Will extended foil on starter allow mold growth?

So I tried to harvest Orval dregs, and stepped it up too quickly. Regardless, I covered my container with aluminum foil and had it on a stir plate. For the first few days there was very little activity, then I started seeing some yeast growth. After a few days of seeing stringy yeast floating around I decided to turn the stir plate off and leave the foil on. About a week in it seemed like the yeast growth definitely had slowed or stopped, with a nice new layer of what I assume is yeast on the bottom (definitely more than I started with), and a clean surface with nothing on it, however I left the aluminum foil on instead of changing to an airlock. A couple days later, I have some white mold growing on the surface. And yes, I am almost 100% positive it is mold (hairy circular colonies) and not pellicle or another type of yeast.

My question is can mold growth be caused by extended aluminum foil coverage after yeast growth/fermentation has stopped by allowing an exchange of air and letting new O2 get into the head space, allowing mold growth on the surface? Or is it caused by poor sanitation and the spores already being in the starter wort?

I'm going to remove the surface mold, decant the liquid, and pour the yeast cake into a new similar sized starter, but do this one 100% with an airlock or add an airlock after a day or two (since the yeast colony should already be the right size for the volume and wont have to grow much) and hope that without CO2 the yeast will take over and the mold won't grow again. Good, bad idea? I can always try to get another bottle of Orval but it was a rare sight at the liquor store that they had them.

Thanks!

i would not try to save yeast that shows signs of being contaminated unless you have a lab where you can isolate the yeast from the other contaminants. the contaminant was probably already inside the vessel or got in when if you lifted the foil off and it happened to be perched nearby waiting. the chances that it made all of the twists and turns to get under the foil are slim.

mattoak said:

My question is can mold growth be caused by extended aluminum foil coverage after yeast growth/fermentation has stopped by allowing an exchange of air and letting new O2 get into the head space, allowing mold growth on the surface? Or is it caused by poor sanitation and the spores already being in the starter wort?

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I exclusively use doubled over heavy duty foil on top of my starters, and I have let some of the wild yeast starter go for a long time, and I have never seen this. The fact is that there will be free exchange of oxygen and gases by simple diffusion and gas moving along it's gradient until a dynamic equilibrium is reached. Mold, however, will not crawl into your starter, and if it was sufficiently covered with aluminum foil, either poor sanitation was used in the flask or foil, the wort was not properly boiled or there was a hole in the foil.

To circumvent any sanitation issues, I pressure cook all my starter media at 121 C and 15 PSI for 15 minutes, which sterilizes my starter container, the media and the foil on top. I usually work with colonies off of a plate, which is why I do this. I would, however, recommend sterile wort with dregs too, as the viable cell count could be incredibly low.

I'll second Biobrewer. We use foil in the lab when growing bacteria. Nothing ever crosses the foil barrier. Your problem is with sanitation.

You might make sure that your foil is sanitized, too, since it could hypothetically carry spores. Either heat-sterilize or spray star-san on the side that faces your wort.

Thanks guys....

I used several month old star-san and wasn't too confident in it. I'll just have to get another bottle And then try again. Will just dump the whole batch.

i'm just going to put it out there but brett will look hairy when starting could be mold but brett may be your answer the pellicules some times look interesting.

I had a starter that I stuck in the fridge (still with tight foil) and I had a sudden death in the family the night before brewing so I didn't take the starter out of the fridge as planned. The foil was tight, with a rubber band around it, but when I came back to it 6 days later, it was moldy.

Just like with food in the fridge, the air in side the jar, even if tightly covered, can allow mold to grow.

Yooper said:

I had a starter that I stuck in the fridge (still with tight foil) and I had a sudden death in the family the night before brewing so I didn't take the starter out of the fridge as planned. The foil was tight, with a rubber band around it, but when I came back to it 6 days later, it was moldy.

Just like with food in the fridge, the air in side the jar, even if tightly covered, can allow mold to grow.

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interesting. i've have a 5L, foil covered erlenmeyer flask with a Brett B. starter in my garage for 3-4 weeks with nothing but a thin pellicle.