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:
All yeast activities can be assigned to three main cycles: 1) Respiration, 2)
Fermentation and 3) Sedimentation.
Respiration is the initial process that yeast undergoes when pitched into the wort. It is an aerobic process through which the yeast cells utilize oxygen. From the available oxygen, the yeast derives energy and other requirements for reproduction, cell construction and fermentation. The energy derived during respiration is almost completely used during fermentation. Respiration will last about 4 to 8 hours and varies quite a bit with conditions. During the respiration cycle, the yeast will reproduce and produce carbon dioxide, water and flavor characteristics (there is no alcohol produced during respiration). The flavor characteristics are by-products of yeast metabolism, the most noticeable ones being esters and diacetyl.
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.