pHeel the Stretch (Recipe Testing, Part 4)
Mozzarella and I go way back. We weren’t very friendly in the beginning. I tried to make it a number of times and never got it right. Then the Milk Maid came along and invited me to her Mozzarella making class where I successfully stretched curd for the very first time. Now we’re friends; Mozzarella, the Milk Maid, and I. So, I think it’s poignant that she assigned Mozzarella as my fourth recipe to test for her upcoming cheese making book.
I took it on a few weekends back and invited my friend, Gretchen, to help out, since Mozzarella is a pretty social cheese. Gretchen is also a pretty social cheese and had a dinner to go to, so it was an accelerated Mozza-making day. You can’t rush good cheese though, and by the time we got around to the stretching, it was almost time for her to go. The curd had ripened for about two-and-a-half hours (the recipe called for two to three), so we boiled some water and gave it a try. No stretch. The strips of curd felt thick, stiff, and breakable as we kneaded them under the hot water. They would not meld together, and taffy-like stretching was not in the… ahem… curds. We managed to smash enough curds together in a ball that Gretchen took home a globe of very squeaky Mozzarella curds, but I was disappointed that the cheese hadn’t worked and she hadn’t been able to experience the fun of Mozzarella stretching.
Frustrated, I left the rest of the curd in its warm pot and went off to watch TV. I came back a couple of hours later and, feeling revived, decided to give it another go. Of course, without witnesses, this time it worked! The curd kneaded out like silly putty, squished together, became a long rope, doubled over on itself, stretched out again, and was eventually twisted into a beautiful knot (my favorite way to finish a Mozzarella.) Now it had the squeak-free consistency of real tender, stringy Mozzarella.
So, what was the secret? Acid. Now, I’m no chemist (my grades in high school chemistry were low and agonizingly earned), but apparently chemistry is pretty important in Mozzarella making. Some recipes say you should even have litmus paper or a pH meter on hand to test the acidity of curd before you stretch. I just kind of wing it, but this particular experience was a good demonstration of the real effect acidity has on curd.
From what I’ve learned, the general acidity of cow’s milk is 6.5, which actually puts it more in the range of acid than base. The natural acid in milk is lactic acid, which is produced and increased by lactic bacteria that occur naturally in milk, but are also introduced when you add a meso- or thermophilic culture (which is especially important if you are using pasteurized milk, which has been sterilized of all its existing bacteria). The bacteria populate and inoculate the milk, feed on the milk sugars in the milk (or more scientifically, ferment the lactose), and convert it to lactic acid. Acid itself has a curdling effect on milk (like vinegar and lemon juice), which is why soft cheeses and dairy products like Crème Fraîche often don’t call for rennet. But it also creates an ideal chemical environment for coagulants like rennet to work in, so that harder cheeses can be achieved. As lactic acid continues to accumulate in the coagulated curd it denatures proteins and causes textural changes. On one website I saw, acid in Mozzarella was compared to gluten in bread, giving it plasticity and allowing it to stretch. In Mozzarella making, allowing the curd to sit lets those bacteria keep working, upping the acidity of the curd (to an ideal pH of 5.2) and enhancing plasticity. My Mozzarella just needed a little extra time for the bacteria to do their work… you just can’t rush microorganisms, and like I said, you can’t rush good cheese!