Pasteurizationorpasteurisationis a process in which packaged and non-packaged foods (such asmilkand fruitjuice) are treated with mild heat, usually to less than 100°C (212°F), to eliminatepathogensand extendshelf life. The process is intended to destroy or deactivate organisms andenzymesthat contribute tospoilageor risk of disease, including vegetativebacteria, but notbacterial spores.[1][2]Since pasteurization is not sterilization, and does not kill spores, a second "double" pasteurization will extend the quality by killing spores that have germinated.

The process was named after the French microbiologist,Louis Pasteur, whose research in the 1880s demonstrated that thermal processing would inactivate unwanted microorganisms inwine.[2][3]Spoilage enzymes are also inactivated during pasteurization. Today, pasteurization is used widely in thedairyindustry and otherfood processingindustries to achievefood preservationandfood safety.[3]

Most liquid products are heat treated in a continuous system where heat can be applied using aplate heat exchangeror the direct or indirect use of hot water and steam. Due to the mild heat, there are minor changes to the nutritional quality and sensory characteristics of the treated foods.[4]Pascalizationor high pressure processing (HPP) andpulsed electric field(PEF) are non-thermal processes that are also used to pasteurize foods.[1]

The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The majority of Earth's atmosphere (78%) is atmosphere nitrogen,[16] making it the largest source of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems.

The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production and decomposition. Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatically altered the global nitrogen cycle. Human modification of the global nitrogen cycle can negatively affect the natural environment system and also human health.


Nitrogen fixation is a process by which molecular nitrogen in the air is converted into ammonia (NH
3) or related nitrogenous compounds in soil.[1] Atmospheric nitrogen is molecular dinitrogen, a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation converts N
2 into ammonia, which is metabolized by most organisms.

Nitrogen fixation is essential to life because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing organic compounds, such as amino acids and proteins, nucleoside triphosphates and nucleic acids. As part of the nitrogen cycle, it is essential for agriculture and the manufacture of fertilizer. It is also, indirectly, relevant to the manufacture of all nitrogen chemical compounds, which includes some explosives, pharmaceuticals, and dyes.

Nitrogen fixation is carried out naturally in soil by microorganisms termed diazotrophs that include bacteria such as Azotobacter and archaea. Some nitrogen-fixing bacteria have symbiotic relationships with plant groups, especially legumes.[2] Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi.[3] It occurs naturally in the air by means of NOx production by lightning.[4][5]

All biological nitrogen fixation is effected by enzymes called nitrogenases.[6] These enzymes contain iron, often with a second metal, usually molybdenum but sometimes vanadium.