A very natural question when you set out to randomly culture fungus and bacteria. How do you know you’re not getting something dangerous? There are some pretty deadly microbes out there, especially bacteria (though several kinds of molds and fungi will kick your can too, now that I think about it). What’s to keep the bad guys from taking up residence in your sourdough starter?
The answer is that it’s not just any kind of microbe that can survive for very long, much less reproduce to any extent, in a mixture of flour and water. Sure, every Tom Dick and Harry will make a go of it in the beginning, but in the end only a very few can succeed, and none of those will do you any harm.
Why? Well as I’ve mentioned many times before, yeasts for the most part do only two things: reproduce and eat. The waste products of all that eating are a) carbon dioxide gas and b) alcohol. The CO2 they give off dissipates for the most part, but the alcohol, it accumulates. So much so that it poisons other types of critters trying to make a living in the starter bowl. As the alcohol level rises, competing yeasts are knocked out (or their activity is suppressed) until only the most alcohol-tolerant yeast is left standing. The vast majority of the time it’s Saccharomyces cerevisiae, a totally harmless — even nutritionally beneficial — microbe that just also happens to be a great leavener. Kind of amazing, no?
The same thing happens on the bacteria side. Not every bacterium can exist on a diet of pure starch. Of the kinds that can, very few indeed can tolerate an environment that contains either a) lots of alcohol or b) lots of acid. A sourdough bread starter is of course chock full of both. The alcohol as you now know comes from the yeast, and the acid from all those lactic acid-producing bacteria I’ve been talking about. The more they make, the more other types of bacteria are either killed or rendered inactive, until just a few viable varieties remain — and none of them can hurt you.
Though just to be on the safe side you never want to actually eat a sourdough starter in the raw (the act of baking essentially sterilizes bread, at least in the short term). As I mentioned yesterday, the system isn’t infallible, and on occasion weird cultures have been known to grow. Know them by their orange-ish and/or blue-ish colors, weird blotches or colored specks, and/or their rank odors.
I should insert here that while no yeast-bacteria leavening combo is actually known to be dangerous, some taste better than others. Some can be downright awful. A good starter should smell like bread with a tinge of alcohol. But should you sniff your starter after a week and it smells rank or vomitous, odds are you’ve cultured some sort of butyric acid-producing bacteria (the stuff that gives some kinds of cheese their vaguely vomit-like smell), and while they won’t hurt you, the bread they makes is unlikely to garner rave reviews from your dinner guests. Throw it out and try again.