Subject: 21. Sourdough Science 101 or How are the sourness and leavening of starters related?
Don't let the subject scare you off. My kids tease me that since I left teaching (biology among other subjects), I have to find other people to listen to me. They are the usual targets. I'll try to be gentle - and practical.
There have been several posts over the last little while asking the same question in different ways - how are the sourness and leavening of starters related? Some starters seem too sour, or not sour enough, or have lost their sourness, or are sluggish or too active (not a problem for most) and folks want to know how to manipulate this. It also has been pointed out that lactobacilli are anaerobes, but this needs expansion. Here are some thoughts on this.
I am a homebrewer, and have read a good bit about yeast growth. Baking and brewing yeast are just different strains of the same species, but wild yeasts are different species, and some are even different genera, so this may not apply to all, but I suspect it does.
Cultured yeast needs oxygen to reproduce, so once it has depleted the oxygen in a starter/sponge/dough, it has pretty much reached the population it's going to have. After this, it shifts its metabolism to anaerobic. Assuming that wild yeast are much the same, this means that letting a starter or sponge sit longer is not going to result in much more yeast, and therefore will not increase its leavening power. It will become more sour (see below).
Lactobacilli are facultative anaerobes (as opposed to obligatory anaerobes), so they will continue to metabolize and reproduce *either with or without oxygen*. However, they only produce lactic acid once the oxygen is depleted, resulting in a more sour starter/sponge/dough the longer you let it sit. I don't think you need to worry about excluding air - the surface above the sponge or whatever is full of CO2 from the yeast, so very little oxygen is going to diffuse into the sponge, especially if you have it covered, and this will keep it from drying out, too. Of course, during this time, the gluten will deteriorate the longer you let it sit.
What does this all mean? If you want a maximally active culture, whip all the air you can into it each time you build it. I add the water first and whip this thin batter to a froth with an electric mixer, then mix in the flour. This results in maximum yeast reproduction. Then, as soon as it has used up all this oxygen, I build it again. Of course, it's hard to tell just when this is, but I generally let a sponge go until it just begins to fall. If you want a more sour bread, let either the starter/sponge/dough go longer. I find that with high protein flour such as bread or hard whole wheat, the dough can withstand two full rises before shaping into loaves, resulting in more flavor (not just more sourness, but that, too).
I hope this little science lesson has practical benefits to your bread baking. If anyone knows more details about how wild yeasts and lactobacilli interact, I'd welcome hearing it, especially if I'm wrong. I suspect the symbiosis of some cultures may change things, but this works with my Poilane (originally) starter.
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