Making food is one thing; keeping it is another. In their natural state, all the organic materials we eat are perishable. After a certain period of time, they go bad - smell bad, taste bad, look bad. Even if they do not become downright unappetising, foods lose nutritional value as vitamins degrade due to heat, strong light, air, or water. In preserving foods, the first order of business is to prevent yeasts, moulds, bacteria, and insects from devouring them before we do. Yeasts and moulds thrive on acidic fruits and convert their sugars to an acidic fizz. Bacteria prefer meat, dairy products and low-acid vegetables; if allowed to proliferate, they create acid and other waste products that destroy food. Certain bacteria can overrun a person's gastrointestinal tract or release dangerous toxins, causing the misery of acute food poisoning. People have tried to prevent food spoilage since they first started cooking meat over a fire - smoking, salting, drying, and pickling have been around for millennia. The science of food preservation, however, advanced considerably in the 19th century with the advent of such processes as pasteurisation.