Preservation of food has been a problem and challenge for the human race since prehistoric times, since natural supplies of food run short in the winter and few foods keep for long without some preservative measures being taken. Those that do, such as nuts, are preserved by animals. But—apart from a few special cases such as bees—the only animal that treats food to make it keep is man.
All fresh foods begin to spoil at once, quickly or slowly. As soon as a plant is harvested or an animal killed, substances in it which were essential to its life begin to break down. It may also be colonized by micro-organisms coming from outside, or be acted on by those already present, such as the bacteria in the intestines of animals and the yeasts which form the ‘bloom’ on fruit.