From: Al Korzonas
Thank you for taking the time to answer all our questions. I'm following up not on my own questions, but on one from another HBD member. In a response about oxygen requirements, you said: "I do not know how you will be able to control the rate of sterol production or the total amount. Yeast can produce the precursor squalene with no oxygen, then with very little oxygen, 10 - 15 ppm, it can move squalene up to sterol."
In Homebrew Digest #1446, the immortal Dr. Fix reported results from some tests he performed using oxygen and verified that the dissolved oxygen levels (unless held artificially high with constant oxygenation) are strongly dependent on the specific gravity of the wort and the levels are quite a bit lower than the ones you have said various yeasts require for a healthy ferment:
SG 54.4F (12.5C) 59F (15C) 68F (20C)
1.030 (7.5P) 8.1ppm 7.5ppm 6.5ppm
1.040 (10P) 7.7ppm 7.1ppm 6.2ppm
1.060 (15P) 6.9ppm 6.3ppm 5.6ppm
1.080 (20P) 5.7ppm 5.5ppm 5.0ppm
Are you suggesting that we need to artificially keep our oxygen levels above the normal solubility for proper yeast growth, or is there such a big gap between theory and practice?
In another answer you said that some wineries will add oxygen near the end of fermentation to restart a stuck ferment when there was insufficient oxygen at the beginning of the ferment. Shouldn't that include a caveat that adding oxygen in the middle or end of a ferment is at the expense of shelf life and will result in an increased amount of aldehydes in the finished beer/wine. Also, although not unwelcome in an ale, increased oxygen means increased oxidation of alpha-acetolactic acid to diacetyl. I have had some bottles of Samuel Smith's beers that had excessive diacetyl, even for an ale. Samuel Smith's uses pumps to get their highly flocculent yeast back into suspension and although their fermentation room relatively quickly fills up with CO2, anytime there are humans in there, they run fans to evacuate the CO2 and replace it with air, so some oxidation is inevitable (excessive, I suggest, on some batches).
Response from Dr. Clayton Cone:
Good to hear from you again.
Oxygen is a difficult topic to address. It is easy to talk about it academically but how do you talk about it practically to the home brewer. How do you know precisely how many ppm of O2 that you need and how does the home beer maker deliver the exact ppm O2 required? You have pointed out one of the variables: O2 saturation is dependent on wort gravity and temperature. The irony is that the higher the gravity the lower the O2 solubility and saturation, yet the yeast requirement is usually higher. Further aeration with air, after the yeast has been added, may be required to achieve the desired amount of O2 needed. The solubility of pure O2 in wort is greater than the O2 from air, so the desired requirement via pure O2 can be achieved with the initial addition.
The yeast requirement varies with the strain. In the wine industry some strains require twice as much O2 to maintain a constant CO2 output as other strains. The same probably holds true with beer and distillery yeast.
The yeast requirement for a particular inoculum is dependent on the quality of the yeast at the time of pitching. A yeast starved for O2 from the previous fermentation will require more O2 than one having received an adequate amount. Most commercially produced Active Dry Beer yeast actually require no O2 addition for a successful average gravity wort fermentation. There is enough lipids built into the cell at the yeast factory. It will need O2 addition on the next re-pitching. Re-pitching yeast held under stressful conditions have a delayed response to O2 uptake. Under pitching requires more O2 than over pitching. Some beer makers do not have adequate refrigeration so they restrict the O2 addition in order to control yeast growth and thus fermentation temperature. O2 delivery system and contact time plays a crucial role in O2 solubility:
porous stone vs. pin hole in pipes
pure O2 vs. air
bubble coalescence vs. bubble integrity
short vs. tall fermenter
The list goes on. I am not sure how defined the over oxidation line is. If the yeast actually requires 6 ppm O2 and you add 7 ppm O2, do you automatically begin to produce unfavorable by-products?
Your caveats regarding late addition of O2 is well taken. Late addition of O2 should be done only as a desperate move to unstick a fermentation. This has become routine first step taken in the wine industry to speed up a sluggish fermentation or unstick a stuck fermentation. Of course the wine industry may not be as sensitive to some of the fermentation by-products as the brewing industry.
Yeast are tremendous O2 scavengers. Live yeast addition to the beer at the time of bottling offer protection from oxygen. I am not sure if it is at the cost of by-product problems. Two or more additions of O2 occurs routinely at breweries that require two or more batches of wort to fill a fermenter. Sometimes highly O2 saturated water is added to the fermenting mash. OG corrected for the added water.
It would have been wiser of me to have given a range of 0 to 15+ ppm O2. Thank you again for your comments. Dialogue is a good thing. After all these years, I am still learning.
