But that flies in the face of thousands of years of brewing history where the water supply was the only zinc source.
Martin, I think history actually testifies to the idea of zinc supplementation being needed.
First, water was not always the only source, as copper and brass brewhouse equipment contributed some small amount. [But this is still a moot point, because (as
@Paulaner notes) textbooks such as Kunze, Briggs, et al., and their numerous primary sources confirm that virtually none of the zinc present in malt and water, including that introduced exogenously upstream in process, survives into the pitching wort. D. G. Taylor, 2006,
may be an outlier contradicting this, but I have not read the original source, only seen the table published in Palmer and Kaminski.] And historically, lager fermentations, for instance, have taken weeks or months to go to completion, and still underattenuated and failed to completely reduce diacetyl (as is still the case especially in smaller, unmodernized, German and Czech breweries,) as zinc is essential for protein synthesis and cell growth, as well as some other metabolic functions. Modern lager fermentations with adequate zinc supplementation will reach 85% AA in 7 days with full diacetyl reduction, and the health of subsequent generations of yeast is greatly improved.
The Germans have long supplemented zinc. Even where brewhouse construction was not from copper and brass, pieces of galvanized equipment have been introduced as sacrificial anodes, and, while propagated Sauergut fortified with spent grain is a relatively recent innovation, as is Servomyces, unlike the sacrificial anodes, these are seen as unqueationably compliant with German law. Use of zinc chloride or sulfate is generally not regarded as Reinheitsgebot compliant, but for those of us not bound by these constraints, is the obvious preferred solution.
