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Old 12-29-2013, 01:51 AM   #41
EarlyAmateurZymurgist
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Originally Posted by g-star View Post
You present a lot of sound theory, but I wonder how much of this (sufficient sterol production via areobically grown starters) is observed in practice for the average homebrewer. I would also like to know at what concentration O2 becomes detrimental (as you indicated in post #25) for the home and professional brewers around the world who follow this well-established practice.

It would seem that if the commonly accepted 8-12ppm concentration of O2 prior to pitching would be good practice to ensure sufficient sterol production, if that concentration is below the threshold for "the shift from anaerobic glycolysis (fermentation) to aerobic utilization of ethanol as a carbon source".
The problem stems from not knowing how much, if any, oxygen remains in solution after glucose and all sugars that can be reduced to glucose have been metabolized by the yeast culture. If excess O2 was not a problem for breweries, then funding for research into ways to eliminate aeration would dry up. The New Belgium study forced a lot of brewing scientists to reassess what they knew about oxygen requirements.

Everything that I know about brewing yeast is the result of scratching a personal itch. In the early days, being able to culture yeast was simply a means to an end because the dried yeast cultures that were available at the time were hit and miss (anyone remember the old Red Star beer yeast?), and Wyeast offered only a handful of cultures that were often difficult to obtain (White Labs did not exist). I was driven to research the effects of excessive O2 on beer flavor because my beers were tasting funky at the end of fermentation when I was using pure O2. Every beer that I oxygenated with pure O2 had a note in the finish that tasted like oxidized alcohol. At first, I thought that I was picking up an infection. However, the plates that I made from affected beer looked no different than those that were made from non-affected beer. The only difference was that I used pure O2 to aerate the affected beer. This discovery forced me to study the affects of diauxic shift. It was while studying diauxic shift that I made the decision to switch from aerating my wort to aerating/agitating my starters and pitching a large mass of cells at high krausen.

With that said, the only way for a brewer to know if he/she is over oxygenating his/her wort is to take dissolved O2 and gravity measurements throughout the fermentation. If there is O2 in solution after the gravity has bottomed out, then a brewer is over aerating his/her wort. I have been saving to purchase a lab-grade stereoscopic microscope to replace the one that I sold back in 2004 as well as few other things that I need for my brewery (Christmas isn't the best time of year to attempt to save money). However, I plan to purchase a dissolved O2 meter after I get these purchases out of the way. I want to know for certain if the oxidized flavors that I was tasting while using pure O2 were simply the result of oxidative chemical reactions that occurred while high-levels of dissolved oxygen were in contact with the wort, diauxic shift, or something entirely different.

As I stated earlier, many amateur brewers underpitch, which increases the need the for dissolved oxygen because the culture has to go through one or more additional divisions in order to reach maximum cell density (the cells that they pitch may not be in top shape as well). The most frequently used pitching calculators on the Internet are based on a pitching rate of 0.75 million cells per milliliter for ales, which is a low pitching rate. Pitching a single White Labs vial directly into five gallons of wort with a gravity of 1.048 or higher is underpitching by at least 50%.

In the end, what matters is if a brewer is happy with his/her results. My method works very well for me. However, I have complete control over my yeast cultures (or at least as much control as mother nature will allow). The average amateur brewer has no desire to go through what I do to use yeast.
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