Originally Posted by baer19d
I don't know if this is even possible so I'm going to ask the question. I propagate my own yeast and was wondering if it's possible to contaminate the yeast in a peatry dish? I know that when foreign microbes get in the dish the will start their own colony along side the yeast, but will they somehow mix with the yeast and ruin it? The last batch that I brewed ended up with a sour taste and I have my doubts that I took enough care when I propagated the yeast even though there doesn't appear to be anything else growing in the dish. I just want to make sure before I through it out and buy more yeast.
Yes and no. It depends on the type of contamination. If a yeast culture is contaminated with another yeast culture, it is possible to separate them, assuming you can tell the difference. All you need to do is plate them out after they have been sufficiently diluted and you can then pick out the desired strain colony.
But, if the infection is a mold, I wouldn't even open the petri dish. Certain molds have a tendency to produce spores. You can sort-of tell if it's producing spores because the colony appears fuzzy on the top. This means there are spores floating around in there, invisible to the naked eye. Your culture is unsavable. You can try, but open at your own risk.
From the sounds of your 'sour taste' I would guess there is some bacteria. Bacteria do not produce spores. Usually you can tell the difference between bacteria and yeast colonies. Yeast colonies should be round and colored like your yeast. Bacterial colonies can vary significantly.
I hope that helps.