I have not made this but the author's note on the recipe may answer your question.
Homemade Ricotta Cheese
From The Savory Way by Deborah Madison (Bantam)
It's very easy to make your own ricotta cheese from a few simple ingredients.
Prep Time: 24 hours
Cook Time: 30 minutes
1/2 gallon milk
3 Tbsp freshly squeezed lemon juice, Champagne vinegar or white wine vinegar
Combine the milk and acid (lemon juice, Champagne vinegar, or white wine vinegar) in a heavy pot and turn the heat to very low. Gradually bring the temperature to 180 degrees F. (At 7,000 feet, bring it to 172 degrees F.) It takes about 30 minutes. What you will notice is a ring of very fine bubbles around the edge of the pan. There will be some movement below the surface of the milk, which will be covered by a fine skin.
When it reaches the desired temperature, turn off the heat, cover the pan, and leave it in a warm spot -- an oven that has been heated briefly or has the pilot light on is perfect -- for 6 hours.
Line a sieve or a basket with a double thickness of cheesecloth or a coarse, porous towl, rinsed first in cold water, and set it over a bowl. Ladle the curds into the sieve and season them with salt, roughly a half-teaspoonful. (The whey, which will drain into the bowl, can be used for baking.) Refrigerate overnight or until the cheese is well drained. For a thicker, firm cheese, tie the ends of the cheesecloth over a wooden spoon balanced over a bowl and let it hang until all the whey has drained out.
Fold back the top layers of the cheesecloth or toweling and turn the cheese carefully out onto a plate. The imprint of the cloth will be left on the cheese. This cheese will stay sweet and fresh for 5 to 7 days.
Author's notes: "Traditionally, ricotta is made with whey, a by-product from cheese making, but since that's not usually available, this recipe uses milk. The same cheese is also called curd cheese or quark. Either whole milk or low-fat (2%) milk can be used. Whole milk yields a richer, creamier cheese." --Deborah Madison
Yield: about 1 pint