Day 1: Make the cheese curd
To understand the science behind cheesemaking, it’s helpful to remember that it began as a way of preserving milk. You start by encouraging milk to curdle so that you can separate the solid portion (the curds) from the liquid (the whey). Rennet, a natural enzyme, is added to cause curdling. You also add live cultures, here in the form of yogurt—these “eat” the milk sugar (lactose) and produce an acid, which lowers the milk’s pH. That acidic environment, along with heat, helps the rennet curdle the milk.
Once the milk coagulates into curds, you cut into it to let the whey flow out. The remaining whey is drained off by hanging the curd in cheesecloth for 24 hours at room temperature. Once drained, the cheese will have reformed into a solid mass, ready to be cut into cubes and then sprinkled with salt to draw out any remaining whey.
Heat the milk and add the rennet and calcium chloride, which help the curds form, and lipase, which lends feta's distinctive flavor.
Remove the milk from the heat, cover, and let sit undisturbed until the curd is firm and has a clean “cleave,” 1 to 3 hours.
With a table knife, cut the curd all the way to the bottom of the pot in a 1/2 -inch crosshatch pattern.
Pour the curd into a cheesecloth-lined strainer and let the whey drain off the whey for 30 minutes. Reserve 1 quart of the whey to make a brine for aging the feta.
Gather the ends of the cheesecloth and tie them loosely at the top of the curd, then tie them around a long spoon or several chopsticks.
Hang the bag inside the pot at room temperature, loosely covering the top with plastic wrap. After 24 hours, you should feel a firm, solid mass of curds.