Editing Deep frying (section)

== Oil deterioration and chemical changes ==
{{See also|Deep fryer#Oil filtration}}
{{More citations needed|section|date=August 2021}} 
Deep fat frying involves heating oil to temperatures in excess of 180&nbsp;°C in the presence of moisture and air. These conditions can induce a series of complex chemical reactions which may impact the quality of both the food and the oil it is cooked in. Examples of different chemical reactions include the production of [[Radical (chemistry)|free radicals]], [[Redox|oxidation]], [[hydrolysis]], [[isomerization]] and [[polymerization]]. The exact reactions are dependent upon factors such as the oil type, frying conditions, and food being cooked. When frying, water can attack the ester linkage of triacylglycerols, resulting in mono- and diglycerols, glycerol, and free fatty acids (a type of [[hydrolysis]] reaction). The aforementioned hydrolysis reaction is enhanced by the produced [[fatty acid]]s and other low molecular weight acid compounds.<ref>{{Cite journal|last1=Mariod|first1=Abdalbasit|last2=Omer|first2=Nuha|last3=Al|first3=El Mugdad|first4=Mohammed|last4=Mokhtar |date=2014-09-09|title=Chemical Reactions Taken Place During deep-fat Frying and Their Products: A review|url=https://www.researchgate.net/publication/270727167|journal=Sudan University of Science & Technology SUST Journal of Natural and Medical Sciences|volume=Supplementary issue|pages=1–17}}</ref>

Overheating or over-using the frying oil leads to formation of [[Rancidification|rancid-tasting]] products of [[redox|oxidation]], [[polymerization]], and other deleterious, unintended or even toxic compounds<ref>{{cite book | title=Household use of solid fuels and high-temperature frying | publisher=Lyon, France : International Agency for Research on Cancer; Distributed by WHO Press, 2010. | author=IARC Working Group on the Evaluation of Carcinogenic Risks to Humans | year=2010 | location=Lyon, France; Geneva}}</ref> such as [[acrylamide]] (from [[starch]]y foods).  Recent research suggests fat deterioration may be worse when fat or oil is fried with food than when fat or oil is tested on its own in a laboratory.<ref>[[BBC]] (London) undated [http://www.bbc.co.uk/programmes/articles/3t902pqt3C7nGN99hVRFc1y/which-oils-are-best-to-cook-with Which oils are best to cook with?]</ref> [[Vacuum fryer|Deep-frying under vacuum]] helps to significantly reduce acrylamide formation,<ref>Granda, C.; Moreira, R.G.; Tichy, S.E. (2004). "Reduction of Acrylamide Formation in Potato Chips by Low-temperature Vacuum Frying Journal of Food Science". 69 (8). Pages 405–411.</ref> but this process is not widely used in the [[food industry]] due to the high investment cost involved.

Some useful tests and indicators of excessive oil deterioration are the following:
* Sensory&nbsp;– darkening, smoke, foaming, thickening, rancid taste and unpleasant smell when heating. This is the most unreliable way to decide when to change oil because those are very individual factors and can depend on different causes.
* Testing strips&nbsp;– decide when to change oil depending on FFA ([[free fatty acid]]s) only<ref name="Peng Morin 1991 p. 86">{{cite book | last1=Peng | first1=S.K. | last2=Morin | first2=R.J. | title=Biological Effects of Cholesterol Oxides | publisher=Taylor & Francis | year=1991 | isbn=978-0-8493-6776-2 | url=https://books.google.com/books?id=lzrCYjxe_swC&pg=PA86 | page=86}}</ref>
* Oil-tester&nbsp;– measurement tool to exactly define the point of change oil by TPM/TPC (Total polar material/compounds)
* Laboratory&nbsp;– [[acidity]], [[anisidine value]], [[viscosity]], total [[chemical polarity|polar]] [[chemical compound|compounds]], polymeric [[triglycerides]].

Instruments that indicate total polar compounds, currently the best single gauge of how deep-fried an object is, are available with sufficient accuracy for restaurant and industry use.