Place the milk in a saucepan over medium-high heat. Bring it just to below boiling (180 degrees F) and then remove the pan from the heat. Stir in the lemon juice and let the mixture stand, uncovered and without stirring, for 15 minutes. After the mixture has sat undisturbed for 15 minutes, pour it into a cheesecloth lined sieve and strain off the liquid. Press down on the curds to remove as much of the liquid as possible. Place the curds in an airtight container and chill well before using. Add salt, to taste, if desired.