Holidays can be a trying time, and when you're preparing a special holiday ham, you might be tempted to skip thawing and stick it right in the oven. Though it seems this would give you more time to focus on side dishes or other holiday tasks, it isn't a good idea. Results can include at best a ruined a dinner and at worst, a trip to the emergency room.
One reason it isn't a great idea to cook a frozen ham without thawing is that the cooking process may not work properly. Ovens heat food from the outside in. According to food network star Alton Brown, ovens work primarily through gradient heat bouncing off the walls and striking the food. Frozen ham is more resistant to heat, so the outside of your ham may burn before the inside is cooked through.
Another problem with cooking frozen hams is potential illness. The U.S. Department of Agriculture recommends cooking "precooked" hams to 140 degrees F and raw hams to at least 160 degrees. Dangerous bacteria multiply quickly at temperatures between 40 degrees and 140 degrees. Since the center of a frozen ham cooks far more slowly than the center of a thawed ham, it may sit in this danger zone long enough for dangerous microbes to grow and cause food poisoning.
If improper cooking and potential food-borne illness aren't enough to convince you to thaw your ham before roasting, the annoyances of working with frozen ham might. If you want to execute the traditional diamond pattern, you're going to have a hard time carving it into a rock-hard, icy ham. Glazes and crusts are no easier. As the ham thaws, it will release moisture, causing whatever you put on the ham to fall off and pool in your roasting pan.
Depending on your specific concerns, you have alternatives to cooking frozen ham that don't have the same potential problems. If convenience is the issue, use cold water thawing, which requires little monitoring. To do this, fill a food-safe bucket with cold water, set it in your kitchen sink, add a wrapped ham, and set your faucet to trickle small amount of water into the bucket. Or, you can by a refrigerated ham, which requires no thawing before cooking.