Sprinkle the citric acid powder (crush tablets if using) into a cool, empty, large non-reactive stockpot. Pour 1/4 cup water over it and stir it to dissolve.
Pour the milk into the citric acid solution and stir it to combine.
Put the pot over medium-low heat and heat it to 90 degrees (use an instant read thermometer.) You may notice the milk beginning to curdle, but this is a good thing!
When the milk reaches 90 degrees, remove the pan from the burner and pour in the rennet mixed with water.
Stir the mixture with a gentle, circular up and down motion for about 30 seconds, then use the spoon to stop the motion of the milk so it will “be still.”
Place the lid on the pot and walk away for exactly five minutes, ensuring that the pan is not moved. After five minutes, the mixture should resemble a very soft custard.
With a spatula long enough to reach the bottom of the pot (we used an offset spatula), cut the curd into a 1-inch checkerboard pattern, making lines across, then lines in the other direction. Return the pot to the the burner over medium heat and stir it gently until the temperature of the whey (the liquid that separates from the curd) reaches 105 degrees.
Use a slotted spoon to transfer the curd to a colander set over a bowl. Slowly spin around the colander to allow the whey to drain from the curd. Remove the cheese and gently squeeze it a few times to drain off more excess whey. IT MAY BE VERY HOT.
Transfer the cheese to a microwave-save bowl and microwave the curd on high for 1 minute. Remove the bowl from the microwave and pour off as much whey as you can. You can reserve the excess way to make bread. Press the curd together with your hands and again drain off the whey that comes out.
Microwave it again on high for 35 seconds, the press the curd together again to drain the whey. Knead and stretch the curd a bit to drain more whey, then microwave it one last time for 35 seconds.
Knead in the salt and roll it under itself until it forms a neat ball, and set the ball into an ice water bath and let it rest in there until totally cool.
Note
It’s best to eat the mozzarella within a day, or it can be kept in a brine (salt water) for up to a week.
You can get citric acid tablets and liquid vegetable rennet on Amazon or in many different places.
If raw milk is unavailable, you may also use pasteurized milk, but will end up with a more firm texture. Raw milk will result in a silkier texture.