Yeast Isolation from Contaminated Batch

[Scut_Monkey]

I was wondering the other day if it would be possible for me to reisolate my yeast from a contaminated batch of cider. The batch is most definately contaminated with acetobacter because it has turned to vinegar. My approach is going to be the following after I bottle the cider/vinegar (hey you never know how it might turn out).

1. Collect the yeast and trub from the secondary carboy.
2. Do a quick yeast wash to remove protein
3. Plate onto sterile wort agar to allow single colony forming units to propagate.
4. After 24-48 hours isolate 1 maybe 2 yeast colonies only if there are no overt signs of bacterial growth around them.
5. Pitch these yeast colonies into 25ml of wort at 1.030 gravity and allow 24 hours on stir plate.
6. Step up 25ml starter to roughly 150ml.
7. Innoculate sample from 150ml starter after 24 hours onto sterile wort agar. to check for no bacterial contamination and healthy yeast colonies.
8. Step up 150ml starter to 500ml then 1000ml.

Now I realize that yeast is not that expensive and most people would simply dump it out and not even risk the chance of carrying the infection to another batch of cider. However, I'm cheap and I like to test things. I think this would be a great way to challenge my sterile technique and as a proof of concept. Besides if I plate the yeast a second time this should be a good way to check to see if I did indeed loose the bacterial infection. Let me know what you guys think or if you have experience with it.

 

[menschmaschine]

I haven't done this specifically, but that sounds good. Step 7 may be overkill... especially if you use a good streak plate method. An acid wash would make steps 2 and 3 go better. A microscope would also be good to check for yeast health.

 

[Joe Camel]

+1 on the acid wash, you could also pick a single colony from the first plate and restreak it on a new plate to ensure that there was no bacteria tagging along

 

[Freezeblade]

what pH should you shoot for on an acid wash?

 

[Scut_Monkey]

Most people say 2.2 for approximately 2 hours. I don't have any experience doing it and don't have any concentrated acid to do it. I think most people recommend using phosphoric acid. For my purposes I will be doing a sterile water wash twice. I think Joe Camel had a good recommendation for my purposes of innoculating the yeast onto wort agar and then transfering a colony onto another wort agar plate to almost ensure a clean yeast culture. I think I'll try this method rather than my step 7.

 

[fratermus]

I'd streak it out until you have a clean colony (a la joe and mensch) then treat it however you normally do.

I have read that acid wash may do more harm than good, especially if you can isolate the yeast cells manually from the streaks.