most food will not keep fresh for every long if left by it self. Fish and meat will quickly do bad and start to smell. Fruit and vegetable will soon rot. This spoilage is caused by chemical changes in the food or by the food or by the air all around us, of very tiny organisms. micro - organisms such as yeasts, moulds and bacteria in the air all around us, and they may grow on all kinds of foods. many of these organisms cause food to ferment and turn sour, to give off smelly gases. some bacteria substances which give people food poisoning.
To preserve food we must somehow prevent all these things form taking place, or at least slow them down. canning is the most important method of food preservation, and it can be used for practically all kind of foods.