I’ve just been browsing a website where the sourdough bread is labelled “Yeast Free”. They couldn’t be more wrong. OK, I accept it’s a genuine mistake, based on lack of information (which I’ve emailed them to put right – nicely!). More worrying is that their baker might have told them that.
A sourdough culture is created from the natural yeasts and enzymes in the wholemeal rye flour used (or sometimes by exposing the dough to a favourable, yeast-rich, environment, like a plum orchard, as one US sourdough baker does – I wonder if he later fishes out the insects and bird crap?).
There is no added yeast in sourdough, as there is in ordinary bread, but there is still yeast, albeit a wild strain, or even several strains (of course, all yeasts once were wild). There’ll be enzymes, too, because before yeast can ferment your dough, some of the starch has to be converted to sugar – that’s the task of enzymes, like diastase, which gives its name to diastatic malt – use too much of that and your dough will be a sticky, sugary blob. That’s also what gives malt loaf its gooey, sticky, texture.
This – being less robust than commercial yeasts, which have been bred for potency – is why sourdough can take longer to prove than, say, plain white. But, just as in the plain white, the leaven is yeast.
If something ferments, you can be pretty damn sure it’s yeast doing the work* (if a fermentation smells beery, it’s yeast, and a sourdough culture smells like essence of brewery**). Which makes me wonder – if not yeast, what do they think is in the sourdough culture to pump up the bread? Tiny men, with really tiny pumps, maybe? Buggered if I know, but if you’re going to put medically-significant labels on your bread, you’d better damn well know…
*Bacterial fermentations normally take place in the gut, particularly of ruminants, and in industrial plants. Not in bakeries.
**Yeast converts sugars to alcohol and CO2 . The former accounts for the smell, and is driven off by the baking process, and the latter, trapped by the gluten, inflates the bread.