I recently was able to take part in a collaboration brew with a neighboring brewery, which was a lot of fun! We were doing a Berliner Weiss, chockfull of our slow growing, but precocious, strain of Lactobacillus delbruckii to help sour the beer naturally.
As a matter of course, we also decided to use acidulated malt to drop the pH of the beer instead of using Calcium salts to achieve proper starch conversion pH. As it turned out, we had trouble getting proper conversion because the acidulated malt dropped the pH too much and we ended up having to add more barley and wheat to the recipe just to bring the pH back up. Long story short, you will have a very difficult time getting ANY starch to convert if your mash pH is too low.
For those curious about how pH and temperature affect mash dynamics, check out my favorite diagram, found on John Palmer’s ‘How to Brew’ website: http://www.howtobrew.com/section3/chapter14-1.html. This picture is actually set as the background on my laptop as a quick and easy reference for mashing during a brew.
As you can see, according to the diagram, the ideal mash pH for all relevant enzyme activity is between 5.1-5.5 pH. Falling anywhere outside of this range will mean losing efficiency somewhere along the line. A pH meter or pH test strips available at homebrew stores are an easy way to tell if you’re on target with mash pH. Conversely, performing an Iodine test will tell you if you have unconverted starch in your mash. However, it won’t tell you why your starch isn’t converting.
In general, here are a couple great ways to lower your mash pH if need be:
Darker roasted malts are typically very acidic, so porters and stouts in general don’t need to be adjusted for pH
Small amounts of acidulated malt or even Calcium Sulfate salts can be added to bring the pH down to acceptable levels
If the pH is too low and you’re in a pinch, you can add Calcium Carbonate (baking soda) to bring the pH back up. Just be careful how much Calcium you add to your brew because an excess of calcium can lead to other problems.