Respiration and Fermentation: Widespread confusion

Frequently on this message board I see posters stating that yeast need oxygen so that they can grow and divide through "respiration" in order to reach the concentrations necessary for a successful fermentation. The idea is that little, if any, yeast growth occurs during fermentation, and that the vast majority of growth is during the aerobic phase in which yeast utilize oxygen for growth and division.

This idea is widespread probably because several published authors, as notable as Papazian, disseminate this idea. For instance, in "The Complete Joy of Homebrewing," page 108, he writes:

As any scientific treatment of the topic will tell you (for instance "Brewing: Science and Practice" or "Brewing Yeast and Fermentation", or even this month's article on yeast metabolism in "Brew Your Own" by Chris White), this is completely incorrect. Aerobic respiration, involving the conversion of wort sugars primarily to CO2 and water, never occurs in a brewery fermentation. When pitched into a wort with glucose, or other hexose sugars that can be converted to glucose intracellularly (such as fructose and maltose, but not sucrose), yeast undergo carbon catabolite repression, better known as the Crabtree Effect, which causes morphological changes (such as the supression of the development of mitochondria) that preclude the possibility of respiration.

Moreover, far from being a period of low yeast growth, fermentation encompasses the exponential (log) phase of yeast growth. The period during which yeast are rapidly budding and exponentially increasing in number is a period of anerobic growth, not respiratory aerobic growth. There is no period of aerobic growth in the production of beer. There is an aerobic phase, at the beginning of fermentation, when the wort is oxygenated and yeast utilize oxygen for the synthesis of sterols and unsaturated fatty acids, which allows future budding, but this is not a period of respiration and no yeast growth occurs during this time. In fact, total yeast mass decreases during this time due to the breakdown of internal glycogen reserves that fuel sterol synthesis.

It is correct to say that you likely won't get enough yeast growth without sufficient O2, because once the yeast run out of sterols they won't bud. It is incorrect to say yeast can't bud without O2 because they can and do (O2 is just a dependency in the chain).

Similarly it's incorrect to say that we homebrewers make beer. We don't, the yeast make the beer -- we just make wort.

This brings up an often missed point, dry yeast is loaded up with sterols before it is packaged which is why I often don't bother to oxygenate a weak wort when pitching dry yeast -- it isn't needed. I just rehydrate and pitch.

The article in BYO is an excellent resource. The yeast culturing guide on the Maltose Falcons website is another really good resource. Both recommended reads for the advanced brewer, IMO.

Good point, if your yeast has already accumulated a sufficient supply of sterols, then you shouldn't need to aerate the wort at all. In fact, several researchers say that this would lead to an inefficient fermentation by contributing to excessive production of biomass (yeast growth). I don't understand why this happens (no one ever explains it), but apparently over-oxygenating yeast will impact the balance between yeast catabolism (and the generation of metabolic by-products such as ethanol) and anabolism (the production of cell mass). The more wort sugars devoted to anabolism, the less ethanol produced. Obviously, to produce ethanol, the cell has to progress through the cell cycle, but apparently excess oxygen effects the rate of biomass production, causing more biomass production relative to ethanol production.

That Maltose Falcons website Yeast Culturing Guide is a great resource. In some serious efforts to educate myself on the proper oxygenation of wort, this guide was the most helpfull I have found. Thanks for the direction and the ongoing discussion.

Fritz