The Microbiology of Bread Starters

So what’s going on in a starter bowl? The answer: lots of stuff. Beneath that happy, bubbly veneer there’s war is going on, between all manner of lower forms of life, all of them vying for supremacy. Sort of like the presidential primaries. Bu-dum-bum.

Starters are not what most people think they are: cultures made up solely of “wild” yeast. Nor are they a simple tag-team of one type of wild yeast and one type of flavor-creating bacteria. Rather they are a brewpot of dozens (even hundreds) of kinds of both (plus the odd mold or two thrown in just for fun). There are however certain strains that dominate according to conditions of temperature, moisture, pH and the availability of food. On the yeast side it’s usually — but by no means always — Saccharomyces cerevisiae, “sugar fungus of the beer-making kind” in translation from the Latin, the fermentation world’s most common (and popular) microbe. On top of the bacteria heap are usually two or three types of lactic acid bacteria, which outnumber the yeast by about 100-1, and vary so dramatically from place to place even the most committed microbiologist could never catalog them all.

But what are all those bugs actually doing in there? Well, if you’re a yeast, you’re consuming simple sugars. Flour as I mentioned yesterday contains enzymes — enzymes which, in the presence of moisture, begin to dismantle starch molecules that the germ (sprout) would normally use to feed itself. Of course in this case there is no germ, it’s either been removed (in the case of white flour) or ground to powder (in the case of whole grain flour). But then the enzymes don’t know that. They’re simple organic molecules with a job to do. They set down to work slicing those big indigestible starches down into easy-to-handle simple sugar molecules. The yeast dig right in, gobbling down the sugars and giving off carbon dioxide and alcohol in return.

In the meantime, the lactic acid bacteria are doing much the same thing. Except when they consume sugar, they give off mostly lactic acid, the tangy tasting compound that gives yogurt its zing and bread quite a bit of its flavor. Pretty neat really, though there is so much more to the story.