April 26, 2021 - 3:52am
science of sourdough feeding
What exactly happens when you feed a starter?
Say you have a starter with population density of X yeast/bacteria cells per gram of starter. After you feed it, the density is X/3 for a 1:1:1 feeding (or X/5 for 1:2:2, etc.). Eventually the density will reach X again. Does that happen when the starter reaches it peak, or before/after? Does the population density fall when the starter falls?
Relatedly, what does it really mean for a starter to be "ready"? Is that referring to population density or something else about yeast/bacteria metabolism?
I've not found a clear answer to these questions. It seems to have some implications for the "lazy sourdough" approach where people don't particularly time their baking to the sourdough cycle.
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Any starter that has matured (reaches the proper balance) is technically ready as it will leaven bread - sooner or later. Concentration will determine the sooner or the later. Highest concentration is when the starter no longer rises after stirring. X high in Y time is irrelevant. Now you have the straight answer. Enjoy!
A starter is ready for use when the recipe says so. For different breads and different starters there are unique requirements. Not all recipes require a mature starter either. And some mature starters can sit refrigerated for weeks and months and are ready to be added to bread dough immediately. They are fully mature and will leaven our bread dough very well.
Majority of starters used in baking are either too stiff ot too liquid to change their volume as they mature. Essentially, what you want from a mature starter is its ability to leaven dough, to make it rise fast enough, and its acidity. If your starter became acidic enough during its fermentation, it will acidify bread dough.
Mature starters have certain acidity expressed as TTA number, and they rise with certain speed after being fed, if they rise at all. For example a soft wheat starter is supposed to double in 2-3 hours when fed 1:1.6 (100 g starter, 100 flour, 60 water) and to double under one hour when mature. Or a stiff starter for French bread is supposed to quadruple in volume in about 6-8 hrs after feeding it 1:2 or 1:4. That means it has enough yeast in it per gram of flour.
Professional bakers use special tests to determine starter leavening ability that have nothing to do with counting yeast cells under microscope or with watching sourdough starter rise, as we do at home. They simply don't work with 100% hydration starters. In bread factories starters are either liquid like milk or so stiff, they don't change volume as they mature.
Those rules usually come with starters, with recipes for starters, and with bread recipes. Bakers know from experience when their starters are mature have target acidity and maximum leavening capacity and write those signs down or teach them to their students.
Population density doesn't peak when the starter peaks. If you add commercial yeast to your bread dough and this dough rises and peaks, does its yeast or bacterial population peak at the same time? Of course not. It either stays the same as at the beginning, doesn't rise at all, or it can rise for hours and days non stop provided there is enough sugar inside that dough, that poolish or whatever. That poolish can rise and fall and deteriorate but its volume has nothing to do with yeast cells, they will continue to multiply until there would be so much alcohol in dough that they would start dying from alcoholic intoxication, from being poisoned by the byproduct of their own metabolism.
...or it might only cause more confusion!
I've made several time lapse videos of my starters, since for a while I was VERY interested to know exactly when "peak rise" was occurring. Besides being really fun to watch, they're also sort of instructive, I think... this one shows my whole wheat / bread flour starter (on the right) rising to a max height, then falling, and then apparently rising again (although not to the former max height). The one on the left is a 100% hydration whole rye starter (not as exciting to watch because it's in a wider container). I don't know how to translate this info about the WW/BF starter into an actual baking recommendation -- maybe someone more experienced than I am could do that? -- but it seems clear that the "perfect" moment to use your starter must be somewhat subjective-- I haven't done the testing, but I'd guess that the flavor imparted to the finished bread would be different depending on when the starter was used, even though the window of viability (for leavening purposes) is probably fairly large...
https://youtu.be/BerEbhDEG-A
My take away is that the usual rule of "use your starter at it's peak" is greatly oversimplified, since that has more to do with gas production than population density. (Thanks, phaz.) I agree that there's probably a fairly broad window when the starter is practical for leavening bread in a reasonable amount of time, with after peak being better than before peak. Of course the smell of a starter changes after collapsing, which probably affect bread taste.
I still don't fully understand how population of yeast/bacteria grows with time though. My understanding is that yeast grow much more slowly in an anaerobic environment. This was discussed last year, but I don't find the answer entirely satisfactory.
Glad I peeked in this one.
Broad window - I look at it as a %. Any % of critters will leaven bread, except 0%. That's a very broad window. All fundamentals still apply.
Understanding population - nothing to understand except the critters take care of themselves.
One over the other - of course there will be imbalances here and there, but eventually ratios come into balance again - that's what the system has been designed to do and it's very good at it. And do note the use of the term imbalance and balance. I mention this as a healthy starter is a balanced starter. An imbalanced starter is a problem. Enjoy!