Bryan, when you say stir well, I envision a few different scenarios, and having had H2S occur for me can you clarify a smidge?
Ahhh ... your telepathy is not working this morning. Ok, I can clarify!
Note -- this is a long one. Get a fresh cup of coffee (or wine) before continuing ...
When doing normal degassing, I run the drill-mounted stirring rod until it builds a small vortex (roughly between C and D in your list) then run it for 20 to 30 seconds. Then reverse the drill and do the same thing,. I typically say "1 minute" as that's the rough time frame for the process, and I've found it's sufficient.
The 1 minute stir is sufficient to disperse K-meta.
I've had 3 personal encounters with H2S, all slightly different. The general treatment was the same, but the details varied. Stirring varied, so I'll explain that first.
2020 Red Blend second run -- This was in 54 liters of wine. We were racking it the first time out of the demijohn when the stench of H2S hit us. My notes state we stirred for 5 minutes. I had a fan running so it blew the stench away. When the wine stopped reeking, I stopped stirring.
2022 Tempranillo -- This one had been in the warehouse too long and had mold. We discarded the worst clusters, hit it with a double dose of K-meta to address remaining mold, and started fermentation using RC-212. Two days later we caught a slight whiff of H2S. So we hit it with another triple-dose of K-meta. This was a grape must, so there was no "stirring". I punched down and mixed the best I could for probably 8 minutes (this is a guess, I didn't time it).
2023 Sangiovese -- Last year caught H2S early in a juice bucket that started fermenting ahead of time. For that one I stirred for probably 2 minutes, and as it was caught very early, 1 minute may have been enough.
Treatment
The exact treatment for each differed because they were caught at different stages.
2020 Red Blend second run -- This one was caught way late, so we added a double dose of K-meta and stirred. That took care of the H2S, but mercaptans had formed, giving the wine an off taste, not exactly bitter, but not good. I ordered Reduless and used the median dosage. 4 days later I fined with K&C.
There was still a residual off-flavor that didn't go away (mercaptans). 4 months later I added 3/4 tsp ascorbic acid to each of three affected 19 liter carboys. The calculation I used said 1-1/4 tsp, but I went with less. As I often say, it's easier to add more than to take some out. It took about 4 months, but the off taste went away and the wine was good.
2022 Tempranillo -- This one was caught early, in the initial part of fermentation, so I added K-meta and nutrient. Then did the punch down as described above. The stench went quickly away, it was caught early to mercaptans didn't form, and the wine was (and still is) good.
Two lessons from this. One -- when using a yeast with high nutrient requirements, use 50% more nutrient than the directions call for. If the wine is still fermenting, regardless of where it is in the process, add nutrient, as stressed yeast is the source of the problem.
Two -- don't go nuts on K-meta. We had already double dosed because of mold, and I added a full teaspoon to the fermenter, which was probably another double dose. This produced a burned match smell. This is not a fatal problem; we didn't add more K-meta to that wine until it had been in barrel for a while.
2023 Sangiovese -- We added pomace from grapes to a pair of juice buckets, and I had to keep the buckets cold enough to not ferment for a week. This didn't quite work, and both buckets started fermenting, even if slowly. One smelled of H2S so I stirred, added 1/4 tsp K-meta and nutrient to each, and sprinkled in 2 tsp Avante yeast. I don't normally inoculate in that fashion, but I figured it would not hurt.
