Anaerobic (without oxygen) or Aerobic (with oxygen)
When someone looks at my simple raisin yeast water method, he or she might have stopped at or might have wondered at Step No. 5: Seal The Jar Very Tightly With The Lid, Leaving No Air. Then, he or she might have asked oneself "leaving no air? Really?" Yes, yeast can grow in an anaerobic environment. This was back in December 2014 when I exchanged emails with Debra Wink, a Microbiologist, a Medical Technologist, and also a Baker about it.
I met Debra when I attended the Grain Gathering Conference during the summer in 2014. She gave a speech about Natural Leavening: Practice and Principles. When I shared with Debra my method of making the yeast water, she immediately understood why I make it this way. In her email, she said, "...there seems to be a belief among bread bakers that yeast can only reproduce in its presence. Clearly that isn't true... So many people think that shaking frequently to aerate is necessary, but it just makes a mess. Also, fermentation doesn't commence to any degree until the oxygen is used up, and fermentation is what we're after..."
I don't have as strong science background as Debra, so I've just learned by making it. But Debra, kindly shared with me her article on aerobic vs. anaerobic environment issue along with her email. Thank you Debra for letting me share your article on my blog!!
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I wish I knew where the myth originated that yeast need an aerobic environment to multiply. Maybe it comes from the beer brewers, or because yeast manufacturers rely on oxygen to produce bakers' yeast. But the oxygen requirement is one of those misunderstood half-truths. What yeast need are sterols and unsaturated fatty acids to build new cells, because these are essential components of cellular membranes. Yeast can manufacture these things in the presence of oxygen, like we humans can manufacture our own vitamin D in skin exposed to sunshine. But like vitamin D, they don't have to manufacture sterols and fatty acids if these are available as nutrients in the growth medium.
Although brewing yeast changes between oxidative and fermentative metabolism according to aerobic or anaerobic conditions, it cannot grow anaerobically indefinitely. As with all eukaryotic cells, yeast cell membranes contain unsaturated fatty acids (UFA) and sterols, which can be synthesized only under aerobic conditions. The amounts of UFA and sterols from malt that are naturally present in wort are too low to support yeast growth, hence the requirement for initial aeration of the wort to allow the yeast to synthesize these compounds. ~ Food Microbiology Fundamentals and Frontiers, 2nd Edition
It's sometimes said that beer is liquid bread, but that isn't really true. Both are fermented by-products of grain, yes, but beer is made from wort which is essentially a tea --- an extract of sugars and other water soluble stuff with all the grain solids removed. The solids are where the sterols and unsaturated fatty acids are. So, flour is a more complete food for yeast than wort, and can support yeast reproduction.
Now here's the other piece. Yeast metabolize sugars to generate energy in the form of something called ATP (their unit of energy currency). They spend ATP, first, in performing whatever tasks they need to in order to survive and overcome adverse conditions they face in their environment. If conditions are reasonably favorable and they can make more energy than they need for survival, the excess is channeled into growth and reproduction. Without oxygen (fermentative metablolism), yeast can only produce 2 ATP from each molecule of a single sugar; whereas they can produce 38 ATP from the same sugar in the presence of oxygen (oxidative metabolism, or respiration). That's A LOT more energy diverted to reproduction, and it's why oxygen is used in manufacturing yeast. The process would take much longer otherwise. With plenty of oxygen, and everything else optimal, yeast can double in as little as every 75 minutes or so. We certainly don't need or want rates like that in bread dough or we wouldn't be able to control it.
For manufacturing purposes I'm sure oxygen is required for yeast reproduction, but in a circular, catch-22 sort of way. They need oxygen for economic efficiency, because yeast reproduce so much faster with, than without it. They cultivate yeast in a liquid nutrient broth (not a flour-based substrate resembling bread pre-ferments/dough). They don't need to add sterols and UFAs---which would add unnecessary cost to the growth medium---since they are already bubbling large amounts of oxygen through it, enabling yeast to manufacture their own. So, without sterols and UFAs in the mix, oxygen is required.
Michael Gaenzle says it simplest (in relation to sourdough cultures):
Yeasts in dough don't have to rely on oxygen for growth: if that were the case, they wouldn't be there, as oxygen is quickly depleted. ~ The Bread Builders, pg 231
In addition to the usual chromosomes in the cell nucleus, yeast cells have an additional chromosome (with copies) that resides in, and drives respiration in the mitochondria where aerobic metabolism takes place:
The genome of S. cerevisiae consists of 16 small chromosomes [in its nucleus]... The yeast mitochondrion, which carries out the enzyme reactions in aerobic respiration, has a separate circular chromosome of which 10–40 copies may be present, depending on the physiological state of the cell. ...the mitochondrial chromosome is quite unstable. In about 1–3% of divisions, the daughter inherits a defective chromosome... Such cells are consequently unable to respire, although they continue to live and divide and can carry out fermentation. ~ Handbook of Dough Fermentations, Chapter 3
When the unstable mitochondrial chromosome becomes defective, it is no longer functional, but the cells and their offspring can still carry on and even reproduce in fermentation mode (anaerobically). It doesn't address the sterol/UFA aspect specifically, but is proof that cells are at least able to generate enough energy to reproduce anaerobically.
~ Debra Wink
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