Do I need to worry about adjusting RO water?

Reverse Osmosis (RO) water has gained popularity since it offers brewers a "blank slate" for their brewing. Most ions are stripped from the water. However, some brewers have found out the hard way that they still can screw up their beer using that water.

RO water doesn't enable the brewer to totally ignore water chemistry in brewing. A case in point was a brewery that John Palmer and I visited in Central Indiana in 2013. That brewery used 100% RO in their brewing with no acid or mineral additions. While that worked for a few of their beers, it was a failure for others. Their IPA was particularly substandard since it suffered from a tannic astringency due to a resulting high mashing pH. The flavor of the beer was also relatively bland due to its lack of flavor ions (Mg, Na, Cl, SO4) in the water.

Using pure RO water, a brewer might be able to mash an amber-colored grist to a proper pH without adding acids or minerals. However, a combination of treatments might be required for RO water in order to get a mash to a proper pH for most other grists. A yellow grist is going to need an external acid and/or a calcium addition to get the pH down. Darker grists might need an alkalinity addition to avoid an overly low mash pH. And in almost any beer, having too little flavor ion content can leave a beer bland or tasteless. (Overdoing flavor ions can screw beer up too!)

Even with RO water, the mash is likely to need a pH adjustment in the form of an external acid, acid malt, calcium and magnesium additions, lime, or baking soda. Achieving an appropriate mash pH is always the critical concern in brewing. Adding flavor ions is an option, but is not required. Flavor ion additions are part of the brewer's art and palate.

Sparging water is one area where the brewer CAN use RO without any additions. The naturally low alkalinity of RO water means that it can always be used for sparging without acid or mineral adjustments, if the brewer so chooses.

RO water is a great starting point for brewing liquor, but it still requires the brewer's attention and adjustment!