One of my favorite science questions came in a phone call that went something like this:

“The power went out and the freezer thawed. When we opened it up, the freezer had flies in it. How did they get in there?”

The answer was a bit unnerving, but it provided an opportunity for a brief lesson on spontaneous generation.

Life does not simply arise from meat in a freezer after a few days or even weeks. Even after millions of years, your home freezer is unlikely to form some primordial ooze capable of life (if it does, I want to know what you were storing in there). However, until the 17th century, it was believed that life could quickly spring forth from rotting meat through a process called spontaneous generation or abiogenesis; maggots were believed to come directly from meat (not from adult flies). This was the case until Francesco Redi showed that maggots only appear on the meat if flies can reach the meat. When the meat was left in an open container, maggots appeared, which is consistent with the concept of spontaneous generation. However, when the meat was placed in a jar covered with gauze, no maggots appeared. He then watched as the maggots developed into flies, which was the first observation of the fly life cycle. This showed that the maggots did not appear spontaneously from within the meat, but were instead placed there by something that could not get through the gauze–namely, the flies. In 1864, Louis Pasteur (you may recognize his name from the term “pasteurization”) helped solidify the fact that contamination comes from exposure to living organisms. After pasteurization/heating a contaminated solution and sealing it from the air, it remained sterile. The very flask he used is still on display and remains sterile nearly 150 years later!

So how does this relate to the flies in the freezer? Well, since the freezer was supposedly sealed shut, no flies came in to lay eggs, which leaves only that stomach-churning answer you may have already realized. There were probably fly eggs on the food that went into the refrigerator. Generally, freezing will kill the eggs, larvae and adult flies; most likely, the flies are a result of dormant eggs in the refrigerator that found their way through to the freezer after the thaw (refrigerators contain a vent connecting the freezer and refrigeration sections). Does this mean that the food was bad or unsafe? Not at all. The insects may not have come in contact with the food and the eggs may have been deposited on the packaging. Nonetheless, I’d hope that any meat in there was going to be cooked before consumption (if everything had not thawed, of course). We are seldom aware of the extra seasoning that sprinkles our food, but rarely does it cause any problems. Certain contaminants can be dangerous and we are made aware of them from time to time, whether it is food poisoning on our chicken (Salmonella) or parasites in our pork (Trichinella), proper handling and cooking takes care of almost all of these. (That can of food with a sudden bulge in the side is another matter entirely; don’t try to salvage that one.)  I’ll go into detail on why you should learn to stop worrying and love your symbiotic bacteria some other time.

Thank you to my parents and their friends for submitting this question.