A July 2018 study in "Science" says that meat consumption is set to rise as the human population increases along with affluence, which will increase greenhouse gas emissions and further reduce biodiversity. The environmental impact of meat production can be reduced by conversion of human-inedible residues of food crops. Manure from meat-producing livestock is used as fertilizer; it may be composted before application to food crops. Substitution of animal manures for synthetic fertilizers in crop production can be environmentally significant, as between 43 and 88 MJ of fossil fuel energy are used per kg of nitrogen in manufacture of synthetic nitrogenous fertilizers. The spoilage of meat occurs, if untreated, in a matter of hours or days and results in the meat becoming unappetizing, poisonous or infectious. Spoilage is caused by the practically unavoidable infection and subsequent decomposition of meat by bacteria and fungi, which are borne by the animal itself, by the people handling the meat, and by their implements. Meat can be kept edible for a much longer time – though not indefinitely – if proper hygiene is observed during production and processing, and if appropriate food safety, food preservation and food storage procedures are applied. Without the application of preservatives and stabilizers, the fats in meat may also begin to rapidly decompose after cooking or processing, leading to an objectionable taste known as warmed over flavor. Fresh meat can be cooked for immediate consumption, or be processed, that is, treated for longer-term preservation and later consumption, possibly after further preparation. Fresh meat cuts or processed cuts may produce iridescence, commonly thought to be due to spoilage but actually caused by structural coloration and diffraction of the light. A common additive to processed meats for both preservation and the prevention of discoloration is sodium nitrite. This substance is a source of health concerns because it may form carcinogenic nitrosamines when heated. Meat is prepared in many ways, as steaks, in stews, fondue, or as dried meat like beef jerky. It may be ground then formed into patties (as hamburgers or croquettes), loaves, or sausages, or used in loose form (as in "sloppy joe" or Bolognese sauce). Some meat is cured by smoking, which is the process of flavoring, cooking, or preserving food by exposing it to the smoke from burning or smoldering plant materials, most often wood. In Europe, alder is the traditional smoking wood, but oak is more often used now, and beech to a lesser extent.