September 17, 2020 - 6:14am
Why do starters peak and fall?
We all take it for granted that a starter or levain rises to a peak volume and then starts to fall - but why?
There must still be plenty of food left in all that flour. Is it lack of oxygen or is it because of the increase in acidity? Does this perhaps inhibit the amylolytic conversion of the flour starches into simpler sugars - and without those sugars the fermentation will come to a halt.
Just wondering....
Lance
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I have always assumed that it's the accumulation of CO2, the product of respiration/fermentation, that limits growth in a static (unstirred) system such as our starter jars and crocks. That is, the bugs transiently "soffocate". Certainly stirring a peaked starter back down -- replacing trapped CO2 with ambient O2/N2 etc -- and watching it rise again supports that hypothesis. I also assume they fall because continued proteolysis (and/or hydrolysis of other polymers) weakens the structures of the bubbles responsible for the volume increase that constituted the "rise". But I wouldn't be surprised if there are other factors that contribute, or even principally cause, the "fall". It could be that the bubbles themselves are just inherently leaky and when the balance between newly generated CO2 vs leaked out CO2 shifts to the latter, the dome slowly begins to collapse. I have noticed how just a gentle shake of my starter's jar when the dome is fully peaked will initiate collapse, suggesting that it wouldn't take much of a shift in CO2 in vs out balance to initiate collapse.
Tom
I'll go out on a limb with 2 possibilities
1 - we know the expansion is due to gas production, we see it in bubbles, that's noticeable. What we don't see are the really small bubbles - microbubbles I'll call them. These little bubbles join to form bigger bubbles, and in doing so take up less space.
2 - it's tired. You would be to if you were climbing straight up for hours and hours, and it's resting.
I'll add, as I've never delved to deep into the subject I have about as much evidence for 2 as I do 1. Enjoy!
As I understand it the carbohydrate (food) supply is limited because most of it is derived from broken starch. Whereas whole / intact granules (native starch) are barely fermentable at all (closed complex structure). Milling grains into flour breaks some of those starch granules releasing fragments of amylose and amylopectin which can now be attacked by native amylase enzymes. If flour contained no damaged starch then there would be little to no fermentation...
For the context of SD it's worth noting that alpha-amylase is optimal at around pH5.5 and its activity declines steeply down to pH4 and below.
Michael
EDIT: relevant reading: The Effect Of Damaged Starch On The Quality Of Baked Good | | Miller Magazine (millermagazine.com)
Thank you for your explanation, Michael . Your linked article also makes interesting reading - it looks like starch damage has to be just the right amount to produce a good flour.
A quick trawl of the literature suggests stone mills produce more starch damage than roller mills and I wonder how the small home mills compare.
Lance