The Difference Between Essential (Volatile) Oil and Fixed (Carrier) Oil: Their Roles in Soap Making

The difference between essential and fixed oils is treated largely within an aromatherapy framework in English-language search results; fixed oil is promoted as a "carrier" that dilutes essential oil when applied to skin. Yet from the soap maker's perspective, the core distinction lies in whether an oil reacts with caustic (NaOH or KOH). This article examines the difference between the two along the axis of "saponifiable" and "non-saponifiable"; using academic sources, current cosmetic regulations, and the example of a single plant (Laurus nobilis — laurel) in which both oils coexist in Aleppo soap, it clarifies the conceptual confusion.

At a glance: Essential oil vs. fixed oil difference (summary table)

The following table summarizes the fundamental distinctions between the two in terms of chemical structure, extraction method, and function in soap.

This table is a summary; the sections below expand each criterion in terms of chemical mechanism and soap-making practice.

Chemical structure: Why is one volatile and one fixed?

Fixed oils consist of triglycerides — three fatty acids bonded via ester linkage to a glycerol backbone. The chain length of the fatty acids (lauric C12, oleic C18:1, linoleic C18:2, for example) and their degree of saturation determine both the physical behavior of the oil and the character of the soap that forms after saponification. Essential oils cannot be reduced to a single chemical class; they are variable-proportion mixtures of low molecular weight, volatile aromatic compounds, primarily monoterpenes and sesquiterpenes. The detailed review by Taşkor Önel and Yaman Akbay, published in the Bayburt University Journal of Science, systematically summarizes this structural difference in terms of structure–activity relationships (DergiPark, 2022).

The structural difference drives behavioral difference: high molecular weight, nonpolar triglyceride chains do not evaporate at room temperature; small, low molecular weight terpenoids readily evaporate due to their low boiling points. The adjectives "fixed" and "volatile" derive directly from this physicochemical distinction.

Extraction method difference

Fixed oils are obtained from oily seeds, kernels, or fruits via mechanical pressure, ideally by cold press method; low temperature preserves the structural integrity of the triglyceride and the natural antioxidant profile. Essential oils are typically obtained by steam/hydrodistillation: plant material is exposed to steam, volatile compounds are carried with the vapor, and after condensation are separated from water. Citrus peel is a known exception; the peel is mechanically expressed using cold pressing on the bead-studded bezels. For this reason, the term "cold-pressed" alone does not prove an oil is fixed — cold-pressed orange peel oil is essential oil.

Yield ratios also clarify the distinction. In a study published in the Molecules journal, Fidan and colleagues reported hydrodistillation yield of laurel (Laurus nobilis) essential oil at 0.78% in fruit, 0.80% in shoots, and 3.25% in leaves (Molecules 24:804, 2019). Fixed oil yield (from oily seeds and fruits) is many times higher than essential oil yield; this mass balance explains why essential oil is used in drop amounts.

Roles in soap making: The core distinction is in saponification

In cold-process soapmaking, the heart of the reaction is saponification: triglyceride + alkali base (NaOH or KOH) → fatty acid salt (soap) + glycerol. The triglyceride structure of fixed oil is precisely the substrate this reaction requires; the terpene/terpenoid structure of essential oil lacks ester linkages and is therefore not a substrate for this reaction. The fundamental soap-making distinction between the two oil families rests on this mechanistic difference.

Fixed oil = saponifiable: SAP value, lye calculation, and soap matrix

Each fixed oil has its own characteristic SAP (saponification) value; this value expresses the amount of alkali (in mg of NaOH or KOH) required to fully saponify 1 gram of the oil. In soap formulation, each fixed oil's SAP value is multiplied by its percentage in the recipe, and total lye quantity is calculated based on this mass balance; tools like SoapCalc, widely used by soap-making practitioners, operate on this principle.

The fatty acid profile, in turn, determines the character of the finished soap: - Lauric acid (C12, coconut/palm kernel): high hardness, strong and quick lather. - Oleic acid (C18:1, olive oil): softness, low cleansing power, high moisture retention. - Stearic–palmitic acid (C18/C16, cocoa/shea butter): hardness, creamy lather balance.

The saponified form of these profiles defines the soap's feel and function on skin. At the Chailea workshop, recipes are built on this fatty acid profile; the detailed role of each carrier oil in the soap matrix — for example, the role of olive oil in soapmaking — is examined separately in Chaileapedia carrier oil monographs.

Essential oil = non-saponifiable: No SAP value, not in lye calculation, added at trace

Essential and fragrance oils have no SAP value; because they do not react with lye to convert to soap, they are not included in the recipe's lye calculation. Instead, they are treated as an additive, expressed as a percentage of the carrier oil mass. In practice, they are added to the soap at the trace stage — the moment the soap mixture begins to thicken — which is just before the peak of the saponification heat curve; this timing limits the duration of exposure of aromatic molecules to heat and alkali stress. Lovin Soap Studio and similar commercial soapmaking references clearly state that this addition is calculated based on carrier oil weight and does not enter the SAP table.

Safe usage rates: Why should essential oil not exceed ~3%?

The rationale for adding essential oil in "small amounts" is not economic alone; safety regulations and sensory threshold jointly determine this limit. In hand-made soapmaking, the widely adopted upper bound is approximately 3% of carrier oil weight (~30 g/kg carrier oil); this rate is both sufficient for a balanced scent profile and helps stay below the component-based limits defined by authorities such as IFRA for the rinsed-off product category (Category 9). Some components (for example, cinnamaldehyde) are additionally restricted in IFRA Standards; cinnamon bark oil, rich in this component, requires component-level accounting when entering a recipe; cinnamon bark oil cinnamaldehyde restrictions are handled separately. Exceeding these rates risks not only amplifying the scent but also causing skin irritation and "seize" (sudden hardening) in the soap. The upper limit is a safety and formulation budget, not a mass budget.

One plant, two oils: Resolving confusion via the laurel (Laurus nobilis) example

Laurel is one of the clearest examples of the fixed oil–essential oil distinction; because two separate oils are produced from the same plant, and these two oils play entirely different roles in soapmaking.

Laurel FRUIT fixed oil (cold press) — the main saponifiable oil of Aleppo soap

Oil obtained from laurel fruits via cold press is a fixed oil; it consists of triglycerides and possesses a SAP value. In traditional Aleppo soap, it is the primary saponifiable ingredient alongside olive oil; its saponification establishes the soap's structure, hardness, and shelf (oxidative) stability. A detailed analysis of laurel fruit oil from a soapmaking perspective is covered in Chaileapedia's laurel oil (Laurus nobilis) monograph.

Laurel LEAF essential oil (distillation) — fragrance only, does not make soap

Oil obtained from laurel leaves via steam distillation is an essential oil. According to Fidan and colleagues' analysis, the leaf essential oil contains 1,8-cineol at 41.0%, fruit essential oil at 33.3%, and shoots at 48.5% (Molecules, 2019); this composition defines the scent character but does not build the soap matrix. Leaf essential oil lacks a SAP value, does not react with caustic, and enters the recipe only at trace for aroma. The classic sector caution neatly summarizes this distinction: "essential oil will not make Aleppo soap" — leaf essential oil alone cannot build Aleppo soap; what saponifies is the fruit fixed oil. The chemical profile and cosmetic use of laurel leaf essential oil is additionally examined in the laurel leaf essential oil monograph.

Two oils from the same plant, two distinct roles in soap: fruit fixed oil builds the matrix, leaf essential oil provides the scent.

Regulation and safety: Though natural, essential oils are subject to oversight

The botanical origin of essential oils does not exempt them from cosmetic regulation. Botanical-source aromatic compounds — chiefly terpenoids such as linalool, limonene, citral, and geraniol — carry allergenic potential and are subject to labeling obligation. Each essential oil entering a recipe must be evaluated against its place on the component-level list of allergens.

Perfume allergens and (EU) 2023/1545: From 24 to ~80 allergens, label deadline 31 July 2026

By Commission Regulation of 26 July 2023, (EU) 2023/1545, the European Commission amended Cosmetic Regulation Annex III, increasing the number of fragrance allergens requiring individual disclosure from 24 to approximately 80 on the INCI list (56 new allergens added). Individual labeling thresholds are preserved: in rinse-off products (soap falls in this category), the relevant allergen concentration must exceed 0.01%, and in leave-on products, it must exceed 0.001%, to require separate component listing. The compliance timeline is clear: for products newly placed on the market, 31 July 2026; for stock already on shelves, 31 July 2028 (EUR-Lex CELEX:32023R1545).

In practice, this means that "contains natural lavender essential oil" alone is insufficient; if the recipe contains linalool, limonene, geraniol and other allergenic components above threshold, these must be separately declared in the INCI list. The compositional profile and allergen load of lavender oil are addressed in a separate Chaileapedia monograph. Chailea recipes are formulated with these thresholds in mind; label disclosure is component-level.

Frequently Asked Questions

Why does essential oil in soap lose its scent over time? Two mechanisms work together. First, the defining property of essential oil — the low boiling point of terpenes and terpenoids — makes them prone to evaporation over time; especially during soap cure, the oil contacts air. Second, the high pH and residual heat during saponification may alter the chemical structure of some aromatic molecules; for instance, citral-rich citrus oils undergo faster change in an alkaline medium. This is why essential oil is added at trace (to largely bypass the heat peak of saponification) and soap is stored in cool, dark conditions.

Are fixed oils and essential oils used together in the same soap? Yes; this is the standard practice in cold-process soapmaking. Fixed oils comprise nearly all of the recipe and saponify to build the soap matrix; essential oil is added as a small percentage of carrier oil weight (typically ~1–3%) at trace for fragrance. The two oils serve different functions side by side: one structure, one aroma.

Is essential oil enough for soap fragrance; why is such a small amount added? Three constraints apply: safety regulations (for example, the EU's fragrance allergen thresholds and IFRA Standards), skin tolerance (high terpene load carries a risk of irritation), and formulation stability (high essential oil ratios can cause the soap to "seize"). Low amounts help both manage legal disclosure thresholds correctly and provide sensory balance in finished soap; the "more oil, more scent" approach is limited by chemistry and regulation.

Is every cold-pressed oil a fixed oil? No. Cold press is an extraction method, not an oil classification. The vast majority of oils obtained from seeds and fruits via cold press are fixed oils; however, oils obtained from citrus peels via mechanical expression (a cold-press variant) — orange, lemon, bergamot — are essential oils. What determines an oil's class is not the extraction method but its molecular structure: if it is triglycerides, it is fixed; if it is volatile terpene/terpenoid mixture, it is essential.

In laurel soap, is "laurel oil" an essential or fixed oil? The primary saponifiable ingredient in Aleppo soap is laurel fruit fixed oil; it builds the soap matrix with olive oil. Laurel leaf essential oil can be added to the same soap only for fragrance; it does not build the soap's structure. The INCI designation matters: Laurus Nobilis Fruit Oil is fixed, Laurus Nobilis Leaf Oil is essential. The same principle applies in Chailea's laurel leaf soap recipes; fixed oils build the matrix, leaf oil is used for aroma.

References

• Taşkor Önel G., Yaman Akbay H.G. (2022). Volatile and Fixed Oils: Structure–Activity Relationship Assessment. Bayburt University Journal of Science 5(1): 104-114. DergiPark. https://dergipark.org.tr/tr/pub/bufbd/article/1021436
• Fidan H., Stefanova G., Kostova I., Stankov S., Damyanova S., Stoyanova A., Zheljazkov V.D. (2019). Chemical Composition and Antimicrobial Activity of Laurus nobilis L. Essential Oils from Bulgaria. Molecules 24(4): 804. DOI: 10.3390/molecules24040804 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6412751/
• Commission Regulation (EU) 2023/1545 of 26 July 2023 amending Regulation (EC) No 1223/2009 of the European Parliament and of the Council as regards the labelling of fragrance allergens in cosmetic products. EUR-Lex. https://eur-lex.europa.eu/eli/reg/2023/1545/oj/eng
• International Fragrance Association (IFRA). IFRA Standards. https://ifrafragrance.org/initiatives-positions/safe-use-fragrance-science/ifra-standards
• SoapCalc — Lye Calculator Help Text (SAP values and lye calculation). https://soapcalc.net/helptext
• Lovin Soap Studio. Essential Oil and Fragrance Oil Usage Rates in Cold Process Soap. https://lovinsoap.com/2014/06/essential-oil-and-fragrance-oil-usage-rates-in-soap/
• Handcrafted Soap & Cosmetic Guild. Essential Oils vs. Fixed Oils. https://www.soapguild.org/how-to/ingredients/essential-oils-vs-fixed-oils.php

Author: Oğuz Kağan Dereci (High Chemist), Chailea Workshop — Rize / Çayeli.