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added two new sections for how old bottles can be and is there a list of bottles
===Potential for Mold Growth===
Mold growth, although rare, is a possibility. If mold begins to grow, throw out all of the beer and start over. See [[Mold]] for examples of identifying mold versus pellicles.
===How Old is Too Old?===
A frequently asked question is, "How old of a bottle can I use to grow dregs?" This is ultimately an impossible question to answer, but the general advice is to try and use a bottle that is as young as possible, preferably less than one year old. Although ''Brettanomyces'' and lactic acid bacteria can survive for long periods of time in bottles, the conditions are still stressful on these organisms and not enough information on viability over time has been collected to answer this question for every given bottle of sour beer. Old bottles might not lead to growth in the starter. Signs of fermentation should be observed, including visual signs like cloudy wort, bubbles or a pellicle on the surface, and measured signs such as a drop in pH and gravity in the starter (measured signs of fermentation are better indicators of active fermentation than visual indicators). That said, ''Brettanomyces'' has been cultured from bottles of beer that are 10+ years old. For example, microbiologist [https://www.facebook.com/groups/MilkTheFunk/permalink/1352210248140519/ Richard Preiss has been able to culture ''Brettanomyces'' from very old bottles of commercial German Berliner Weisse]. Culturing microbes from very old bottles like these often requires agar plates and streaking techniques. If there are not enough viable cells in an older bottle of commercial sour beer, using a simple starter to grow them may fail because the starter may fail to ferment fast enough before other microbes such as mold take hold. If this is the case, for safety reasons do not consume the wort and consider growing the microbes from a different, fresher bottle. Using autoclaved or pressure cooked wort and jars might help with preventing other microbes from growing.
===Is There a List of Viable Bottles?===
The only list of viable commercial bottles that we know of is [http://www.themadfermentationist.com/p/dreg-list.html Michael Tonsmeire's list of unpasteurized commercial sour beers.] Tonsmeire has stated that he will no longer maintain this list because of the effort needed to do so. For this reason, we also do not track unpasteurized commercial sour beers. If you are curious if a particular brewery pasteurizes or kettle sours their sour beer, it is recommended that you contact the brewery and ask them if the bottles dregs can be used to make sour beer. Most breweries will be friendly enough to answer your question.
==See Also==