Lots of people are buying toaster ovens so that they don't have to turn on their energy hogging full-sized oven for small meals. Then they notice that most prepackaged frozen meals have this warning on the back: "Do not cook in a toaster oven." These toaster ovens are just smaller versions of normal ovens, so is it really a bad idea to cook frozen meals in them?
First, lets look at the differences between toaster ovens and full-sized conventional ovens. Both of these ovens produce dry heat and in general, both of these ovens have the same basic temperature range. The method of heating varies from oven to oven, with conventional models using either gas or electricity as the primary energy heat source. Toaster ovens rely solely on electric heat, but this is no different from many regular ovens out there. So why would you not be able to cook a frozen meal safely in a toaster oven?
I had some idea of why this might be, but rather than rely on my assumptions I went straight to the source. I contacted three major food production companies whose frozen meals warned against using a toaster oven. All three gave me the same basic answer: uneven and unpredictable heating in a toaster oven could lead to improperly cooked food. That is dangerous and can lead to food poisoning.
I have tested many toaster ovens throughout my lifetime and I can tell you that they do not all have uneven and unpredictable heating, but many oaster ovens do. The issue with some toaster ovens is a discrepancy between the temperature you set it at and the actual internal toaster oven temperature. You may have set the oven to 400 degrees F as directed by the frozen food package instructions but have an actual temperature of much less than that. This is especially true with cheap toaster ovens that are poorly insulated. This can lead to undercooked foods which can be dangerous, sometimes even fatal, to eat.
Is there a way around this awful problem? There certainly is! Accurate instant-read thermometers are easy to find and inexpensive. Many of these thermometers have probes that can be left inside the toaster oven while it is in use. This will allow you to ensure the internal toaster oven temperature is where it needs to be. This will take care of any issues regarding unpredictable heating or temperature when cooking in your toaster oven.
Uneven heating is a much more difficult problem to overcome. Many cheap toaster ovens cook food very unevenly, leaving some portions of the food burnt while others are raw or undercooked. This isn't just an issue with frozen meals; it is problematic with anything you cook. The problem is just amplified when you start with food that is frozen rather than room temperature or close to it. I would never attempt to cook a frozen meal in any appliance that has issues with uneven heating. I would also not want to use that appliance for any of my other cooking needs.
So is it safe to cook a frozen meal in a toaster oven? I would say the answer to that is "maybe." You probably should not risk it unless you are absolutely sure the toaster oven you are using cooks things evenly and accurately. Use a thermometer to monitor your actual toaster oven temperature and check the internal temperature of the food once it is done. Most packaged foods will tell you what the internal temperature of the food needs to be for that food to be safe. There are also temperature guidelines provided by most local health departments. Use those if you are unsure of what the target temperature of the finished product needs to be.