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.