Why is oxygen so essential to the yeast?

[truebe]

I was always confused about why the yeast need oxygen before they ferment. Do the yeast respire during the growth phase and then start fermenting later? TIA

 

[SporkD2]

Usually I dont do this, but you need to use the search function

 

[Clonefarmer]

Taken from Wyeast website:

In this phase, oxygen is extremely important. Oxygen is used by yeast for synthesis of sterols and unsaturated fatty acids that are necessary growth factors. Without oxygen, these lipids can’t be biosynthesized and growth will be very limited. The sterols and fatty acids produced are also very important in the structure of the cell membrane and the ability of the yeast to respond to external and internal stresses. Adequate oxygen in the wort when the yeast is pitched is essential to a successful fermentation and long-term health of the culture.

 

[Dr Malt]

Yeast can exist in environments with oxygen (aerobic conditions) and without oxygen (anaerobic conditions). When they are in aerobic conditions, their metabolism of food (sugars) produces much more cellular energy than in anaerobic conditions to do cell repair, cell division, etc. Thus, under aerobic conditions, yeast are healthier, metabolize sugars more efficiently, and grow and divide rapidly. This is something we want them to do when they are initially pitched into our wort. We want the cell numbers to increase in our wort so they are plentiful when they move to anaerobic conditions and they out compete any unwanted bacteria for the food supply. Once they use up the oxygen, they go into anaerobic fermentation where they live but produce less cellular energy. Hopefully by this time, they have increased in numbers sufficiently that all they need to do is survive and genrate alcohol and CO2 for us brewers. Thus, when you prepare your wort for the initial pitching of the yeast, you want to increase the oxygen levels so you get healthy yeast growth and reproduction.

Thanks.

Dr Malt

 

[Ouroboros]

When they are in aerobic conditions, their metabolism of food (sugars) produces much more cellular energy than in anaerobic conditions to do cell repair, cell division, etc.

I'm pretty sure that yeast, at least in the conditions present in aerated wort, are still carrying out fermentation. The Crabtree effect. Aerobic respiration generates a lot more ATP per glucose molecule, but it isn't as rapid as glycolysis and fermentation.

Like the Wyeast site says, sterol and unsaturated fatty acid synthesis require oxygen. To introduce double bonds into a saturated fatty acid, the cell uses an electron transport chain (not the same as the one used in aerobic respiration) that ultimately sends the electrons to O2. The same sort of thing happens in the biosynthesis of the common precursor to all sterols. Without O2 in your wort, these essential membrane components can't be synthesized. I imagine the yeast that you pitch can't multiply as rapidly or to as high of a concentration as it could in aerated wort.