Could zinc be toxic to yeast above 0.5 mg/ml?
The first time I remember seeing a discussion about zinc in brewing was around 2000 or so. This was part of a much longer discussion on stuck fermentations and high alcohol styes. The question was which of the necessary nutrients was the "limit" factor. Meaning, which mineral, when exhausted, caused the yeast to slow and shut down.
The limit mineral quickly was thought to be Zinc, with the next two being Manganese and Copper. All of them being related to how much specialty malt and/or kettle sugars (i.e. Belgian Quads) was in the recipe. This went further into malt extracts and their notoriously low FAN levels.
There's a sweet spot of Zinc that is around .2 mg/l for ales and .3 mg/l for lagers. In that range, beer will finish faster and drier than without it. Both yeasts can handle amounts above .5 but the side effect is a longer lag time and a slightly metallic aftertaste.
Generally, an all malt beer doesn't need any nutrient to finish fermentation. All the nutrients needed are already in pale malt, along with the enzymes and starches. ...That's the thing about malt, it wants to become beer.
What the nutrient does is aid the yeast with flocculation and stasis. Basically, a preventative for autolysis and health enhancement for repitching. Kinda like a protein bar on a long hike.
The main thing is that once you feed the yeast with nutrients, the limiting factor is the sugar content. From that point forward, excess nutrient becomes food for contamination organisms.
