Pour the milk into a heavy stainless-steel or enameled saucepan and stir in the white vinegar or lemon juice.
Set the pot over very low heat and bring the milk very slowly to a simmer (a reading of 200 degrees on a thermometer).
There will be fine beads around the edge of the milk, which will look foamy but will not appear to be boiling.
Remove the pot from the heat and set it, covered, in a spot where it can remain undisturbed and where the temperature will remain fairly uniform at a reading between 80 and 100 degrees. (An unheated oven, without a pilot light, is a good spot. ) Let the milk stand for about 6 hours, or until a solik curd floats above the liquid (the whey).
More or less time may be required, depending on the temperature of the environment and the characteristics of the milk.
Line a fine sieve with doubled, dampened cheesecloth (or, better yet, two layers of very fine-mesh nylon curtain netting, dampened) and set it over a bowl.
Dip the curds and whey into the sieve and allow the whey to drain off until the ricotta is yogurtlike.
If you want firmer cheese, tie the corners of the cloth to form a bag and hang it up to drain further. (In warm weather, the draining might well be completed in the refrigerator. ) When the texture of the ricotta is to your liking, add a little salt (from 1/4 to 1/2 t) if you wish.
Store the cheese, covered, in the refrigerator.
It will be at its best after it has chilled for 24 hours, and will keep well for 4 or 5 days.
Author's notes: Unlike most other fresh cheeses-cottage and cream cheese, for example-the curd of this bland, light cheese is fromed from the direct addition of acid to the milk, not by fermentation.
For that reason the time required to make it is generally short.
If you haven't used this Italian favorite before, try it in place of cottage cheese, as well as in Italian recipes for such dishes as lasagne and manicotti.
You'll find it is a bit creamier than most cottage cheeses, with a much finer curd.
For a pleasant light milk dessert, sweeten ricotta slightly and top it with a sprinkling of grated chocolate or cinnamon.