So I did some reading from Google Books...here is some info from Brewing: Science and Practice by Briggs et al.
http://books.google.com/books?id=zV...t&pg=PA163#v=onepage&q=zinc beer wort&f=false
Zinc Concentrations in Sweet Worts from Published Studies (from Table 4.23):
Wort A: 0.12 mg/L zinc
Wort B: 0.07-.16 mg/L zinc
Wort C: 0.1-0.3 mg/L zinc
Briggs et al. recommend a Zinc level of 0.1-0.2 mg/L and Zainasheff/White recommend 0.1-0.15 mg/L (this is from JonK331's post earlier in the thread).
So if we assume if Worts A, B, and C are representative of your typical worts, then you can argue that zinc supplementation is not necessary. All of these worts have the recommended amount of zinc in them (or pretty darn close in the case of the range in Wort B).
Briggs then goes on and states that zinc supplementation may be necessary (cites three references), and for the following reasons:
1. Inorganic ions (such as zinc) may not be adequately extracted from the grains during mash. Only 5% of zinc may end up in the wort post-mash.
2. Of the zinc that is extracted, some may be chelated or otherwise unavailable to the yeast.
3. Consequently, analysis of zinc content is unreliable for determining if there is zinc deficiency. So I'm not sure what this means for the numbers in Table 4.23...does it mean those aren't valid?
My take on it (FWIW): I have no doubt that zinc is important for proper yeast growth and fermentation. But right now I'm skeptical that zinc supplementation would make all that much difference. Even if a wort was zinc deficient, it sounds as if analytical tests would have trouble picking it up. Furthermore, based on the numbers in Table 4.23, it appears that wort may have the proper levels of zinc anyway, without supplementation.
That being said, I can see where it could be a problem though as I can envision a scenario where zinc is poorly extracted from the mash and/or not available for uptake by yeast, leading to a zinc deficiency.
I don't have the time to look right now, but I'd like to see some an instance of where a wort tests and actually has a zinc deficiency (less than 0.08 mg/L). I'd also like to see what effect that has on yeast fermentation (if any).