Respiration vs Fermentation

Respiration is almost negligible. There just isn't enough oxygen to even bother worrying about it. The little oxygen available will quickly be used up by the yeasts.

If you whip a high hydration sourdough starter, it will perform better because you whipped in some air which provides oxygen. But that boost quickly fades.

... as far as I know.

I admire your inquiring mind, Danny.  I don't have all the answers, although I did teach university level biology for 30 years, so I know a little about respiration vs. fermentation.  But exactly how they apply to and occur in doughs and baking -- I could stumble there.  Regardless...

Respiration is 30x or so more efficient than fermentation in generating the coin of the realm, ATP, that drives energy-requiring biological chemistry.  So most organisms (i.e., all but obligate anaerobes whose lifestyles are decidedly unambitious, but have succeeded in surviving on this planet since before there was oxygen, so don't knock 'em) have evolved to exploit any circumstance in which oxygen is present to perform respiration and not fermentation.  I assume that when we fold our doughs, we are introducing oxygen between the layers and bugs that now have access to it respire away, until they sort of pollute their environment with CO2 and choke themselves off of further respiration.  Then on to fermentation for the piddling level of ATP it yields - enough to get by on.

You can demonstrate that "CO2 pollution" effect by stirring down a fully mature and collapsed starter and returning it to its happy place.  It will grow up again, consistent with the hypothesis (read: not proving - there are no "proofs" in biology, but that's a whole 'nother thread) that it was accumulation of some volatile waste product (i.e., CO2 - a safe assumption) that was the limiting factor in its first grow-up.

Respiration comprises far less biochemically creative pathways than the bogglingly diverse versions of bacterial fermentation out there.  It's the creativity of the latter that infuse our doughs and breads with the rainbow of flavors that make SD baking worthwhile, as opposed to CY baking. 

It isn't so much that we're incubating our doughs in "closed" vessels that reduces the bugs' access to oxygen.  It's the dough itself that keeps oxygen from diffusing in once that which was trapped by the last mix, slap or fold is used up by respiration.

Make sense?

Tom

Dan, I have a peripheral but maybe important contribution to make.

It's about the "limited understanding".

On this topic, everyone - everyone - is of limited understanding, and yes that is meant in exactly the way you meant it. In fact, on this topic, beware of anyone who claims not to be of limited understanding.

(There certainly are topics for which the answers are known and clear-cut, and in which it's easy to spot an expert simply by their superior knowledge. But in your topic, the real expert is the one who makes great bread; words don't taste very good. ☺️)

NB: That's not to say that the person making great bread knows why their bread is great; many great bakers have misguided and even stupid ideas regarding why their process works. Copy their process. Disregard their comments.

You this a bit in her book 'Bread Science'....My brain is just not wired for the science behind bread but her hand drawings describing the chemistry behind bread and clear approach has started to clear my foggy brain. She has studied chemistry and turned bread maker and that is an amazing combination, I find. Also, her book is very reasonable as a Kindle book at the moment...

• "In dough, respiration occurs until the oxygen is used up and fermentation begins."
• "The basic reaction of respiration is glucose + oxygen -> carbon dioxide + water. This produces lots of carbon dioxide gas, causing the dough to rise faster. It does not produce any alcohol, however.
• "In the absence of oxygen, yeast performs fermentation. There are different fermentation pathways with different products. The products depend on which enzymes are present - different yeast contain different enzymes.. With bread-making yeast, the reaction is: glucose -> ethanol + carbon dioxide. This process produces less carbon dioxide than respiration but also produces ethanol. This ethanol increases the flavor of the bread."
• "Adding air (and therefore oxygen) to dough while kneading enables more respiration to occur, resulting in faster-rising but less flavorful dough. This effect can be easily be amplified when mixing in a mixer. Commercial bakeries often strife for more respiration because it speeds up the bread-making process. The balance of the two processes found in home made dough results in a dough that rises in a timely manner and also has good flavour."
• "As a side note, the microorganisms in sourdough starter perform a fermentation reaction that produces lactic acid,  resulting in a different flavor. "

She then shows a diagram which demonstrates the different pathways of fermentation, which I've found really useful as a simple picture of this complex process.

So, in my simple mind I have come to the following conclusions...with regards to some of the 'science'that contribute to flavor:

1. AL longer as this gives time for the enzymes such as protease to break down protein chains and create amnio acids which I believe are important for flavor...(Emily Buehler has also produced an excellent easy to understand text about the work of enzymes)

2. Now - my conclusion here might be totally wrong and is based on my very simple deductions...but if I develop the gluten upfront during mixing (with not too much oxygen but just the right amount) then 'respiration' happens  and

3......I may need less folds in order to build structure and therefore introduce less oxygyn into the dough aka maximising fermentation i.e. better flavours. I can also extend this process using temperature and longer bulk??

I am intrigued as from what I am reading there are different stages when 'flavor' is developed...

1. During mixing and longer AL with enzyme protease by creating amnio acids which can be processed by yeast resulting in organic molecules that add flavors to bread.

2. During Fermentation via yeast pathway, where ethanol is created, which affects flavour

and 3. Sourdough Bacteria's pathway where lactic acid is created

Hah, now I have to apply this to my baking and proof that there really is a difference in flavour.....Good to exercise that brain though...  Kat

p.s. she is on the Sourdoughpodcast in an interview https://www.thesourdoughpodcast.com/episodes/2019/2/12/emily-buehler-author-of-bread-science and there is a link to her article on enzymes too...

This has been written for the brewers, but it applies to baking as well: https://www.morebeer.com/articles/how_yeast_use_oxygen