Your kit will have everything you'll need to make mozzarella, except milk. Grocery-store whole milk is fine, as long as it's NOT ULTRAPASTEURIZED. The best milk, though, is from a local dairy farm, if you can find it. Look for non-homogenized, full-fat milk. It will yield the best-tasting cheese — and more of it.
Heat the milk gently to 85°F. While that's happening, prepare your rennet by mixing it with a quarter cup of cold, non-chlorinated water.
Acidifying the Milk
In order for the curds to eventually stretch properly, they have to have the proper acidity. Both the kits I've used and almost all of the recipes for mozzarella I've seen online achieve this through the addition of citric acid (about 1.5 teaspoons of it, dissolved in 1 cup of cold water, per each gallon of milk). There are more traditional ways of amping up the acidity in your curds without citric acid; one of the best recipes I've seen for it is Professor David Fankhauser's recipe for fresh mozzarella.
Once the milk comes to 85°F, you add the citric acid. The milk solids will begin to coagulate at this point. You continue to heat the milk gently to 100°F — at which point you will add the rennet mixture.
Add the rennet mixture at 100°F, and then stir it in using a gentle up-and-down motion with a slotted spoon. Continue heating the milk until it reaches 105°F. At this point, turn off the heat, cover the pot, and wait for at least 10 minutes.
The curd is ready for draining when it pulls away from the side of the pot and when the back of a teaspoon leaves an indentation in it when gently pressed into it.
When the curd is ready, you transfer it to a colander or fine-mesh sieve over a large mixing bowl. Expel the whey by pressing down gently with the palm of your hand. This can take a while. I hope you enjoy becoming intimately familiar with the feeling of curd between your fingers.
The Home Stretch
Once you've got the whey pressed out, you'll have a lump of curd. Both these kits recommend putting it in a microwave-safe bowl and using that to heat it to stretching temperature — 135°F. I don't have a microwave, so I use an alternative method, which is to simply pour hot salted water (140°F) over the curd and work it for a bit until it starts to melt. At that point, I form it into a rope and stretch it, folding the rope over on itself again and again. You're looking for the cheese to reach a shiny, satiny appearance. At which point, you form it into a ball and plop it in cold water to set.
This is my second ball of mozzarella from my day of cheesemaking. Honestly, it was not that great. I know I can make it better with practice, but you know ... Beyond being fun and educational, it's really not worth it to me personally to get great at making mozzarella. I'd rather perfect my dough and leave the cheese to the pros.
Still, now that I've got the bug, I do want to make a great mozzarella ball before retiring from cheesemaking entirely. Watch Slice for more mozzarella-making in the future.