What Is Saponification? How Do Oil and Lye Turn into Soap (Chemistry of the Reaction and SAP Value)

In Turkish search results, the answer to the query “what is saponification” is mostly a repetition of the same textbook definition using different words. This article does not repeat that definition; instead, it explains the reaction mechanism, stoichiometry, and the saponification (SAP) value—the practical application of this mechanism in soap making—from a chemist’s perspective. A separate resource is available for the canonical explanation of the definition, covering natural soap ingredients, production methods, and regulatory guidelines; the purpose of this page is not to re-answer the question “what is natural soap,” but to present the underlying chemistry of the reaction in a readable format.

Saponification Reaction: General Equation for Fat + Lye

A vegetable or animal fat is, chemically speaking, an ester known as a triglyceride. A triglyceride is formed when three long-chain fatty acids are bonded to the three hydroxyl groups (–OH) of glycerol via ester bonds. Saponification is the hydrolysis of these three ester bonds in the presence of a strong base.

The stoichiometry of the reaction is as follows:

1 mol triglyceride + 3 mol NaOH (or KOH) → 1 mol glycerol + 3 mol fatty acid salt (soap)

The number 3 is not arbitrary. There are three ester bonds in a triglyceride molecule; one equivalent of hydroxide ion is required for the hydrolysis of each ester bond. Therefore, one mole of triglyceride consumes three moles of alkali and produces three moles of fatty acid salt. The basic description of this reaction is covered in standard organic chemistry course materials under the heading “basic ester hydrolysis”; the products are glycerol and fatty acid carboxylate salts (LibreTexts, Hydrolysis of Triglycerides, 2024).

Why Glycerol Is an Inevitable Byproduct

Glycerol (glycerin) is not a component added from outside the reaction; it is the backbone of the triglyceride. When the three ester bonds break, this three-carbon alcohol—which previously carried the fatty acid chains—is released. Therefore, glycerol is present wherever saponification occurs; it is more accurate to ask not “Is glycerol formed?” but “What happens to the glycerol that is formed?” In the industrial boiling method, glycerol is separated into the brine phase and converted into a separate raw material, whereas in the cold method, it remains within the soap bar (Cosmébio, Cold Saponification, 2024). For the cosmetic implications of this distinction, a comparison between the cold process and the boiling method establishes a separate point of reference.

Step-by-Step Mechanism: Basic Ester Hydrolysis

Saponification is a three-step, repeating process of basic ester hydrolysis for three ester bonds. Each of the ester bonds in the triglyceride is processed in the same sequence:

1. Nucleophilic attack. The hydroxide ion (OH⁻) in solution attacks the carbonyl carbon of the triglyceride as a nucleophile. The carbonyl carbon is partially positively charged and serves as an electrophilic target for the hydroxide ion, which carries a lone pair of electrons.
2. Formation of a tetrahedral intermediate. As a result of the attack, the carbonyl carbon’s hybridization changes from sp² to sp³; in this unstable intermediate, the carbon is bonded to four different groups (–OH, –O–R, –R’, and the negative charge on oxygen). The negative charge is now on the carbonyl oxygen.
3. Alkoxide elimination. The tetrahedral intermediate collapses: the carbonyl oxygen regains its electron pair to re-form the double bond, and the alkoxide (–OR) on the other side of the ester bond leaves as the leaving group. What remains is the fatty acid carboxylic acid (RCOOH).
4. Carboxylate formation (the irreversible step). The departing alkoxide is a strong base; it immediately deprotonates the carboxylic acid in the solution, converting the fatty acid at the end of the carbon chain into a negatively charged carboxylate (RCOO⁻). The negative charge in the carboxylate anion is highly stable because it is delocalized between two oxygen atoms via resonance; this thermodynamic stability is the reason the reaction proceeds unidirectionally and irreversibly (LibreTexts, Organic Chemistry, 2024).

The same four-step sequence occurs for each of the three ester bonds in the triglyceride. Ultimately, three fatty acid carboxylates (soap molecules) detached from the glycerol backbone and free glycerol are obtained.

NaOH or KOH? The Chemical Distinction Between Bar Soap and Liquid/Soft Soap

The cation of the hydroxide used in the reaction—whether sodium or potassium—directly determines the physical form of the finished soap.

The difference lies in how the cation behaves in solution. Sodium salts are more tightly packed in their crystal structure; therefore, soaps made with NaOH are hard. Potassium salts dissolve more easily in water due to their larger cations and produce a finer, creamier lather; soaps made from these salts are soft or even liquid in form (LibreTexts, Hydrolysis of Triglycerides, 2024). The choice of base depends on the desired consistency of the final product; the reaction mechanism is the same in both cases.

SAP (Saponification) Value: Each Oil’s Unique Lye Requirement

The chemistry of saponification works the same for every triglyceride, but not every oil consumes the same amount of alkali. This measure is called the saponification value (SAP or SV).

Definition. The SAP value is the amount of potassium hydroxide, in milligrams, required to completely saponify 1 gram of oil. In industry, the standard determination is performed using the AOCS Cd 3-25 method, involving KOH titration and the phenolphthalein indicator (AOCS Official Method Cd 3-25).

Calculation. The formula specified in AOCS Cd 3-25 is:

SV = [(B − S) × M × 56.106] / W

Here, B is the blank titration volume (mL HCl), S is the sample titration volume, M is the molarity of HCl, W is the sample mass (g), and 56.106 is the molar mass of KOH (g/mol). The result is expressed directly in "mg KOH/g fat."

SAP – average molecular weight relationship (inverse proportion). The SAP value reflects the average molecular weight of the fatty acids in the fat. The more ester bonds there are in one gram of fat, the more hydroxide is consumed; the number of ester bonds increases as the fatty acid chains become shorter. In practice, oils rich in short- to medium-chain fatty acids (such as lauric and caprylic acids), like coconut oil (Cocos nucifera), have high SAP values, while long-chain oleic acid-dominant oils, such as olive oil (Olea europaea), which are dominated by long-chain oleic acid, have lower SAP values. This relationship can also be expressed approximately by the following equation:

MW (average) = 168,318 / SV + 38.049

(Wikipedia, Saponification Value, 2025, based on primary refs. 14–16.)

Reference SAP values used in soap making. In cold process formulations, the SAP value for each oil—both for NaOH (for bar soap) and KOH (for soft/liquid soap)—is read from the table, and the total alkali requirement for the formula is calculated. Typical reference values for common oils are as follows (Alfa Chemistry, Reference Guide to Saponification Values, 2025; cross-verified with Wikipedia’s primary references):

Wide reference ranges are also reported in the literature for the same oils; for example, 248–265 mg KOH/g for coconut oil, 184–196 mg KOH/g for olive oil, 190–209 mg KOH/g for palm oil, 182–193 mg KOH/g for canola oil, and 188–196 mg KOH/g for flaxseed oil (Wikipedia, Saponification Value, 2025). This range reflects natural variation (harvest year, geography, degree of refining); for recipe calculations, it is methodologically safer for the producer to work with the certificate for their own oil batch. Chailea In our workshop, cold-process recipes are formulated based on the SAP range reported on each oil batch’s certificate and under the conditions specified at GMP; thus, the lye calculation is verified on a lot-by-lot basis, and we do not blindly rely on average values from the literature.

The Relationship Between SAP Value and Iodine Value

The SAP value and the iodine value (IV) are often discussed together but measure different properties:

• SAP value: reflects the total ester content and the average molecular weight.
• Iodine value: measures the degree of unsaturation of the oil; it is defined as the grams of iodine absorbed by 100 g of oil.

A fat may have a high SAP value but a low degree of unsaturation (e.g., coconut oil: high SAP, low IV); or the opposite (e.g., flaxseed oil: moderate SAP, very high IV). For this reason, both parameters are evaluated together in oil characterization and the prediction of oxidative stability; the SAP value and the iodine value are complementary analyses determined by separate standard methods in the official AOCS method compendium.

Base Oil Sample: High Oleic Profile and Low SAP

One of the lowest SAP values in the table belongs to olive oil. This is because olive oil consists primarily of long-chain, monounsaturated oleic acid (C18:1); long chain = low SAP. For a detailed monograph on the behavior of this oil in soapmaking (hardness, foam profile, curing time), reference can be made to olive oil’s 5,000-year history as a raw material. Coconut oil, which is dominated by short- to medium-chain lauric acid, is at the opposite end of the spectrum; for details on its chemical behavior, the role of coconut oil as a foam source can be examined.

The Course of Saponification in the Cold Process: Exothermic Reaction and Trace

The cold process is a production method that carries out saponification at room temperature without an external heat source. The reaction itself is exothermic: the breaking of ester bonds and the formation of more stable carboxylates clearly release heat; therefore, the oil-lye mixture heats up as the reaction progresses (Cosmébio, 2024). Three key differences from the industrial boiling method:

• No external heat is applied; the reaction proceeds using its own energy.
• Superfat / surgras: the amount of oil is kept slightly higher than the amount of lye needed to complete the reaction; this ensures that all the lye participates in the reaction and no free lye remains in the finished soap, while the excess oil is retained as a moisturizing buffer.
• The glycerol formed during the reaction remains in the soap; it is not removed.

“Trace” is defined in formulation literature as the point at which the mixture transitions from the appearance of oil plus lye solution to a pudding-like consistency—that is, when the saponification reaction has progressed to a measurable degree. Pouring into the mold is done at this stage. The main body of the reaction is completed within the first day and the few days that follow; the curing process that begins afterward (3–6 weeks) is largely due to water evaporation and the settling of the crystal structure, not the saponification itself. For the differences between the cold process and traditional soap-making, the cold process soap guide is covered in a separate section.

Saponification and Alkalinity: “As the Degree of Saponification Increases, Alkalinity Decreases”

The alkalinity measured in a finished soap (pH and total alkali content) is an indirect indicator of how complete the saponification process was. In a study published in the journal Heliyon in 2025, which evaluated the physicochemical, antioxidant, and antimicrobial properties of 22 commercial soaps (19 bars + 3 liquid hand soaps), the measured pH values ranged from 7.01 to 10.17, and the total alkali content ranged from 0.20% to 1.17%; and total alkali content decreased as the degree of saponification increased (Nova, J.F. et al., Heliyon, 2025, DOI: 10.1016/j.heliyon.2024.e41614).

In summary: high free alkali in the finished soap indicates that the reaction is incomplete; in a complete saponification, the lye is bound to carboxylate salts and is not present in free form. The detailed chemistry of free alkali/residue measurement and the superfat buffer is the subject of a separate section; how the soap’s surface pH is buffered by the skin is discussed in the article on soap pH and the skin barrier. Here, the focus is solely on alkalinity as an indicator of saponification completion.

Frequently Asked Questions

How long does saponification take to complete? The bulk of the reaction occurs within the first 24–48 hours in the cold process method. The subsequent 3–6-week curing period is not saponification itself; most of this time is spent on the slow evaporation of water and the stabilization of the soap’s crystalline structure (Cosmébio, Cold Saponification, 2024). The exothermic heating during production indicates the active phase of the reaction.

What is the difference between NaOH and KOH? Both are strong bases and carry out the same saponification mechanism; the difference lies in the physical behavior of the resulting salt. NaOH forms sodium salts of fatty acids and produces a hard, solid soap. KOH forms potassium salts; these are more soluble in water and result in a soft or liquid soap (LibreTexts, 2024). The choice between them is a decision based on the desired product form, not the chemistry of the reaction.

Why does the SAP value differ for each oil? Because the SAP value is inversely proportional to the average molecular weight of the fatty acids in the oil. In oils containing short-chain fatty acids (such as lauric acid in coconut oil), there are more ester bonds per gram of oil, and more hydroxide is required for saponification; therefore, the SAP is high. In olive oil, where long-chain oleic acid is predominant, there are fewer ester bonds, resulting in a lower SAP value (Alfa Chemistry, 2025; AOCS Cd 3-25).

Does lye remain in the soap after saponification? The calculated lye is consumed in the reaction, and the superfat in the cold process ensures that all the lye is bound; therefore, no free lye remains in the finished soap. The measurement of residues and the detailed chemistry of the superfat buffer are the subject of a separate article.

Where does glycerin (glycerol) come from? From the backbone of triglycerides. In a fat molecule, three fatty acid chains are attached to the three –OH groups of glycerol via ester bonds; saponification breaks these three ester bonds and releases the glycerol. In the cold process, this glycerol remains in the soap and acts as a natural humectant; in the industrial boiling method, however, it is separated by being transferred to brine (Cosmébio, 2024).

Sources / References

• LibreTexts (American River College, CHEM 309). Neutralization of Fatty Acids and Hydrolysis of Triglycerides, 2024. https://chem.libretexts.org/Courses/American_River_College/CHEM_309%3A_Applied_Chemistry_for_the_Health_Sciences/11%3A_Lipids_-_An_Introduction/11.05%3A_Neutralization_of_Fatty_Acids_and_Hydrolysis_of_Triglycerides
• LibreTexts. Organic Chemistry — Basic Ester Hydrolysis (Saponification Mechanism). https://chem.libretexts.org/Bookshelves/Organic_Chemistry
• AOCS Official Method Cd 3-25. Saponification Value of Fats and Oils. American Oil Chemists' Society. https://myaccount.aocs.org/PersonifyEbusiness/Store/Product-Details/productId/111542
• Nova, J.F., Smrity, S.Z., Hasan, M., et al. Comprehensive evaluation of physicochemical, antioxidant, and antimicrobial properties in commercial soaps: A study on bar soaps and liquid hand wash. Heliyon, 2025. DOI: 10.1016/j.heliyon.2024.e41614 PMC11867263. https://pmc.ncbi.nlm.nih.gov/articles/PMC11867263/
• Alfa Chemistry. Reference Guide to Saponification Values of Common Oils & Fats in Cosmetics, 2025. https://cosmetics.alfa-chemistry.com/resources/reference-guide-to-saponification-values-of-common-oils-fats-in-cosmetics.html
• Wikipedia. Saponification value. 2025. https://en.wikipedia.org/wiki/Saponification_value
• Cosmébio. Cold Process Soap Making (Cold Saponification), 2024. https://www.cosmebio.org/en/reports/cold-process-soap-making-cold-saponification/