Last week we filmed the video for Craft Beer & Brewing’s upcoming online course on quick sour beers. As we were discussing some of the more technical aspects of the material, it struck me that we often use the term “pH” with abandon, but we rarely stop to really think about what it is. If, like me, some time has passed since your last chemistry class, here is a simplified way to understand pH and, more importantly, why it matters in the brewery.
Almost everyone knows that the chemical formula for water is H2O. [Ed note: Apologies to the precise among you; the system doesn’t do subscripts.] This simply means that every molecule of water consists of two hydrogen atoms and one oxygen atom. The reason that two hydrogen atoms pair up with one oxygen atom has to do with the readiness with which each is willing to either (1) part with its own electrons or (2) steal electrons from other atoms. A hydrogen atom has but one lonely electron, which it is more than willing to offload, much as one ditches one’s suitcase in the hotel room before heading down to the lobby bar for a well-deserved beer.
Oxygen, however, is a greedy little jerk. In fact, it’s twice as greedy as hydrogen is generous, which means that oxygen isn’t satisfied with just one electron: It wants two. And so, a sort of atomic ménage à trois takes place, in which two hydrogens shack up with one oxygen, thus giving us water.
But, alas, oxygen isn’t always content to commit itself to just two hydrogens, and sometimes it seeks the companionship of a third hydrogen atom. And it does this in the most crude way possible. It simply walks up and steals one away from another water molecule. Like I said, oxygen is a jerk (not to mention that it can also ruin your beer).
The end result is that instead of just H2O, we also have some HO (or, equivalently, OH) and some H30 mixed in there. Written in the language of chemists, this is expressed as
2H2O ⇌OH⁻ + H3O⁺
which just says that for every two water molecules, there is some infidelity from time to time. OH⁻ and H3O⁺ are ions that are given the special names hydroxide and hydronium, respectively. The superscript minus sign and plus sign mean that these ions are unsatisfied in their relationships and are prepared to invite into or expel from the arrangement a hydrogen atom, respectively.
The thing is, it’s not just water that exhibits this behavior. Other substances do this sort of thing as well, but it almost always involves the same kinds of interatomic relationship dynamics. Scientists have come up with a way to describe the likelihood of a particular substance to do so, which is called the equilibrium constant. It is a measure of how many hydroxide and hydronium ions are being created and destroyed when a solution is in equilibrium. And, if you measure this tendency for pure water, you find that the numbers of hydroxide and hydronium ions are precisely equal and represent 0.0000001, or 0.00001% of the solution. The remaining 99.99999% is just the H2O we know and love.
A substance that is rich in hydroxide ions is called a base, and a substance that is rich in hydronium ions is called an acid. How much is “rich?” Well, it’s defined relative to plain old water. If there’s a higher hydroxide concentration than there is in water, then we say it is basic. If there’s a higher hydronium concentration than there is in pure water, then we say the substance is acidic. Pure water itself is neutral.
It’s hard to work with numbers that have so many zeros (chemists are just as lazy as the rest of us), so instead of looking at the numbers themselves, they use a logarithmic scale, which just tells you how far to the left or right of the decimal point the number is:
- The logarithm of 100.0 is 2.
- The logarithm of 10.0 is 1.
- The logarithm of 1.0 is 0.
- The logarithm of 0.1 is -1.
- The logarithm of 0.01 is -2.
- And so on…
So, we finally come to the definition of pH. pH is nothing more than the logarithm of the hydronium ion concentration, or in symbols:
pH = – log [H30⁺]
- A pH of 1 means the hydronium ion represents 10% (0.1) of the solution because –log [0.1] = 1.
- A pH of 7 means the hydronium ion represents 0.00001% (0.0000001) of the solution because –log [0.0000001] = 7.
- A pH of 14 means the hydronium ion represents 0.000000000001% (0.00000000000001) of the solution, because – log [0.00000000000001] = 14.
As it happens, the pH scale is defined from 1 to 14. A substance with a pH between 1 and 7 is acidic. If the pH is between 7 and 14, it is basic. And if it’s exactly 7, it’s neutral.
So what does all of this have to do with beer?
Well, there are many things, but one of the most important concerns is the enzymes upon which we rely to convert our malt starches into sugars during the mash. These enzymes prefer to work within a relatively narrow range of pH values. The two most important enzymes for starch conversion, alpha amylase and beta amylase work best in the pH range of about 5.2 to 5.5 (which, in terms of absolute numbers, means the hydronium ion represents about 0.00032% to 0.00063% of the mash liquid).
But pH also affects the way we perceive the final beer’s flavor. Low pH values in the finished product tend to be associated with characteristics such as sharpness, dryness, and bitterness, while higher final pH values can lead to flavors that some describe as soapy or metallic. If you enroll in our course on quick sour beers (look for it at learn.beerandbrewing.com in early October), you’ll learn that pH isn’t the whole story when it comes to your palate. But, understanding it is an important first step in evaluating all parts of the brewing process, from mashing to fermentation, to the final sensory profile you taste in the glass.