In chemistry, acid value (AV, acid number, neutralization number or acidity) is a number used to quantify the acidity of a given chemical substance. It is the quantity of base (usually potassium hydroxide (KOH)), expressed as milligrams of KOH required to neutralize the acidic constituents in 1 gram of a sample.[1][2][3][4] The acid value measures the acidity of water-insoluble substances like oils, fats, waxes and resins, which do not have a pH value.

The acid number is a measure of the number of carboxylic acid groups (−C(=O)OH) in a chemical compound, such as a fatty acid, or in a mixture of compounds.[2] In other words, it is a measure of free fatty acids (FFAs) present in a substance. In a typical procedure, a known amount of sample dissolved in an organic solvent (often isopropanol) and titrated with a solution of alcoholic potassium hydroxide (KOH) of known concentration using phenolphthalein as a colour indicator.[2] The acid number for an oil sample is indicative of the age of the oil and can be used to determine when the oil must be changed.[5]

A liquid fat sample combined with neutralized 95% ethanol is titrated with standardized sodium hydroxide of 0.1 eq/L normality to a phenolphthalein endpoint. The volume and normality of the sodium hydroxide are used, along with the weight of the sample, to calculate the free fatty acid value.[3]

Acid value is usually measured as milligrams of KOH per gram of sample (mg KOH/g fat/oil), or grams of KOH per gram of sample (g KOH/g fat/oil).[5]

Calculations

For example, for analysis of crude oil:[6]

Where KOH is the titrant, wherease crude oil is the titrand.
Veq is the volume of titrant (ml) consumed by the crude oil sample and 1 ml of spiking solution at the equivalent point,
beq is the volume of titrant (ml) consumed by 1 ml of spiking solution at the equivalent point,
56.1 g/mol is the molecular weight of KOH,
Woil is the mass of the sample in grams.

The normality (N) of titrant is calculated as:

Where WKHP is the mass (g) of potassium hydrogen phthalate (KHP) in 50 ml of KHP standard solution,
Veq is the volume of titrant (ml) consumed by 50 ml KHP standard solution at the equivalent point,
204.23 g/mol is the molecular weight of KHP.

Applications

An increment in the amount of FFAs in a fat or oil sample indicates hydrolysis of triglycerides. Such reaction occurs by the action of lipase enzyme and it is an indicator of inadequate processing and storage conditions. The source of the enzyme can be the tissue from which the oil or fat was extracted or it can be a contaminant from other cells including microorganisms.[1]

For determining the acid value of mineral oils and biodiesel, there are standard methods such as ASTM D 974 and DIN 51558, and especially for biodiesel the European Standard EN 14104 and ASTM D664 are both widely used worldwide.[2] Acid value of biodiesel should be lower than 0.50 mg KOH/g in both EN 14214 and ASTM D6751 standard fuels. This is because the FFAs produced can corrode automotive parts, hence these limits protect vehicle engines and fuel tanks.[5]

Low acid value indicates good cleansing by soap.[7]

When oils and fats become rancid, triglycerides are converted into fatty acids and glycerol, causing an increase in acid value.[8] A similar situation is observed during aging of biodiesel through analogous oxidation and when subjected to prolonged high temperatures (ester thermolysis) or through exposure to acids or bases (acid/base ester hydrolysis).[5]

Transesterification of waste cooking oil, having high acid value and high water content, can be performed using heteropolyacids such as dodecatungstophosphoric acid (PW12) as a catalyst.[9][10]

In 2007, Sahoo et al. made biodiesel consisting of mono-esters of polanga oil extract of the plant Calophyllum inophyllum produced by triple stage transesterification and blended with high speed diesel, which was then tested for its use as a diesel substitute in a single cylinder diesel engine.[11]

Testing

Total acidity, fatty acid profiles, and free fatty acids (FFAs) can be determined for oils such as sunflower and soybean oils obtained by green processes involving supercritical carbon dioxide (scCO2) and pressurized liquid extraction (PLE). The identification and separation of the primary fatty acids responsible for acidity can ensure higher quality of fat and oil products.[12]

In 2020, Dallas Group of America (DGA)[13] and American Oil Chemists' Society (AOCS) devised a standard method (5a-40) for testing free fatty acid in cooking oils.[14][15] The DGA FFAs hand-held test kit was produced from the AOCS test method, but without the burets, flasks, and laboratory hardware. Its portable nature is convenient for both small and large frying operations. Testing next to the fryer or in the comfort of a laboratory setting is simple with the DGA FFAs test kit. It gives accurate results for cooking oil used in potato chips, corn dogs, meat browning, bread products, roasted peanuts, and more.[15]

Acid values of various fats and oils

Fat / oil Acid value (mg KOH per g sample)
Beeswax 17 – 36[16]
Canola oil 0.071 – 0.073[17]
Maize oil 0.223 – 0.224[17]
Soyabean oil 0.60 – 0.61[17]
Virgin olive oil 0.8 – 2[18]
Used frying oil 0.1 – 2.5[19][20]

See also

  • Amine value – Measure of an organic compound's nitrogen content
  • Bromine number – Mass of bromine absorbed by 100 grams of a given substance
  • Epoxy value – Measure of the epoxy content of a substance
  • Hydroxyl value – Mass of KOH needed to neutralize 1 gram of acetylized substance
  • Iodine value – Mass of iodine absorbed by 100 grams of a given substance
  • Peroxide value – Measure of peroxide content of a fat or oil
  • Saponification value – Milligrams of a base required to saponify 1g of fat
  • Redox – Chemical reaction in which oxidation states of atoms are changed
  • EN 14214 – Fuel standard for biodiesel
  • Rancidification – Spoilage of fats & oils into foul-smelling substances

References

  1. 1 2 "14.10.1: Foods- Acid Value and the Quality of Fats and Oils". Chemistry LibreTexts. May 26, 2016. Retrieved October 28, 2022.
  2. 1 2 3 4 Ahuja, Satinder (January 25, 2015). Food, Energy, and Water: The Chemistry Connection. Elsevier. p. 301. ISBN 9780128003749. OCLC 900781294.
  3. 1 2 Nielsen, S. Suzanne (March 20, 2010). Food Analysis Laboratory Manual, 2nd Edition. Springer Science & Business Media. pp. 108–109. ISBN 9781441914637. OCLC 663096771.
  4. ↑ O'Brien, Richard D. (December 5, 2008). Fats and Oils: Formulating and Processing for Applications, 3rd Edition. CRC Press. pp. 220–221. ISBN 9781420061673. OCLC 367589246.
  5. 1 2 3 4 "Acid Value Number or Neutralization Number of Oil". www.engineersedge.com. Retrieved October 28, 2022.
  6. ↑ CN103776825A, Junmin, Ji; Dongmin, Wang & Huamin, Liu, "Determining method of acid value of deep-color grease", issued 2014-05-07
  7. ↑ "Acid Value - an overview". ScienceDirect. Archived from the original on January 31, 2022. Retrieved October 28, 2022. link to original article
  8. ↑ Fernando, Sandun; Karra, Prashanth; Hernandez, Rafael; Jha, Saroj Kumar (May 1, 2007). "Effect of incompletely converted soybean oil on biodiesel quality". Energy. 32 (5): 844–851. doi:10.1016/j.energy.2006.06.019. ISSN 0360-5442. The acid number can become a serious issue when feedstocks with high free fatty acids...
  9. ↑ Cao, Fenghua; Chen, Yang; Zhai, Fengying; Li, Jing; Wang, Jianghua; Wang, Xiaohong; Wang, Shengtian; Zhu, Weimin (September 1, 2008). "Biodiesel production from high acid value waste frying oil catalyzed by superacid heteropolyacid". Biotechnology and Bioengineering. 101 (1): 93–100. doi:10.1002/bit.21879. ISSN 1097-0290. PMID 18646228. S2CID 205497850.
  10. ↑ US 8962873 B2, A, Summers William; Rebecca, Williams & Danny, Gulledge et al., "Systems And Methods For Making Bioproducts", issued 2015-02-24
  11. ↑ Sahoo, P. K.; Das, L. M.; Babu, M. K. G.; Naik, S. N. (February 1, 2007). "Biodiesel development from high acid value polanga seed oil and performance evaluation in a CI engine". Fuel. 86 (3): 448–454. doi:10.1016/j.fuel.2006.07.025. ISSN 0016-2361.
  12. ↑ Medeiros Vicentini-Polette, Carolina; Rodolfo Ramos, Paulo; Bernardo Gonçalves, Cintia; Lopes De Oliveira, Alessandra (December 30, 2021). "Determination of free fatty acids in crude vegetable oil samples obtained by high-pressure processes". Food Chemistry: X. 12: 100166. doi:10.1016/j.fochx.2021.100166. ISSN 2590-1575. PMC 8604745. PMID 34825173.
  13. ↑ "Products & Applications". The Dallas Group of America. Retrieved October 28, 2022.
  14. ↑ "Validation of the Free Fatty Acid Test Kit for the Measurement of the Free Fatty Acid Content of Vegetable Oils, Fish Oils, Animal Fats (Tallows), Meat and Fish Meals, and Potato Chips and Grain-Based Snack Products: AOAC Performance Tested Method 052004". Journal of AOAC International. Oxford University Press (OUP); AOAC International. November 26, 2022. Retrieved October 28, 2022.
  15. 1 2 "How to Test for Free Fatty Acid in Cooking Oil". DALSORB. March 2, 2021. Archived from the original on October 12, 2022. Retrieved October 28, 2022.
  16. ↑ Min, David B. (March 17, 2008). Food Lipids: Chemistry, Nutrition, and Biotechnology, Third Edition. CRC Press. ISBN 9781420046649. OCLC 213371194.
  17. 1 2 3 Kardash, Elena; Tur'yan, Yakov I. (March 24, 2005). "Acid Value Determination in Vegetable Oils by Indirect Titration in Aqueous-alcohol Media". Croatica Chemica Acta. 78 (1): 99–103. ISSN 0011-1643. Archived from the original on February 26, 2021.
  18. ↑ Grossi, Marco; Lecce, Giuseppe Di; Toschi, Tullia Gallina; Riccò, Bruno (May 2014). "Fast and Accurate Determination of Olive Oil Acidity by Electrochemical Impedance Spectroscopy" (PDF). IEEE Sensors Journal. 14 (9): 2947–2954. Bibcode:2014ISenJ..14.2947G. doi:10.1109/JSEN.2014.2321323. ISSN 1558-1748. S2CID 10659764.
  19. ↑ Park, Jung Min; Koh, Jong Ho; Kim, Jin Man (September 1, 2020). "Determining the Reuse of Frying Oil for Fried Sweet and Sour Pork according to Type of Oil and Frying Time". Food Science of Animal Resources. 40 (5): 785–794. doi:10.5851/kosfa.2020.e54. ISSN 2636-0780. PMC 7492171. PMID 32968730.
  20. ↑ Park, Jung-Min; Kim, Jin-Man (October 31, 2016). "Monitoring of Used Frying Oils and Frying Times for Frying Chicken Nuggets Using Peroxide Value and Acid Value". Korean Journal for Food Science of Animal Resources. 36 (5): 612–616. doi:10.5851/kosfa.2016.36.5.612. ISSN 1225-8563. PMC 5112423. PMID 27857536.
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