Prepare a sanitizing solution of 1 quart of water and 1 tablespoon of household bleach.
Rinse a large stainless steel or enameled stockpot or soup pot with the sanitizing solution and pour the milk and cream into the pot. Place over medium-high heat and bring to a boil, stirring as necessary with a sanitized spoon so the milk doesn't scorch on the bottom of the pot. Reduce the heat to low and stir in the lemon juice. Continue stirring slowly until the milk curdles and white curds float to the top. Remove from the heat and cover. Wait for 5 minutes while the curds solidify slightly and become firmer.
Rinse a colander and a large piece of muslin or several layers of cheesecloth in the sanitizing solution. Wring out the cloth to release the excess water. Line the colander with the cloth and set the colander in the sink or over a large bowl.
Using a sanitized perforated spoon, gently ladle the curds into the colander. Let the whey drain for 30 minutes, or until the ricotta is still moist but fairly dry.
After about 15 minutes, lift the edges of the cloth toward the center of the colander to loosen the cheese from the cloth and facilitate draining. If the ricotta is still too liquid and runny in texture after 30 minutes, gather the edges of the cloth together using 1 hand, wrap a piece of kitchen string around the gathered cloth to form a bag, and tie it closed. Hang the bag containing the cheese from the faucet or the handle of a kitchen cabinet door and allow the excess whey to drip from the bag into the sink or a bowl.
If adding salt, transfer the ricotta to a sanitized bowl and stir in the salt. Place the ricotta in an airtight container and store it in the refrigerator. Use within 5 days.