Saponification value

Saponification value or saponification number (SV or SN) represents the number of milligrams of potassium hydroxide (KOH) required to saponify one gram of fat under the conditions specified.[1][2][3] It is a measure of the average molecular weight (or chain length) of all the fatty acids present in the sample as triglycerides. The higher the saponification value, the lower the fatty acids average length, the lighter the mean molecular weight of triglycerides and vice-versa. Practically, fats or oils with high saponification value (such as coconut and palm oil) are more suitable for soap making.

Example of saponification reaction of a triglyceride molecule (left) with potassium hydroxide (KOH) yielding glycerol (purple) and salts of fatty acids (soap).

Determination

To determine saponification value, the sample is hot-saponified with an excess of alkali (usually potassium hydroxide dissolved in ethanol), in standard conditions, generally for half an hour under reflux. Alkali is mainly consumed by glycerides : triglycerides, diglycerides, monoglycerides but also by free fatty acids, as well as by other ester-like components such as lactones.[4] At the end of the reaction the remaining quantity of alkali is titrated against standard solution of hydrochloric acid (HCl). Therefore, the SV (mg KOH/ g of sample) is calculated as following:[2]

 

 

 

 

(1)

Where :
(B - S) is the difference between the volume of HCl solution used for the blank run and for the tested sample, in mL ;
M is the molarity of HCl solution, in mol · L−1 ;
56.1 is the molecular weight of KOH , in g · mol−1;
W is the weight weight of sample, in g.

Standard methods for SV determination in vegetable and animal fats are for example: ISO 3657:2020, ASTM D5558 (fats and oils), ASTM D94 (petroleum products) and DIN 51559 (mineral oils).

The SV can also be calculated from the fatty acid composition as determined by gas chromatography (AOCS Cd 3a-94).[5]

Handmade soap makers who aim for bar soap use sodium hydroxide (NaOH), commonly known as lye, rather than KOH (caustic potash) which produces soft paste, gel or liquid soaps. In order to calculate the lye amount needed to make bar soap, KOH values of SV can be converted to NaOH values by dividing KOH values by the ratio of the molecular weights of KOH and NaOH (1.403).[6]

Relationship with the average molecular weight of fats and oils

The theoretical SV of a pure triglyceride molecule can be calculated by the following equation (where MW is its molecular weight) :[7][8]

 

 

 

 

(2)

Where : 3 is the number of fatty acids residues per triglyceride, 1000 is a conversion factor (mg/ g) and 56.1 is the MW of KOH.[8]

For instance, triolein, a triglyceride occurring in many fats and oils, has three oleic acid residues esterified to a molecule of glycerol with a total MW of 885.4 (g · mol−1). Therefore, its SV equals 190 (mg KOH · g−1).[7] In comparison, trilaurin with three shorter fatty acid residues (lauric acid) has a MW of 639 and an SV of 263.

As it can be seen from the above formula (2) the SV of a given fat is inversely proportional to its molecular weight. Actually, as fats and oils contain a mix of different triglycerides species, the average MW can be calculated according to the following relation :[7]

 

 

 

 

(3)

This means that coconut oil with an abundance of medium chain fatty acids (mainly lauric) contain more fatty acids per unit of weight than, for example, olive oil (mainly oleic). Consequently, more ester saponifiable functions were present per g of coconut oil, which means more KOH is required to saponify the same amount of matter, and thus a higher SV.[7] The calculated molecular weight (Eq. 3) is not applicable to fats and oils containing high amounts of unsaponifiable material, free fatty acids (> 0.1%), or mono- and diacylglycerols (> 0.1%).[8]

Unsaponifiables

Unsaponifiables are components of an fatty substance (oil, fat, wax) that fail to form soaps when treated with alkali and remain insoluble in water but soluble in organic solvents. For instance, typical soybean oil contain, by weight, 1.5 – 2.5% of unsaponifiable matter. Unsaponifiables include nonvolatile components : alkanes, sterols, triterpenes, fatty alcohols, tocopherols and carotenoids as well as those that mainly result from the saponification of fatty esters (sterols esters, wax esters, tocopherols esters, ...). This fraction may also contain environmental contaminants and residues of plasticizers, pesticides, mineral oil hydrocarbons and aromatics.[9]

Unsaponifiable constituents are an important consideration when selecting oil mixtures for the manufacture of soaps. Unsaponifiables can be beneficial to a soap formula because they may have properties such as moisturization, conditioning, antioxidant, texturing etc. On the other hand, when proportion of unsaponifiables is too high (> 3%), or the specific unsaponifiables present do not provide significant benefits, a defective or inferior soap product can result. For example, shark oil is not suitable for soap making as it may contain more than 10% of unsaponifiable matter.[10]

For edible oils, the tolerated limit of unsaponifiable matter is 1.5% (olive, refined soybean), while inferior quality crude or pomace oil could reach 3%.[11][12]

Determination of unsaponifiables involves a saponification step of the sample followed by extraction of the unsaponifiable using an organic solvent (i.e. diethyl ether). Official methods for animal and vegetable fats and oils are described by ASTM D1065 - 18, ISO 3596: 2000 or 18609: 2000, AOCS method Ca 6a-40.

Saponification values and unsaponifiables of various oils and fats
Fat Saponification value (mg KOH/ g) [13][14] Unsaponifiable matter (%) [8][13][15]
Beeswax 60 – 102 > 52
Canola oil 182 – 193 < 0.2
Cocoa butter 192 – 200 0.2 – 1
Coconut oil 248 – 265 0.1 – 1.4
Corn oil 187 – 195 1 – 3
Cottonseed oil 189 – 207 < 2
Fish oil [16] 179 – 200 0.6 – 3
Lanolin [17][18] 80 – 127 40 – 50
Lard [19] 192 – 203 < 10
Linseed oil 188 – 196 0.1 – 2
Mineral oil 0 100
Olive oil 184 – 196 0.4 – 1.1
Palm kernel oil 230 – 254 < 1
Palm oil 190 – 209 < 1.4
Peanut oil 187 – 196 0.2 – 4.4
Rapeseed oil 168 – 181 0.7 – 1.1
Safflower oil 188 – 194 < 1.6
Shea butter 170 – 190 6 – 17
Soybean oil 187 – 195 1.5 – 2.5
Sunflower oil 189 – 195 0.3 – 1.2
Whale oil [4] 185 – 202 < 2

See also

References
  1. "Saponification Value of Fats and Oils". Retrieved January 18, 2018.
  2. "Saponification value of Fat and Oil" (PDF). kyoto-kem.com. Retrieved July 8, 2016.
  3. Klaus Schumann, Kurt Siekmann (2005). "Soaps". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a24_247. ISBN 3527306730.CS1 maint: uses authors parameter (link)
  4. Chakrabarty, M. M. (2003). Chemistry and Technology of Oils & Fats. New Delhi: Allied Publishers. pp. 89, 183. ISBN 978-81-7764-495-1.
  5. Knothe, Gerhard (2002). "Structure indices in FA chemistry. How relevant is the iodine value?". Journal of the American Oil Chemists' Society. 79 (9): 847–854. doi:10.1007/s11746-002-0569-4. ISSN 1558-9331. S2CID 53055746.
  6. "Saponification Chart". www.fromnaturewithlove.com. Retrieved 2020-09-13.
  7. Gunstone, F.D.; Harwood, J.L. (2007). The Lipid Handbook (Third ed.). Boca Raton, FL: CRC Press. p. 424. ISBN 978-1-4200-0967-5. OCLC 327018169.
  8. Nielsen, Suzanne (2014-09-04). Food Analysis. Springer Science & Business Media. pp. 247–248. ISBN 978-1-4419-1477-4.
  9. Belitz, H.-D.; Grosch, Werner; Schieberle, Peter (2013). Food Chemistry. Springer Science & Business Media. ISBN 978-3-662-07279-0.
  10. Fryer, Percival J.; Weston, Frank E. (2013-12-19). Technical Handbook of Oils, Fats and Waxes. Cambridge University Press. ISBN 978-1-107-68731-8.
  11. "Trade standard applying to olive oils and olive pomace oils (COI/T.15/NC No 3/Rev. 14)" (PDF). internationaloliveoil.org. 2019. Retrieved 2020-09-15.
  12. "USDA commodity requirements document for bulk oil and tallow" (PDF). fsa.usda.gov. 2013. Retrieved 2020-09-15.
  13. Gunstone, Frank (2009). Oils and Fats in the Food Industry. John Wiley & Sons. ISBN 978-1-4443-0243-1.
  14. Akoh, Casimir C.; Min, David B. (2008). Food Lipids: Chemistry, Nutrition, and Biotechnology, Third Edition. CRC Press. ISBN 978-1-4200-4664-9.
  15. "Physical Properties of fats and Oils" (PDF). Deutsche Gesellschaft für Fettwissenschaft e.V. Retrieved September 14, 2020.
  16. Turchini, Giovanni M.; Ng, Wing-Keong; Tocher, Douglas Redford (2010). Fish Oil Replacement and Alternative Lipid Sources in Aquaculture Feeds. CRC Press. ISBN 978-1-4398-0863-4.
  17. "Lanolin - CAMEO". cameo.mfa.org. Retrieved 2020-09-14.
  18. Wilkie, John M. (1917). "The estimation of unsaponifiable matter in oils, fats, and waxes". Analyst. 42 (495): 200–202. Bibcode:1917Ana....42..200W. doi:10.1039/AN9174200200. ISSN 1364-5528.
  19. "SECTION 3. Codex Standard for Fats and Oils from Animal Sources". www.fao.org. Retrieved 2020-09-14.
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