Yeast from Commercial Beers
There are many quality microbrewed beers on the market that are bottle conditioned, i.e. naturally carbonated and unfiltered, much the same as homebrewed beers are. The yeast layer from a bottle conditioned beer can be harvested and grown just like the yeast from a liquid yeast packet. This is a common practice among homebrewers because it allows for the use of some special yeast strains in homebrew that would not otherwise be available. This method can be used for cloning some of the specialty styles, such as Belgian Wit, Trappist Ales, or everyone's favorite - Sierra Nevada Pale Ale.
Harvesting yeast from a bottle conditioned beer is quite simple.
Step 1. After opening the bottle, thoroughly clean the bottle neck and opening with sanitizer to prevent bacterial contamination.
Step 2. Simply pour the beer into a glass as you would normally, leaving the yeast layer on the bottom of the bottle intact.
Step 3. Swirl up the sediment with the beer remaining in the bottle and pour the yeast sediment into a prepared starter solution as described in the previous section- Preparing a Liquid Yeast Starter.
For best results, add the sediment from 2-3 bottles and be sure to use the freshest beer you can find. The starter should behave the same as any other liquid yeast pack starter, though it may take longer to build due to the smaller amount of yeast that you start out with. In fact, you may not notice any activity in the starter for the first couple wort additions until the amount of yeast builds to higher levels. Add more wort as necessary to build the yeast slurry to pitching level.