Zinc and RO Water
One of the reasons that brewers like to employ reverse osmosis (RO) water in their brewing is due to its lack of dissolved solids and nearly blank palate. However, that isn't always a good thing.
An ion that is critically important for yeast health is zinc. Typical wort provides everything yeast need for their metabolism, except zinc. In most raw water supplies, zinc is present at the trace levels needed by the yeast. But the RO process takes that trace amount and reduces it well below the level needed for yeast health and duplication. Therefore when brewing with significant percentages of RO water, it is important to supplement the water with zinc.
The preferred zinc content in brewing water is 0.2 mg/L for ale yeasts and 0.1 mg/L for lager yeasts. The difference in zinc content is likely due to the lower pitching rate for ales and its subsequent greater yeast growth. The problem with working with zinc compounds is that the need for zinc is so small and the dose at the homebrew size can be 'microscopic'. Food-grade zinc is typically available as zinc sulfate or zinc chloride. Since solid zinc chloride absorbs moisture from the air and you can't be sure how much its absorbed, zinc sulfate is an easier solid to work with. Zinc sulfate is typically available in a heptahydrate (7 water molecule) form. For zinc sulfate heptahydrate, the dose of that solid is 1 gram per 10 barrels of ale or 1 gram per 20 barrels of lager. As you can see, those doses would be exceedingly small at a typical homebrew batch size. An important factor with zinc use is that you CAN overdose your water and that can result in zinc's metallic taste coming through in your beer.
Ways to dose your water with the proper amount of zinc include: working with teeny amounts of zinc salt, diluting a known amount of zinc salt in water and figuring out how much of that solution to add, using a commercial yeast nutrient that includes zinc, or recycling spent yeast into the boil kettle. All are workable alternatives. Some require potentially tricky math and chemistry knowledge. Working with commercial yeast nutrients is easy since the provider has already figured out the nutrient dose you should use. Do make sure that the yeast nutrient that you use has zinc in it. Not all do! Recycling spent yeast is also easy, but it can impart rubbery, meaty flavor into your beer from yeast autolysis if the yeast dose is too large.
In any case, be sure to consider your water's zinc content (or lack of it) when brewing. RO users should employ some sort of zinc supplement in their water.
