Aeration And Foul Smell
Over the past few years, I have practiced a crude form of yeast ranching. After stepping up a smack-pack (1/3 cup DME in one pint of water, boiled 10-15 minutes) in preparation for brewing, I would inoculate a second flask and allow it to grow to krausen and then place it in the refrigerator for the next brewing session. Sometimes, this brewing session would occur months later. Essentially, the yeast was stored under refrigerated beer. Prior to brewing, I decant the supernatant and pitch the slurry into a starter. Sometimes it takes 2 steps before a good krausen. I would repeat the process several times with the same culture, storing successive cultures and growing them up as needed with no detectable adverse effects.
Recently after reading about the benefits of oxygenation/aeration, I bought a stir plate, an aquarium pump, and an in line HEPA filter (used for IV infusions). Additionally, I began adding 1 TSP of a "yeast nutrient" which I interpret to be yeast hulls (yellow granular appearance) to my cup of starter solution during the boil. I then fed the sanitized air line from the aquarium pump through the HEPA filter and into the hole in the rubber stoppered flask and stirred the culture at a moderate rate using filtered room air for aeration.
On two occasions, once after pitching yeast sediment from one of my refrigerator cultures and once after pitching one of the new pitchable yeast tubes, I noted a very foul smell, much like burning plastic wire insulation or burnt rubber. The culture from the refrigerator made acceptable beer, no adverse tastes but a tremendous amount of yeast sediment (I believe it was California ale yeast I used for a wheat beer so it was difficult to evaluate whether the haze was due to poor flocculation from a wild yeast infection). The culture from the pitchable tube (Kolsch) made two horrible batches, one alt and one Kolsch. In all fairness, I have never attempted these styles before and now understand that this yeast needs to be cold conditioned despite being called ale yeast. Both batches just wouldn't clear and never developed a clean taste.
This weekend, I stepped up another pitchable yeast tube, a Bavarian Wheat for a Hefeweizen. I used only 1/4 TSP of the yellow yeast nutrient and added 1/4 TSP of "yeast energizer". I stirred for 48 hours without using the aquarium pump. Instead, I inserted the HEPA filter directly into the stopper to filter incoming ambient room air. I detected the same foul smell, but it was very faint.
Is this smell due to autolyzed yeast? Is it due to one of the yeast nutrients? How about an infection from the aquarium pump? Am I stirring too long and exhausting the media and nutrients causing yeast autolysis?
A couple of things you indicate follow a pattern to some extent. One key to the questions is the aroma that was noted to varying degrees from three different sources. The burnt rubber aroma which appears to be a common thread from 3 different yeast strains may be an indication. This aroma is often associated with what some describe as 'phenolic'. This is somewhat a catch all for a number of compounds usually associated with wheat beer yeast, many Belgian yeast strains and wild yeast. The compounds can vary in strength and character depending on the strain itself or how it is handled. This would include temperatures and, aeration levels. Some of these yeast have relatively high levels of phenylethylene, a plastic resinous like odor, like the synthesized compound known as styrene.
Since ale yeast and Kolsch yeast typically do not have measurable amounts of phenylethylene, or 4-vinyl guaiacol, which is another common wheat beer associated compound, it may be likely that it was introduced by another yeast strain or wild yeast. Even in the Bavarian Wheat beer strain you noted "the same foul smell", which typical will have detectable levels of the above compounds. This could be from the wheat beer strain itself, another organism, or a combination.
Whether from a known wheat beer yeast strain or a wild yeast, the relative levels of these compounds vary and can be perceived quite differently. I find phenolic compounds from wild yeast and bacteria to have a much different (undesirable profile) as compared to German style wheat beer strains and many Belgian yeast strains.
Regarding other possible sources you inquired about, autolysis typically can be better described as sulphery, and dirty diaper. I have not seen where yeast nutrients contribute the type of odor you describe either.
If you place the yeast directly from the package into fully boiled cooled wort do you get the same foul smell?
