Day 1: Make the cheese curd:
Sterilize all the equipment you will need for this first day of work. Clean all counters with hot soapy water or an antibacterial wipe.
In a small bowl, mix the yogurt with 1/2 cup of the milk.
In a deep 8- to 10-quart pot, heat the remaining milk over medium-low heat, stirring occasionally with a slotted spoon, until it registers 90°F on an instant-read thermometer, 10 to 12 minutes. Stir in the yogurt mixture. Turn off the heat (leave the pot on the burner), cover, and let sit for 45 minutes.
Meanwhile, in a small bowl with a soupspoon, stir the lipase with 1/4 cup water until blended—it doesn’t matter if the lipase stays a little lumpy. Let sit for 20 minutes. Stir in the calcium chloride and rennet until the mixture is smooth and blended.
Turn the burner under the milk mixture to medium low, add the lipase mixture and stir with a slotted spoon for 1 minute. Stop the movement of the milk with the spoon and hold a thermometer in the center of the milk—the temperature should be at least 96°F; if necessary, continue heating until it comes up to temperature.
Remove the thermometer, turn off the heat, cover the pot, and let sit undisturbed until the curd is firm and has a clean “cleave,” 1 to 3 hours. To determine a clean cleave, wash your hands with soap and hot water and insert a finger (or a sterilized spoon) 1 inch diagonally into the curd and pull straight up. If the cleave is clean, the curd will split with sharp edges and whey will start to fill the split.
Cut the curd:
With a table knife, cut the curd all the way to the bottom of the pot in a 1/2 -inch crosshatch pattern. Turn the heat to low and heat for 5 minutes. Stir the curd with the slotted spoon and insert a thermometer; it should read at least 96°F. If not, continue heating, stirring occasionally, until the curds come up to temperature, increasing the heat to medium low, if necessary.
Turn off the heat, cover the pot, and let sit for 1 hour, stirring every 10 minutes to break up large chunks.
Drain the curd:
Set a large colander over a large bowl and line it with 2 layers of cheesecloth. Pour the curd into the strainer and drain off the whey for 30 minutes. Put 1 quart of the whey in a sterile 1-quart liquid measuring cup, cover, and set aside at room temperature.
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 for 24 hours, loosely covering the top with plastic wrap. After 24 hours, you should feel a firm, solid mass of curds; if not, let the curd hang for another few hours and check again for firmness.
Day 2: Salt the feta:
Sterilize the equipment you’ll need for this day of work. Clean all counters with hot soapy water or an antibacterial wipe. Untie the cheesecloth and transfer the feta to a cutting board.
Cut the feta into 2- to 3-inch pieces. If you see small, uniform, round holes throughout the cheese when you cut it, and it feels spongy, that means undesirable bacteria have contaminated it and you should throw it out. Otherwise, arrange the squares in a single layer in a sterile shallow container with a tight-fitting lid. Sprinkle about 1/2 oz. salt over all sides of the cheese. Cover and let sit at room temperature for 3 days. Turn the feta daily and resalt with 1/2 oz. salt on days 3 and 4. Each day, pour off the whey as it collects in the bottom of the container.
Day 5: Brine the feta:
Sterilize a 3-quart covered container. Transfer the cheese pieces to the container—it’s fine to stack them at this point. Stir the 2 oz. kosher salt into the 1 quart of reserved whey until it is dissolved. Pour this brine over the cheese, covering it completely. Cover and refrigerate for 1 to 4 weeks. The longer the feta is aged, the stronger the flavor and crumblier the texture will be.