Considering the average size of a Thanksgiving turkey is 15 pounds, a 21.5-pound turkey is quite large and thus requires a relatively long cook time (expect about 4.5 to 5 hours). However, extended cooking times present two main challenges: preventing the meat from drying out and preventing the skin from overcooking before the meat is done. The key is to prep your turkey properly, cover the turkey for the first part of the cooking process, refrain from basting and to avoid opening the oven frequently. To cook a 21.5-pound turkey at 325 degrees F, take the following steps to ensure a tasty finished product.
SET UP THE OVEN: Position an oven rack on a low enough tier that the center of such a large turkey will be centered in the oven chamber. Preheat the oven to 325 degrees Fahrenheit.
PREPARE THE TURKEY: Remove the turkey neck and giblets from the body cavity of the turkey if necessary. Though conventional wisdom says to rinse the bird before cooking, this might actually help spread harmful bacteria all over your kitchen sink and counter. Cooking should burn off all the bacteria, as long as you follow the timing listed below. If for some reason you do need to rinse your turkey, make sure you also wash anything that comes in contact with the raw turkey to prevent spreading bacteria to other dishes you might be cooking. Use hot water and soap, especially on your hands.
POSITION THE BIRD: Insert a cooking rack into a shallow roasting pan. Place the turkey on the rack, breast side up. Tuck the ends of the legs under the loose skin at the tail area. Tuck the ends of the wings under the shoulders.
KEEP THE SKIN MOIST: Brush the surface of the turkey with melted unsalted butter, olive oil or another vegetable oil. Sprinkle salt and ground black pepper over the bird. Add any other seasonings according to taste or your recipe.
TO STUFF OR NOT TO STUFF: Though many people like cooking the stuffing inside the turkey, the USDA recommends that you cook it in a separate casserole dish. If you insist on cooking it in the turkey, keep wet and dry ingredients separate until just before you stuff the turkey. Pack the turkey loosely with stuffing and cook the turkey immediately after stuffing to avoid contamination. Alternatively, you can let the turkey cook for half the recommended time, and then stuff the turkey. This method allows you to prep the stuffing while the turkey starts to cook.
FINISH THE PREP: Pour a half cup or one cup of chicken broth or water into the bottom of the roasting pan. Stick an oven-safe meat thermometer into the deepest area of one thigh muscle. Make sure the thermometer is not in contact with bone, and that its tip is in the middle of the muscle. Either place the roasting pan lid on the turkey or tent the bird with a large sheet of aluminum foil. Put the turkey in the oven.
COOK THE TURKEY: Rotate the roasting pan once an hour to promote more even cooking for a turkey this large. This helps account for temperature variances throughout the oven, explains the Science of Cooking website. Keep the roasting pan from touching the edges of the oven and obstructing heat flow, if possible. After 60 to 90 minutes, remove the aluminum foil.
CHECK IN ON THE BIRD: Replenish the chicken broth or water at the bottom of the pan as needed when it evaporates. If you want moist, tender skin, refrain from basting the turkey, advises the University of Illinois. Basting doesn't make the turkey more moist; it promotes browning of the skin. Such a large turkey's skin often browns too fast relative to the required cooking time. Opening the oven for frequent basting also lets heat out, extending what is already a long cooking time. If you have an oven light, use that to check in on the cooking process, rather than opening the door.
OBSERVE COOKING TIME: Roast the turkey until the meat thermometer indicates the internal temperature in the thigh muscle is at least 165 degrees Fahrenheit. Use the meat thermometer to test the center of the stuffing if your turkey is stuffed and make sure the temperature there is also at least 165 degrees Fahrenheit. Expect it to take 4.5 to 5 hours to cook a 21.5-pound turkey at 325 degrees Fahrenheit if your turkey is unstuffed. If it is stuffed, add at least 15 minutes to the total cooking time. Determine when the turkey is done by internal temperature only and don't rely solely on the fact that your pop-up thermometer has "popped up," as cooking time always varies.
GET READY TO EAT: Move the turkey onto a serving platter or large cutting board when it's finished cooking. Let such a large bird rest 20 to 30 minutes before carving to make it easier to slice and serve. If you stuffed your turkey, remove the stuffing before carving.