my liquid sourdough starter. Are you suggesting that lactobacilli can't cope with the pH they create, or the levels of lactic acid specifically? I could add lemon juice or vinegar if it's the pH you're referring to.

If I let the must/wort ferment to sourness in order to kill off the bacteria, how do I know if it's worked? Do I simply keep feeding it until it tastes sour, then hit it with an acid (lemon juice, vinegar, or winemaker's acid blend), then wait a day or two, then re-feed? Would that work?

Regards,

Caleb

 

PS: In beers, the hops added has an antibacterial element to it so that the yeast gets to ferment the wort, so theoretically I could just add a bunch of hops to the starter sample and feed it as normal, but this would be more expensive that the above stated possible solution.

 Right the lactobacilli if they keep producing acid will make the pH so low they will kill themselves off.  This is one reason that sourdough starters need to be fed.  When the starter is fed the pH rises.  You can lower the pH by adding acid but I really don't think this is the direction you want to go.  What did brewers do 200 to 300 years ago to make beer?  They had to have a way to keep the beer from going sideways.  I'm a completely ignorant when it comes to beer making.

"He is no fool who gives what he cannot keep to gain what he cannot lose." Jim Elliot

 

The home brewers know how to propagate yeast strains.  It's all over the home home brew sites.  I forget what it's called atm, but they wash their yeast with an ingredient that kills off any clinging bacteria without adverse effects on the yeast.  Makes for a pure yeast culture.  I've also read that Italian bakers wash their starters precisely for the same reason you'd like to.

Ps - I've fermented molasses washes with my sourdogh culture and it can make for a difficult fermentation.  Two  ways around potential problems is to mix a low gravity wort coupled with a rather large and very acitve 12 hour starter.  You want the whole thing over in a hurry, say 2-3 days.  Then rack off of the trub and then the lees.  A standard beer wort would work best, say 5-6% abv.  If the fermentation goes sideways, as in it takes too long to complete because of low temp or an excessively sour mash/wash, the bacteria will win.   It always does with me.  Good luck.

[quote=Panevino]

The home brewers know how to propagate yeast strains.  It's all over the home home brew sites.  I forget what it's called atm, but they wash their yeast with an ingredient that kills off any clinging bacteria without adverse effects on the yeast.  Makes for a pure yeast culture.  I've also read that Italian bakers wash their starters precisely for the same reason you'd like to.

Ps - I've fermented molasses washes with my sourdogh culture and it can make for a difficult fermentation.  Two  ways around potential problems is to mix a low gravity wort coupled with a rather large and very acitve 12 hour starter.  You want the whole thing over in a hurry, say 2-3 days.  Then rack off of the trub and then the lees.  A standard beer wort would work best, say 5-6% abv.  If the fermentation goes sideways, as in it takes too long to complete because of low temp or an excessively sour mash/wash, the bacteria will win.   It always does with me.  Good luck.

[/quote]

Thanks for the reply, Panevino.

Do you mean this (see link)? That could work, and is in essence what I was suggesting in my previous post.

acid washing and ammonium persulfate

Thanks for the tip about pitching, too. I'll be sure to get my starter fired up and ready before pitching the jar into a wort, but I'll transfer the yeast to a fresh starter made with the actual ingredients first. :)

I don't actually know first hand but it sounds like you have a handle on the situation.  I was just pointing.

That could work, and is in essence what I was suggesting in my previous post.

I see, I thought you were saying to increase the acidity of the wort.

 

Good luck