Many cheeses experience a brief rise in pH after they’re made, the so-called “buffering” phase
Life of a Cheese
When one thinks about the lifespan of a cheese two main phases come to mind. (1) The initial steps the cheese maker takes to produce the cheese, and (2) the affinage process that allows cheese to develop its characteristic flavor and texture. The former happens in a matter of hours, and the latter can take weeks/months/years depending on the cheese variety. While both steps are discussed at-length in many different blogs/books/articles, the phase in between is nowhere to be found outside of scientific literature.
If a cheese at day zero is considered an infant, and a fully mature cheese is an adult, we’re going to discuss a cheese’s adolescence. This is better known by the “buffering” or “equilibrium” phase. It’s important to know this phase can vary dramatically across cheeses.Top
Before we dive in, let’s recap.
If you recall, cheese is made up of casein protein chains. And in this spongy mesh of casein, fat and moisture are dispersed. The “glue” that is holding this protein mesh together is calcium. In reality it’s a little more complicated. Specifically, calcium phosphate is the glue bridging those caseins together. See below for brief foray into calcium phosphate chemistry.
Acid is crucial to many cheeses. You may recall acid can be introduced into cheese a few different ways. Mainly, starter culture metabolism, by direct addition of acidifying agents, and combinations of the two. In the case of starter culture, the culture bacteria of consuming lactose and converting it to lactic acid. You may recall the “acidic” part of an acid is a proton, which is usually depicted as H+. If you see an H+, just think of that as acid. Acid is measured in terms of pH. It’s an inverted scale, so lower pH means more acid. Higher pH means less acid.
What does “buffering” actually mean?* In our case, it describes the slight rise in pH after a cheese is made. This is counterintuitive upon first thought -- acid is produced by the starter culture, which is lowering the pH, what would make the pH go back up?
The diagram above shows the pH curve of mozzarella as an example. Mozzarella's pH may get as low as ~5.1, but then buffer back up to ~5.4 the day after it's made. Buffering is due, in large part, to the chemistry of the casein protein that’s in the cheese. Portions of the protein can “absorb” some of the acid thereby raising the pH. Another way of thinking about it is the casein binds some of those H+’s we talked about earlier. With less of them freely floating around, there is less acidity and thus a higher pH. Read on to learn more.
*Buffering, buffers, etc. have different definitions depending on your discipline: chemistry, computer science, etc.Top
It’s been almost a year since I’ve launched cheesescience.org. In that time, I’ve droned on and on about how “calcium is the glue that holds cheese together”. While that is true to some degree, it’s also an over simplification. Another important mineral is involved -- phosphate. Calcium phosphate forms the cross-links between caseins proteins binding them together. I’d love to say that’s the end of the story, but unfortunately even that is an over simplification to some degree. Things get more complicated when you consider some of the phosphate in milk is bound directly to casein, some is freely floating in the milk (soluble), and some forms insoluble clusters with calcium. The latter is referred to as colloidal calcium phosphate (also known as CCP, which makes it even more confusing since the American Cheese Society decided to use that abbreviation for their own purposes). So… that’s really the “glue” I’ve been referring to for all this time.
What does this have to do with buffering? We’ve already learned that when acid is created during the cheese making process, calcium is dissolved from the cheese structure. This removal of calcium allows some of those acidic H+’s we’ve talked about to get sucked up by those phosphates. Now that they’re no longer freely floating around, the pH goes up. I know this sounds confusing, hopefully the animation below helps.
In addition to the pH rising slightly, several other phenomena may occur during the buffering phase. Going back to mozzarella as an example, the cheese behaves different before the buffering/equilibrium phase has finished. Very young mozzarella has a lot of free moisture, melts poorly, and can be very white. After buffering, melting improves, water is absorbed, and the color gets creamier.Top Disclaimer
All of the figures shown in this post are highly schematic and none of them are to scale. Some liberties were taken with the calcium phosphate section in order to shorten it.
For More Information
- Importance of Calcium and Phosphate in Cheese Manufacture: A Review
- CDR Dairy Pipeline - Cheese pH—What’s behind the rise and fall?