A tender cut of meat, boneless pork sirloin doesn't require much advance preparation. Grilling, baking or broiling steaks straight from the refrigerator renders the meat juicy. Marinating pork sirloin tenderizes the meat even more, and gives the dish an extra burst of flavor. Use milk as a marinade ingredient when you want a mild flavor.

Dairy products, such as yogurt and milk, impart a mild flavor when used as marinades. The mild flavor of milk complements the delicate flavor of pork loin. Buttermilk, which is slightly sour or acidic, gives the meat a slightly tangy taste. Marinades provide flavor by penetrating to the inner layers of the meat. Oil helps impart flavorings. Use at least 1/2 cup of milk-based marinade per 1 pound of boneless pork sirloin steak to ensure the entire surface of the meat is absorbing the flavor. Generally 1 to 2 tablespoons of oil per 1 cup of marinade is enough.

Plain milk alone is not an adequate tenderizer for pork, beef, chicken or fish because it is not acidic. Acid tenderizes by breaking down the fibers -- a process called "denaturing." Buttermilk, with its acidic content, performs the dual task of softening and flavoring pork sirloin. If you prefer to use plain milk, add an acid to your marinade to tenderize the meat, such as lemon juice or vinegar. The acid will also add flavor to the meat. Pork sirloin is already tender and usually does not need much extra help.

Marinate pork steaks in a plastic or glass container. Bowls, containers with airtight lids and zip-top plastic bags all work well. Metal bowls and some ceramic containers may react with the acid in the milk marinade, changing the appearance and odor of the liquid.

Place closed containers of marinating pork in the refrigerator for as few as 30 minutes up to overnight. Do not leave the pork on the counter, which encourages the growth or bacteria, especially when when milk-based products are involved. Cook the marinated pork after 24 hours to ensure freshness. Pork sirloin steaks are cooked enough when a meat thermometer reads 160 degrees Fahrenheit. The center of the pork should be light pink.