I always use RO water to brew. I have never checked mash pH, but have become concerned since my brews have come out darker than predicted by BrewSmith.
I thought (I read in How to Brew by Palmer) that adding some DME to the mash will normalize mash pH; so that is what I have done.
Is RO water not the way to go? Or should I be paying more attention to mash pH on lighter beers? Is there a rule of thumb?
For the best results, you should always check your pH and adjust if necessary. Using RO water doesn't change anything. Remember, it's the pH of the mash, not the water, that matters. As pointed out above, you will also need to adjust the mineral content of your water for flavor. That will likely also have an effect on the pH.
You'll get more browning with higher pH, but there are also plenty of other reasons for producing a darker wort, so you'll need to at least check the pH before deciding to do anything about it. Ideally your pre-boil pH should be around 5.3-5.5 - lower is better.
While adjusting pH may help you with the color, you should be adding minerals to the RO water for the benefit of mash efficiency, flavor, stability and yeast health. In particular Calcium a key constituent in brewing liquor.
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