MOST food will not keep fresh for very long if left by itself. Fish and meat will quickly ga bad and start to smell. Fruit and vegetable will soon rot. This spoilage is caused by chemical changes in the food or by the growth of very tiny organisms. Micro-organisms such as yeasts, moulds and bacteria are 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 produce substances which give people food poisoning.