Editing Citric acid (section)

==Natural occurrence and industrial production==
[[File:Citrus fruits.jpg|alt=|left|thumb|Lemons, oranges, limes, and other citrus fruits contain high concentrations of citric acid.]]
Citric acid occurs in a variety of fruits and vegetables, most notably [[Citrus|citrus fruit]]s. [[Lemon]]s and [[Lime (fruit)|limes]] have particularly high concentrations of the acid; it can constitute as much as 8% of the dry weight of these fruits (about 47&nbsp;g/L in the juices<ref>{{cite journal |vauthors=Penniston KL, Nakada SY, Holmes RP, Assimos DG| title=Quantitative Assessment of Citric Acid in Lemon Juice, Lime Juice, and Commercially-Available Fruit Juice Products | journal=Journal of Endourology | volume=22 | issue=3 | year=2008 | pmid=18290732 | pages = 567–570 | doi = 10.1089/end.2007.0304 | pmc=2637791}}</ref>).{{efn|This still does not make the lemon particularly strongly acidic. This is because, as a weak acid, most of the acid molecules are not dissociated so not contributing to acidity inside the lemon or its juice.}}  The concentrations of citric acid in citrus fruits range from 0.005&nbsp;[[molarity|mol/L]] for oranges and grapefruits to 0.30&nbsp;mol/L in lemons and limes; these values vary within species depending upon the [[cultivar]] and the circumstances under which the fruit was grown.{{Cn|date=May 2025}}

Citric acid was first isolated in 1784 by the chemist [[Carl Wilhelm Scheele]], who crystallized it from lemon juice.<ref>{{cite journal |last1=Scheele |first1=Carl Wilhelm |title=Anmärkning om Citron-saft, samt sätt at crystallisera densamma |journal=Kungliga Vetenskaps Academiens Nya Handlingar [New Proceedings of the Royal Academy of Science] |date=1784 |volume=5 |pages=105–109 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015009215438;view=1up;seq=115 |series=2nd series |trans-title=Note about lemon juice, as well as ways to crystallize it |language=sv}}</ref><ref name="book1">{{cite book|last=Graham|first=Thomas|title=Elements of chemistry, including the applications of the science in the arts|url=https://archive.org/details/elementschemist00grahgoog|access-date=June 4, 2010|year=1842|publisher=Hippolyte Baillière, foreign bookseller to the Royal College of Surgeons, and to the Royal Society, 219, Regent Street.|page=[https://archive.org/details/elementschemist00grahgoog/page/n973 944]}}</ref>

Industrial-scale citric acid production first began in 1890 based on the Italian citrus fruit industry, where the juice was treated with hydrated lime ([[calcium hydroxide]]) to precipitate [[calcium citrate]], which was isolated and converted back to the acid using diluted [[sulfuric acid]].<ref name="ullmann" /> In 1893, [[Carl Wehmer (chemist)|C. Wehmer]] discovered ''[[Penicillium]]'' [[Mold (fungus)|mold]] could produce citric acid from sugar.<ref>{{cite book |url=https://books.google.com/books?id=fdBMcYg_xGYC&pg=PA140 |title=A History of Lactic Acid Making: A Chapter in the History of Biotechnology |author=H. Benninga |date=30 June 1990 |publisher=Springer Science & Business Media |isbn=978-0-7923-0625-2 |pages=140–5}}</ref> However, microbial production of citric acid did not become industrially important until World War I disrupted Italian ''Citrus'' exports.{{Cn|date=May 2025}}

In 1917, American food chemist James Currie discovered that certain strains of the mold ''[[Aspergillus niger]]'' could be efficient citric acid producers,<ref>{{Cite book |last=Currie |first=James |url=https://books.google.com/books?id=7q8BAAAAYAAJ&pg=PA15 |title=The Journal of Biological Chemistry |publisher=American Society for Biochemistry and Molecular Biology |year=1917 |pages=15–27 |language=en}}</ref> and the pharmaceutical company [[Pfizer]] began industrial-level production using this technique two years later, followed by [[Citrique Belge]] in 1929. In this production technique, which is still the major industrial route to citric acid used today, cultures of ''Aspergillus niger'' are fed on a [[sucrose]] or [[glucose]]-containing medium to produce citric acid. The source of sugar is [[corn steep liquor]], [[molasses]], hydrolyzed [[corn starch]], or other inexpensive, [[carbohydrate]] solution.<ref>{{cite journal |first1=Walid A. |last1=Lotfy |first2=Khaled M. |last2=Ghanem |first3=Ehab R. |last3=El-Helow |year=2007 |url=https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=Citric+acid+production+by+a+novel+Aspergillus+niger+isolate%3A+II.+Optimization+of+process+parameters+through+statistical+experimental+designs&btnG= |format=PDF |title=Citric acid production by a novel ''Aspergillus niger'' isolate: II. Optimization of process parameters through statistical experimental designs |journal=Bioresource Technology |volume=98 |issue=18 |pages=3470–3477 |doi=10.1016/j.biortech.2006.11.032 |pmid=17317159|bibcode=2007BiTec..98.3470L }}</ref> After the mold is filtered out of the resulting [[Suspension (chemistry)|suspension]], citric acid is isolated by [[Precipitation (chemistry)|precipitating]] it with calcium hydroxide to yield calcium citrate salt, from which citric acid is regenerated by treatment with sulfuric acid, as in the direct extraction from citrus fruit juice.

In 1977, a patent was granted to [[Lever Brothers]] for the chemical synthesis of citric acid starting either from aconitic or isocitrate (also called alloisocitrate) calcium salts under high pressure conditions; this produced citric acid in near quantitative conversion under what appeared to be a reverse, non-enzymatic [[Krebs cycle reaction]].<ref>{{cite web |last1=Lamberti |first1=Vincent |last2=Gutierrez |first2=Eddie N. |title=Chemical processes for preparing citric acid |url=https://patents.google.com/patent/US4056567A/en |website=Google Patents |access-date=4 August 2024 |date=1 November 1977}} (US 4056567)</ref>

Global production was in excess of 2,000,000 tons in 2018.<ref>{{cite web |url=https://www.prnewswire.com/news-releases/global-citric-acid-markets-report-2011-2018--2019-2024-300814817.html |title=Global Citric Acid Markets Report, 2011-2018 & 2019-2024 |date= 19 March 2019 |website=prnewswire.com |access-date=28 October 2019}}</ref> More than 50% of this volume was produced in China. More than 50% was used as an [[acidity regulator]] in beverages, some 20% in other food applications, 20% for detergent applications, and 10% for applications other than food, such as cosmetics, pharmaceuticals, and in the chemical industry.<ref name="ullmann">{{Ullmann | title = Citric Acid | first1 = Frank H.|last1=Verhoff|first2=Hugo|last2=Bauweleers|year=2014|doi=10.1002/14356007.a07_103.pub3}}</ref>