You can probably find citric acid at a good grocery store, but rennet is harder to come by. You can order it online at several retailers; I’ve purchased it from New England Cheesemaking Supply and Grape and Granary.

I prefer to use liquid vegetarian rennet, which New England Cheesemaking Supply sells at double-strength, so I use 1/8 tsp. for this recipe.

1 gallon Milk, not ultra-pasteurized (Depending on the fat percentage of your milk, you’ll get a very different cheese at the end. Whole milk produces a very rich, soft mozzarella, whereas 1% will make a harder, more string-cheese-like cheese. Fat free can get a bit too rubbery, so I don’t recommend it)
1 1/2 tsp. Citric Acid powder, dissolved in 1/4 cup room-temperature water
1/4 tsp. Liquid Rennet or 1/2 tablet Rennet, dissolved in 1/4 cup room-temperature water
1 tsp. Cheese (Flake) Salt or Kosher Salt

Pour the milk in to a large pot. On medium-low, heat slowly to 55 degrees Fahrenheit. Stir slowly and continuously to keep from scalding. Once the milk reaches 55 degrees, pour in the citric acid mixture and stir well. Keep heating. When the milk hits 88 degrees, add the rennet mixture and stir well. Right around this time the milk will start to thicken, and you’ll see little white flecks stick to your spoon as it starts curdling. Once the milk is in the 90-degree range, it should be noticeably curdled. Stir very gently at this point, if at all — you want to encourage the curds to knit together. Between 95 and 105 degrees, the curds will be quite thick. Turn off the heat once they start separating from the sides of the pot, and there’s a very clear distinction between the curds (white clumps) and whey (yellow liquid). Let the curds rest for 5 minutes.

With a perforated or slotted spoon, ladle the curds into a bowl. The curds will continue expelling whey once they’re in the bowl, which is fine. Once you have pulled most of the curds out of the pot (some little bits will probably still be floating about), pour any excess whey back in the pot. Using a microwave, heat the curds for 60 seconds. Drain off any excess whey, then fold the curds over once, then once again. This is to distribute the heat evenly. Microwave again for about 30-40 seconds, depending on the strength of your microwave. Pour off the whey. Sprinkle the salt onto the cheese, and then fold the curds over twice again. Put them back into the microwave for another 30-40 seconds. Pour of any excess whey.

At this point, the cheese should be very hot, and look like melted mozzarella! The curds will get quite hot — it’s really helpful to have a pair of clean kitchen gloves to protect your hands. Stretch the cheese, and then fold it back on itself. If it tears when you try to stretch it, the cheese is not hot enough; just repeat the microwaving process. Stretch it again once or twice. If you want a more string-cheese like cheese, do it a few more times. You can then twist or braid the cheese, or tear off pieces and roll them into small balls. If you’re going to refrigerate the cheese for later, drop it in a bowl of ice water to get the temperature down quickly. Otherwise, just dig in while it’s still warm!