Acidity and pH: Two fermentation values with two different meanings

Introductory note: I’m not a scientist, just a ferment geek. If I misstate information in this post, please leave a correction in comments or email stacie@multicultured.org. 

Fermenters have a specific interest in pH changes in the food and beverages we’re fermenting. After all, the lactic and acetic acids that microbes create during the process drop the pH of things like sauerkraut into safe range (below a pH of 4.6, and often quite a lot lower), and increase the acidity. Conceptually, you’d think that pH and acidity are the same thing, but in reality, they represent two different measures that don’t necessarily align.

What is pH?

The value “pH” literally stands for “power of hydrogen,” and is a way of describing how many free hydrogen ions (described as “H+”) are floating around in a solution. A solution that’s jam-packed with hydrogen ions is acidic; one with relatively few hydrogen ions is basic. Another way to describe this is to say that an acid is a substance that donates hydrogen ions, and a base is a substance that receives hydrogen ions.

pH is measured on a scale between 0-14, with 0 being a strong acid, 7 being neutral (deionized water has a pH value of 7), and 14 being strongly alkaline. As noted, in the fermentation of most foods, a pH value in the low 4’s or 3’s is attained by the work of lactic acid bacteria, and aside from the acidic environment inhibiting the growth of most pathogens, organic acids like the lactic and acetic acids found in ferments are what preserves the food.

How do I measure the pH of a ferment?

There are several easily available methods to measure pH. Litmus paper, or pH test strips, are the cheapest but also the least accurate. When shopping for pH test strips, make sure they measure the full spectrum, as many are intended for particular ranges. I use these and like them just fine. Here they are in action:

Litmus paper shows pH using a color spectrum.

For more precision, you can buy an electronic pH meter. There are plenty of inexpensive options available via Amazon, and I’ve had good luck with a $20 version staying calibrated between uses. Check out your options here if you’re considering getting one.

So low pH means high acidity, right?

Well, no, not necessarily. When you buy a bottle of many mass market vinegars, you’ll notice they have a “strength” description, usually 5%. This is a statement of the “titratable acidity,” which is technically telling you the concentration of acid in the solution. Importantly, pH and acid concentration do not clearly correlate, and for certain purposes, like canning, the strength of the vinegar (percentage of total acid in solution) in a recipe is every bit as critical as the final pH of the mixture.

Acidity matters for wine makers, too, and is tested and adjusted as needed by professionals and diligent homebrewers. For the home fermenter looking to bring some science into the process, an acid test kit normally deployed by home winemakers can be an extraordinarily fun bit of kitchen magic. Essentially, you slowly add a measured amount of a strong base (sodium hydroxide) to a known quantity of brine, vinegar, wine, or what have you, mixed with a reagent, and wait for a chemical reaction to occur that changes the color of the mixture. In the picture at the top (and here so you don’t have to scroll), that pink liquid – vinegar diluted in water, with an indicator fluid mixed in – started clear. Based on the amount of base it took to turn it pink, and using some math, I was able to calculate the acidity of the vinegar in the back.

Titrating the acidity of vinegar.

I’d add that the moment the reagent’s color changes is pretty unexpected and exciting. Even when you know it’s going to happen, it’s still… well, I did not excel at chemistry in high school, so my adulthood is filled with delayed gratification of stuff like this.

Why does any of this matter?

For most of us, in most of our endeavors, it doesn’t matter at all. However, I find the science of fermentation fascinating, and have used pH strips and acid titration to evaluate my ferments against published standards, like this paper by Steinkraus, which is full of interesting facts about a variety of common (and uncommon, in America) ferments.

For instance, according to Steinkraus, these are common pH and total acidity values for several ferments:

As you can see, pH and total acidity don’t clearly align, with something like a cucumber pickle developing roughly the same pH as sauerkraut with potentially around one-quarter of the total acidity.

So there you go, a quick primer on pH and acidity in home fermentation, what each value means, and a bit on what that looks like in the actual practice of fermentation.