Editing Rancidification (section)

==Pathways==
Five pathways for rancidification are recognized:<ref>{{Cite book | last1 = Freeman | first1 = I. P. | chapter = Margarines and Shortenings | doi = 10.1002/14356007.a16_145 | title = Ullmann's Encyclopedia of Industrial Chemistry | year = 2000 | isbn = 978-3-527-30673-2 }}</ref>

===Hydrolytic===
[[Hydrolysis|Hydrolytic]] rancidity refers to the odor that develops when [[triglyceride]]s are hydrolyzed and free fatty acids are released. This reaction of lipid with water may require a [[catalyst]] (such as a [[lipase]],<ref name="koon" /> or acidic or alkaline conditions) leading to the formation of free [[fatty acid]]s and [[glycerol]]. In particular, [[short-chain fatty acid]]s, such as [[butyric acid]], are [[odor|malodorous]].<ref name=":0">{{Cite book|title=Chemistry : course companion|last=Sergey|first=Bylikin|others=Horner, Gary; Murphy, Brian; Tarcy, David|isbn=978-0-19-839212-5|edition= 2014 |location=Oxford|oclc=862091138|date = January 2014}}</ref> When short-chain fatty acids are produced, they serve as catalysts themselves, further accelerating the reaction, a form of [[autocatalysis]].<ref name=":0" />

===Oxidative===
[[Organic redox reaction|Oxidative]] rancidity is associated with the degradation by oxygen in the air.
====Free-radical oxidation====
{{Main|Lipid peroxidation}}

The [[double bond]]s of an [[Fatty acid#Unsaturated fatty acids|unsaturated fatty acid]] can be cleaved by [[Radical (chemistry)|free-radical]] reactions involving molecular oxygen. This reaction causes the release of malodorous and highly [[Volatility (chemistry)|volatile]] [[aldehyde]]s and [[ketone]]s. Because of the nature of free-radical reactions, the reaction is catalyzed by sunlight.<ref name=":0" /> Oxidation primarily occurs with unsaturated fats. For example, even though meat is held under refrigeration or in a frozen state, the poly-unsaturated fat will continue to oxidize and slowly become rancid. The fat oxidation process, potentially resulting in rancidity, begins immediately after the animal is slaughtered and the muscle, intra-muscular, inter-muscular and surface fat becomes exposed to oxygen of the air. This chemical process continues during frozen storage, though more slowly at lower temperature. Oxidative rancidity can be prevented by light-proof packaging, oxygen-free atmosphere (air-tight containers) and by the addition of [[antioxidant]]s.<ref name=":0" />
====Enzyme-catalysed oxidation====
A double bond of an unsaturated fatty acid can be oxidised by oxygen from the air in reactions catalysed by plant or animal [[lipoxygenase]] enzymes,<ref name="koon" /> producing a [[hydroperoxide]] as a reactive intermediate, as in free-radical peroxidation. The final products depend on conditions: the lipoxygenase article shows that if a [[hydroperoxide lyase]] enzyme is present, it can cleave the hydroperoxide to yield short-chain fatty acids and [[dicarboxylic acid]]s (several of which were first discovered in rancid fats).

===Microbial===
[[Microorganism|Microbial]] rancidity refers to a water-dependent process in which microorganisms, such as bacteria or [[Mold (fungus)|mold]]s, use their enzymes such as [[lipase]]s to break down fat.<ref name="koon">{{cite web|url=https://www.naturalproductsinsider.com/regulatory/understanding-rancidity-nutritional-lipids|title=Understanding rancidity of nutritional lipids|author=Robin Koon|publisher=Natural Products Insider|date=4 August 2009|access-date=7 April 2019|archive-date=3 August 2018|archive-url=https://web.archive.org/web/20180803074034/https://www.naturalproductsinsider.com/regulatory/understanding-rancidity-nutritional-lipids|url-status=live}}</ref> [[Pasteurization]] and/or addition of [[antioxidant]] ingredients such as [[vitamin E]], can reduce this process by destroying or inhibiting microorganisms.<ref name="koon" />