I never thought I did an especially good job of leading my students at The Culinary Institute of America to understand that knowing why food behaves in the ways that it does when we manipulate it, is just as important as knowing how to cook. I think at best I probably reinforced the belief that consistently positive outcomes in the kitchen are not possible if you don't understand the science to a few who already believed it. But I'm sure that my entreaties to question everything that happens and to not take anything for granted was lost on the majority of students who, in all fairness, were mostly of the popular opinion that an Institute was not, like a college or university, supposed to encourage skepticism as much as train them in a specific set of tasks.
However, expectations of the school and my role as a teacher there aside, the truth remains that unless you always cook from carefully vetted recipes in the same place on the same equipment with ingredients that are produced and stored under the same conditions, unexpected things are going to happen. And forget it if you, like most cooks, like to fiddle around with or develop your own recipes.
This point was driven home to me this morning during a reading of a method for making sour dough starter at Michael Ruhlman's blog. The method he described involved adding a cabbage leaf to a mixture of flour and water, letting sit overnight, dosing it with flour and water again and letting it sit for another night before using it. I don't doubt for a moment that the method produces the vigorously bubbling starter that he describes, but I'm not sure why it bubbles faster than a simple mixture of flour and water left to ferment for the same period of time.
The obvious answer is that there is yeast on the cabbage leaves that is introduced to the starter. But why would we assume that the yeast that lives on cabbage is capable of colonizing wheat? There are thousands of species of yeast, many of them quite host specific and not all of them capable of digesting starch. There is very little starch in cabbage so why would cabbage host significant number of starch digesting yeast?
Another possibility is that the cabbage is adding in invert sugar which the yeast gobble up. However, neither Ruhlman nor the person he learned the method from (Carri at Two Sister's Bakery in Homer, Alaska) report crushing the leaves to release sugar from the cells which, I think would be required to extract enough of the cabbages' measly <3% sugar content to produce the dramatic bubbling they report.
It is possible that what is introduced to the starter by the cabbage is some type of bacteria. Bacteria will produce gas just like yeast, and if the right kind are introduced will drop the pH and make the starter sour. One type of bacterium that is always found on cabbage that has not been cooked or fumigated is Leuconostoc bacteria which produces prodigious amounts of bad smelling gas. However, if Leuconostoc bacteria is present in Ruhlman's starter culture he reports no off odors. Not yet at least.
Not knowing what the cabbage is doing in his starter is not great. Professional bakers have been making starter in very specific ways from flour only for generations because that is the best way to assure a microflora that be built exclusively of a specific population of yeasts and lactic acid producing bacteria. I would not be at all surprised to find that in a few days he discovers that his starter has a big colony of funky smelling bacteria blooming on its surface. I've had that happen to me a few times after I've added something unique to my starter.