Asked by: Jacob Hipkiss, Southwell
As cells die, their membranes degrade and enzymes start leaking out. The cell digests itself, then neighbouring cells, and the process cascades. Without any immune response to stop them, bacteria and fungi will also begin eating the food and multiplying. As they munch away, they alter the texture of the food and release waste products that change the taste. There are purely chemical rotting mechanisms too, like fat oxidation, which makes it taste rancid.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.