When I was about six years old, I remember my mum accidentally defrosted too much meat for dinner. She was swearing away, and I suggested “can’t you put it back in the freezer?” She said no. Doing that causes food poisoning.
And for many years, that was the end of it. I never really did much cooking until I moved to Newcastle University, and then I was very careful to not re-freeze my food. After all, it causes food poisoning. Everyone knows that.
It wasn’t until my second year when something changed. I had specialised from regular Biomedical Sciences to Biomed with Microbiology, a course which spends a lot of time working with diseases. One of our lectures was about Salmonella enteritidis, a bacterial subspecies which causes food poisoning. Our lecturer asked us “why don’t we re-freeze food?” Obviously, it’s food poisoning, right? Wrong.
Food poisoning occurs more easily if meat is defrosted outside the fridge. When left on a counter, S. enteritidis can quickly multiply if the temperature of the room is above 5°C. It’s not going to be a problem if you then thoroughly cook that meat that very same day, but leaving it too long will make it almost impossible to kill all the bacteria on the items in question. Just 1,000 Salmonella bacteria can cause an infection, while a piece of meat can have billions or even trillions of them on it. Dettol will not save you.
That’s all very well and good, but what about re-freezing food that has been defrosted in the fridge? That’s perfectly safe. The fridge is too cold for Salmonella to reproduce easily in, so you won’t have too much growth there. Same goes for the freezer. The real reason that we don’t re-freeze food too often is the freeze-thaw effect.
Ask any passing geologist about freeze-thaw and they’ll go all excited and tell you about how water seeps into cracks in rocks, freezes and expands, and then widens the crack in the rock. A similar process occurs in the cells our food is made from (unless you literally drink primordial soup for your meals). Every time food is frozen, sharp little ice crystals form and poke a few holes in the cell membrane. Even a dead cell can survive a few little holes in its membrane, so re-freezing food once or twice is no big deal.
After that though, the holes will eventually damage the membrane to the extent that the cell will burst and leave its contents all over the place. In a lab this can be used to release proteins from inside a bacterium. In a kitchen, it creates a weird mush that used to be a chicken breast or a stew or anything else you might have.
And that is why we don’t re-freeze food.