How long are you mashing? If, for example, you mash for 60 minutes using American 2-Row Pale as a base and for the same amount of time with Munich, you might not be converting all of the sugars you need to.
Why? Enzymes. To put it simply, Munich malt (and to a lesser extent, Vienna) has lower diastatic power than American 2-Row. It will take longer to convert starch into fermentable sugars.
Weyermann's Munich II has a diastatic power of 25 Lintner. Briess 2-Row Pale has a diastatic power of 85 Lintner (Weyermann Vienna has 50).
I've seen multiple sources--though admittedly they might all come from the same origin--that say you should try to aim for about 25-35 °L per pound of grain in your mash. My results tend to show that you can completely ignore any grain that doesn't need to be mashed (e.g. crystal, chocolate, etc.) when calculating.
The quick way to calculate this:
- Add up how many pounds of grain you have that needs to be mashed.
- Find out the diastatic power of each grain (available online or in an app like BeerSmith). Multiply that diastatic power by how many pounds of each grain you are using and add them all up.
- Divide the diastatic power by the pounds of grain. If it's above ~30, you should be fine, but you might want to increase your mash time to ensure proper conversion.
To use an example, I have a beer I'm brewing tomorrow that uses Munich II as a base and includes Aromatic, which must be mashed. All my other ingredients don't require mashing, so I'm ignoring them for my calculation.
- Pounds of Grain: 8 pounds Munich II + 0.5 lbs Aromatic = 8.5 lbs
- Total Diastatic Power: (8 lbs x 25 °L) [for Munich II] + (0.5 lbs x 30 °L) [for Aromatic] =
- Diastatic Power for this Mash: 215 °L / 8.5 lbs = ~25.3
That's on the low end. Sometimes that means I'll add a little something with a lot of diastatic power (such as Briess 6-Row Brewers Malt, which has a diastatic power of 160 °L). What I usually do, though, is extend my mash time by 30-60 minutes. That usually serves me well and it doesn't alter the flavor of my beer.