Use fresh whey leftover from making 30-Minute Mozzarella, that is no more than 3 hours old.
Heat the whey in a pot until foam appears. This usually happens just prior to boiling; if the mixture boils, it will taste burned.
Turn off the heat; let the whey set for 5 minutes.
Gently skim off the foam and place the whey in a colander lined with butter muslin.
Let drain for 15 minutes, then refrigerate. This ricotta will keep for up to 1 week in the refrigerator.
Yield: About ½ pound per gallon of whey.