Thursday, October 11, 2012

Mash pH

It's hard to find any good rules of thumb out there for how much lactic acid to add to the mash to adjust the pH, and there's good reason for that.  The starting mash pH is very dependent on a number of complicated factors and interactions.  The main driver of the starting pH is the pH of the grain itself and the dissolved minerals in the water interacting with the grain.  These will stabilize at some pH value when you have thoroughly stirred in your grains.  How this all works is... complicated, and you really don't need to understand it to brew great beer.  The fact is you really aren't going to know what your pH is until you get to dough it.
...but there is an easy way to figure out how much acid to add after dough in!
Once you have doughed in you can then measure and adjust the pH.  While the starting pH is hard to determine, the strength of  the buffering agent is notBecause there are so many more dissolved particles of grain in the water than dissolved particles of minerals, the buffer power of the mash is almost entirely driven by the amount of grain that you have added to your water.  The more grain added to the water, the stronger the buffer.  The stronger the buffer, the more resistant to change the pH will be.  Because it is the grain driving the strength of the buffer, the pH change of a volume of acid will not be effected by the amount of water in your mash tun.  Meaning that whether you have 2 gallons or 20 gallons of water, it will not affect how much 1mL of acid changes the pH.  The fact that the mash pH is not related to the volume of water may seem very strange, so let me explain further.  pH is a function of the ratio of positive ions to negative ions.  The water itself has very few ions, so while it does have a pH value, it is a very weak buffer.  The grain, however, has tons of particles that dissolve into the solution, meaning thousands of times more ions than the minerals in the water.  Therefore, the ratio of ions is driven by the grain, not the water.  To beat a dead horse, because this was such a light bulb moment for me, let's use some number to explain it one more time.  If the water had equal number of ions, five positive and five negative, (5+, 5-)  the ratio would be one to one (1:1).  If we add grain with ten thousand positive ions, and five thousand negative ions (10,000+, 5,000-) the ratio is now ten thousand and five to five thousand and five, (10,005:5,005) or about two to one (2:1) which is roughly the same as the grain alone.
Using the EZ-Water calculator, (1) and several brewing experiments, I have come up with a simple equation:

 mL of lactic acid needed = change in pH * weight of grain in lbs

For 10 lbs of grain you would need 1mL for every 0.1pH point.
For 5 lbs of grain you would need 0.5mL for every 0.1pH point
This is for 88% lactic acid solution which seems to be pretty common. To make measuring simple without a pipette use a normal eye dropper.  It will hold about 1mL and each drip is about 0.1mL

pounds of grain

milliliters of 88% lactic acid to add  to achive a hot mash pH of 5.2

pounds of grain
61/23/43/411 1/41 1/41 1/21 1/2
5.91/21/23/43/411 1/41 1/41 1/2
hot5.81/41/21/23/43/4111 1/4
mash5.71/41/21/21/23/4111 1/4

teaspoons of 88% lactic acid to add to achive a hot mash pH of 5.2


  1. I've been looking for something explaining this for a while. Thanks for the write up!

    1. Thanks! The first time you give this a go, you might want to start with half the recommended amount. It's better to be short and have to add a little more than to end up needing to add baking soda to back track.