The more I learn about it, the more I think zinc is the key. And now that I've been adding actual zinc to my repitches for close to a year, it looks like the data is backing that up.
As day_trippr rightly noted, dead yeast is good yeast food. It's not a raisins/olive oil fad. If you think about how yeast used to be handled pre-biochemistry, it makes sense and it worked for a very long time.
The next thing that I'm thinking about doing is figuring out how to denature and keep all that yeast that I dump down the garbage disposal. Sure, yeast nutrient is often called "cheap insurance," but I'll posit that free insurance is even better.
You are right about the zinc. According to the most
recent issue of BYO, it isn’t the dried yeast that is a good yeast food, it’s the zinc they add to the yeast in the Servomyces that allows them to get around the Reinheitsgebot:
“Servomyces is another convenient way to add zinc to wort. Like many biologically produced products used in German brewing, Servomyces allows German brewers to be Reinheitsgebot-compliant and to deliver the yeast dose that helps fermentation (direct addition of zinc from zinc sulfate and zinc chloride is not permitted by the Reinheitsgebot). Servomyces is made by growing yeast cells in an enriched media. Zinc and other trace minerals are brought into the cell, the cells are dried, packaged like other dried yeast, and added to the wort during the last 10 minutes of the boil to kill the yeast cells.”
Now as for the raisin water, I just happened to be
reading about sourdough cultures and this reference had caught my eye because I hadn’t heard about the raisin thing:
“The growth of yeast and bacteria depend on three key factors: availability of nutrients, acidity, and temperature. Because growth can happen exceptionally fast, species and strains that aren’t adapted to a specific diet (like flour) can quickly be overwhelmed and die out. This is precisely why the inoculants, such as raisin water, that some bakers use to jump-start their levain don’t make a difference. (We think flour, which is chock-full of microbes, and water work just fine.)”
Sourdough cultures have evolved to do well in flour while our beer yeasts have evolved to thrive in wort. Not sure if zinc is needed for bread yeast, but the zinc does seem to be key for fermenting beer.