What happens when you don't Preheat the oven?

Online Answer
By not preheating your "stove and pan" before placing your meat in the oven, the meat will steam before it can sear at that ripping hot temperature it should be roasting in. The high heat is the reason for the wonderfully browned tops of roast chickens, pork loins, and roast beef.
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Hang an oven thermometer in the center of the middle rack and preheat the oven to 350˚F (176.67˚C). This will tell you if the oven is even reaching the desired temperature from the beginning.
In general, it's also ok if you do want to preheat the oven for these items. "It won't make much of a difference," he says, "as the total cook time is fairly long and the oven temperature is relatively low.