Lactic Acid vs. Rennet Coagulation

There were many great questions asked by attendees at counter culture Charlotte a few weeks ago (#countercultureCLT).  Here, we deep dive into one: what's the difference between lactic acid and rennet coagulation?

 Lactic Acid Fermented Cheese - Photo by:  The Mobile Monger

Lactic Acid Fermented Cheese - Photo by: The Mobile Monger

To understand the difference between lactic acid coagulation and rennet coagulation, first you’ll need to understand the composition of milk, and how it changes as it’s transformed into curd. 

Milk is basically water with solids suspended in it—fat globules, proteins, minerals, vitamins, and enzymes. To make milk into cheese, the first step is to separate a good portion of those solid components from the water in a controlled way: to “curdle” the milk. 

We can do that by changing the structure of specific types of milk proteins. Called casein proteins, they organize themselves into balls called micelles that float around in the milk. Surrounding the outside of each micelle, there are specific types of casein protein molecules, called kappa caseins, that look like little hairs. They have a negative charge, so the micelles repel one other while suspended in the liquid—never bonding together, never falling apart. 

Until! We add a magic ingredient that breaks down the micelle structures and reorganizes the proteins into a firm network. Well, there are actually two magic ingredients that achieve this: acid and rennet. And they both work in different ways. 

Acid neutralizes the negative charge of those kappa casein molecules, which causes the micelles to break down until the casein proteins re-organize themselves into a kind of loose network. You’ve seen this happen if you’ve squirted lemon juice or citric acid into milk; it curdles right away. But cheesemakers have another method of acidifying milk: lactic acid bacteria. These little buggers consume lactose (milk sugar) and convert it into lactic acid. Add them to milk and leave it at a lukewarm temperature over several hours to a couple of days, and it’ll become acidic enough that it transforms into a “lactic” curd. 

There’s a second method of coagulating milk, though, which most cheesemakers use—often in conjunction with lactic acid bacteria. The secret is an enzyme called chymosin, the active ingredient in rennet—which is derived from the stomach of young ruminants. It works in a different way than the acid; instead of neutralizing the negative charge, of those little hairs, it chops them off. With their new haircuts the micelles are let loose, and float towards one another to form a sturdy protein network. 

The networks that form from both acid and rennet coagulation trap delicious fat globules and minerals inside, as well as a bunch of liquid—so in either case, the cheesemaker’s next step is to set more of that liquid free, and to manipulate the moisture content, pH, and size of the curd (which all varies according to the recipe). 

Curd made from lactic acid bacteria is very loose and delicate. With a higher acidity, much of the calcium that would otherwise support a firm network runs off with its whey. One can’t do much with this type of curd, aside from gently scooping or pouring it into cheesecloths or molds, leaving the whey to drain slowly. Cheeses made in this way are soft, delicate, high-moisture, acidic products that are usually consumed young.

Rennet curd is firmer; it’s more robust. The maker can do more with it, can take more steps to ensure that the liquid whey is separated from solid curd. The strength of rennet curd allows us to chop it into little pieces (curds), stir the curds around, and do all kinds of other fancy additional steps. Those subsequent options result in endless variety in recipe potential. That’s why almost all cheesemakers use rennet—particularly in any firm cheese that needs to be aged.  

 Cheeses made with rennet coagulation - Photo by:  The Mobile Monger

Cheeses made with rennet coagulation - Photo by: The Mobile Monger