Despite being easily the most rudimentary stuff in bread making and heavily discussed and written about, the relationship between oxygen and rising still appears to be shrouded in myth and speculation. Having read various articles on this topic--frankly most information online is vague at best and at times totally contradictory to each other--I am still puzzling over the basic stuff: Does a dough need oxygen to rise or not? Or is an anaerobic environment better for the process in any way? If so, how exactly?
In the bread-making process, it is the yeast that undergoes cellular respiration. Anaerobic respiration -- also known as fermentation -- helps produce beer and wine and happens without the presence of oxygen, while aerobic respiration requires oxygen to be present. During bread production, yeast starts off respirating aerobically, creating carbon dioxide and water and helping the dough rise. After the oxygen runs out, anaerobic respiration begins, although the alcohol produced during this process, ethanol, is lost through evaporation when the bread is exposed to high temperatures during baking. (source)
In the simplest of terms, fermentation is what happens when yeast cells eat and poop. Specifically, it's what happens when yeast cells consume sugars and produce ethanol and other derivative chemicals. The alcohol produced by the yeast during fermentation—along with a multitude of other reactions—are what give great bread its characteristic flavors and aroma. Generally speaking, more fermentation means tastier bread. In the most technical, terms fermentation is an anaerobic reaction (meaning it happens in the absence of oxygen) that the yeast performs after respiration, which is aerobic and requires oxygen. (source)
Per these articles, the process starts with aerobic respiration and ends with anaerobic respiration when the oxygen runs out, and both reactions are necessary for the end product to carry a pleasant and appetizing flavor. So aerobic -> anaerobic.
But that runs counter to what people say about covering: What should I cover bread dough with while it's rising? This old Seasoned Advice discussion seems to suggest the major reason the dough is covered is to keep it from drying out, and the most upvoted answerer suggests they "haven't noticed a difference in the bread produced" with plastic wrap and wet towels. If oxygen really plays a part in the fermentation process, access to it most certainly should affect the end result. Plastic wrap limits air access, whereas wet towels don't.
So is limiting air access necessary at all in any part of the rising process? Also with this question I am seeking accurate, scientifically sound, informed answers.
Eddie KalEddie Kal
I can't give an official answer but hopefully this helps. I'm more familiar with yeast in terms of brewing. Beer yeast is a cultured strain which doesn't convert ethanol into ethyl acetate. Other than that I think they are similar. Oxygen is necessary for reproduction, but co2 is created in an anaerobic environment. The general rule with beer is that you want lots of oxygen in the beginning but none after that. I would think bread would be similar. – mreff555 Jun 21 '20 at 2:00
@mreff555 That makes sense and agrees with a lot of sources. That is why I don't think the dough should be covered with just a wet towel. – Eddie Kal Jun 21 '20 at 2:05
I can't give the following as an answer because I'm not 100% confident it's what's going on. Most of the bread dough is sealed off from the outside air by the outer layer of dough. Any oxygen used by the yeast for reproduction is trapped in the dough during mixing & kneading. Sealing the dough in plastic has no effect on the oxygen levels inside the dough, but will affect moisture in the outer layer. – LSchoon Jun 21 '20 at 8:08
Letting the dough rise in a plastic bag does not create an anaerobic environment: there is ample oxygen in the bag. (Instead using a bag helps maintain humidity and a stable temperature.) Moreover, some plastics are quite permeable to oxygen, and unless you use a special seal, oxygen may get in anyway. As LSchoon says, the anaerobic environment is created inside the dough. – Mark Wildon Jun 21 '20 at 10:38
I can confirm Mark Wildon and LSchoon said. I actually tested this same theory once. If you don't cover the dough, evaporation (which will be especially bad if refrigerated) will cause the dough to dry out. From my experience, this prevent the rise. Most likely because the hardening limits oxygen during the growth phase. – mreff555 Jun 21 '20 at 14:22
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