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Old 06-11-2013, 12:55 AM   #1
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Default Bad genetics or contamination in starter?

Hey,

So I made a starter a few days ago from slants I had prepared a few months ago from Wyeast English Ale. I took a couple inoculating loop-fulls of yeast from the slants and stirred them around in a sterilized flask with about 200mL wort from DME. I put the flask on a stir plate and let it stir overnight. The wort looked cloudy the next day so I transferred to a 1000mL flask to build up a bigger starter. When I was going to brew today, I sniffed the starter and it sure had a funky smell, not very yeasty at all. I don't remember the English Ale smelling that funky, so I opted not to use the starter.

My question is, was the flask likely contaminated? I feel that I am pretty good at sterile culture and have not had a contamination problem ever before. And I don't see any signs of it like floating mats of bacteria and the like. Or is it likely that I could have ended up with some foul smelling yeast due to the founder's effect of the initially small number of cells on the slants and used to innoculate?

Also, any easy tests that can indicate that what you've got in your flask is indeed yeast?

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Old 06-11-2013, 04:17 PM   #2
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My first question is how did you store your slants and at what temp?

Your nose is a good indicator, but you can taste a sample as well. It is surprisingly easy to contaminate cultures, so it is certainly possible.

The easiest ways to check your culture would be to streak a plate or to look under a microscope. Both of which still require a fair amount of equipment and effort.

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Old 06-13-2013, 07:14 PM   #3
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Thanks for the reply!

My slants were stored in a cardboard box packed with paper in the fridge at ~32F. Is that right?

I did take a little taste and it had some off flavors, pretty ester-y imho. It didn't taste absolutely disgusting though =/

What would I look for in a scope? I'm assuming the biggest indicator would be size differences (really big yeast cells in comparison to bacteria). So would I just make a w.m. and look between the yeast cells with a high objective to see if I see any bacteria? Would a gram stain be appropriate to indicate the presence of bacteria, or would it stain the yeast too?

Also, I remember learning in biology that two organisms cannot share the same niche. In the experiment, two species of microorganism were placed in a petri dish and eventually one out-competed the other and was the only remaining species in the dish. I'm curious if this applies to brewing: if I've got a contaminated starter (multiple organisms in one niche), will one organism eventually out-compete and come to predominate the starter?

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Old 06-13-2013, 08:21 PM   #4
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Excellent post, and good questions.

Freezing or below is the best condition for long term storage of yeast, although you can't do that on slants. I bet you lost a fair amount of viability during storage, which might explain why your starter yeast seemed stressed. Good decision not to use it, why use bad yeast on good wort, right!

Under the scope you would look at size and morphology of the cells. A wet mount and high objective would be best, like you mentioned. You could gram stain, it may provide a better image. Unfortunately, yeast spores look a bit like round bacteria and will stain negative, while active cells will stain positive and be larger.

Your last point is basically accurate. Many brewing techniques (especially sours) simultaneously use multiple genera of microbes to achieve various flavors. In most beers, the yeast are able to eventually out-compete the contaminating bacteria by producing ethanol, although the bacteria are faster growers and could take hold before the yeast. If you pitched a contaminated starter, the bacteria could have a chance to grow first, before the yeast start producing alcohol.

Have you thought about freezing yeast? It is probably less work than slanting, and you get better long term viability.

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Old 06-15-2013, 05:59 AM   #5
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Yes, I have thought about freezing yeast, but all of the info I have found on it has been a little iffy. From what I understand, you freeze the yeast in a liquid medium with some sort of cryo-protectant (glycerine?). What technique do you recommend? How do you revive the yeast after their time in the freezer? I've wanted to do this because, as you mentioned, making slants and reslanting yeast after each batch is a little labor intensive.

Thanks for your informative replies!

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Old 06-18-2013, 01:59 AM   #6
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That's basically the procedure.

Make a starter at about 1.040, and let it ferment until it's done.
Cold crash and decant off the wort.
Add glycerol/glycerin to 10% of the total volume that's left.
Transfer to tubes and freeze as cold as you can get.

I make 50ml samples, so I get about 10 out of each batch.

A week before brewing, thaw one vial and make a starter. The volume depends on the beer you are planning to make (mr.malty.com is good for this).

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Old 06-21-2013, 04:04 PM   #7
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Cool, seems much less labor intensive than slanting! So rather than make a starter, I am assuming it's ok to take the yeast cake from my primary, wash it, then use that for my tubes. Would I be right in my assumption? I'm going to try it out as soon as one of my primaries is done. Thanks for the help!

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Old 06-22-2013, 03:58 AM   #8
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Washing the yeast out of primary is an option. Unfortunately, it requires quite a bit of time. Also, the yeast have already lost a generation, and the general environment of a full beer, as opposed to a starter, can result in less than the optimally healthy yeast you would be looking for if you were making yeast stocks.

For culturing your freezer stocks, I would make a starter and create stocks from that. Washing yeast is another animal.

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Old 06-25-2013, 08:35 PM   #9
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Thanks for all your help! I'm making a starter today, then plan on freezing some stock in the next few days.

About yeast generations... how many gens is too much? I get the idea (genetic drift/bottleneck effect/etc.), but how do companies like white labs keep culturing their yeast over and over again without having the same problems? Is it simply that they have such a huge population to begin with (compared to the homebrewer's) that genetic drift isn't an issue?

Thanks again for all the informative biology!

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Old 06-25-2013, 08:37 PM   #10
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Thanks for all your help! I'm making a starter today, then plan on freezing some stock in the next few days.

About yeast generations... how many gens is too much? I get the idea (genetic drift/bottleneck effect/etc.), but how do companies like white labs keep culturing their yeast over and over again without having the same problems? Is it simply that they have such a huge population to begin with (compared to the homebrewer's) that genetic drift isn't an issue?

Thanks again for all the informative biology!

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