After a couple of batches of 'less than stellar' beer, I've done some thinking about what they had in common and have come down on the topic of harvested yeast. Just like a couple of cleaning and sanitizing threads here lately, I don't think anything I'm sharing is new but it's always good to have this discussion once in awhile. I've tried harvesting yeast from bottles of a commercial beer a few times with mixed success. The first time was from a seasonal beer (Bells Oberon) that was really fresh. Probably less than a month. I put it through a stepped starter and got enough to pitch a 5 gallon batch and enough left over for a couple more batches. I just washed the yeast, kept it in the fridge, and used a starter when I needed it. However, I had mixed success with these subsequent pitches of yeast. Just about a week ago I picked up a 12 pack of a year-round brew from the same brewery (Two Hearted IPA). I harvested the yeast from 3 bottles and went through the same process as before. However this time it didn't work out... After a couple days of no activity in the started I checked the date on the bottles and the beer was about 4 months old. While they still tasted good, the yeast wasn't viable anymore. After a few days, I did see some foam forming but it didn't look right. There was no dark schmutz on top like krausen and when I swirled it, the little bit of air that sneaks out around the foil stunk. After my first spoiled batch just a couple of months ago, I recognized the stink right away. It was infected! Correct me if I'm wrong, but my understanding is: there are infectious bugs EVERYWHERE, including your meticulously sanitized brewhouse. All that cleaning/sani work we do is to minimize the risk of infection, not eliminate it. The point is to have enough healthy yeast that it easily outcompetes everything else and ends up being the only microorganism left after a few days of fermentation. My thought with the spoiled starter is that the yeast either wasn't viable, or it was too old and sluggish that it lost the competition for resources and another wild yeast or bacteria took over. The Takeaway While harvesting yeast from commercial beers is a good way to try and get yeast that might otherwise be unavailable, it's a mixed bag. First of all, you have to make sure the beer is relatively fresh. Seasonals are good for this but a really fresh year-round beer certainly shows up at the bottle shop on occasion. Secondly, watch your starter closely. If fermentation doesn't get rolling right away you may want to think again. Even if the yeast wins the competition, a sluggish start might indicate an unhappy or tired yeast. Third, If you get a good pitch of yeast from commercial beer, don't bother with trying to save some for another batch. Again this goes back to the possibility of tired yeast. Coming from a commercial brewery there are a lot of uncertainties. Unlike somewhere that yeast production is the sole priority.