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Head of the Oils and Fats Department at the Food Industries and Nutrition Research Institute, National Research Centre.
Fixed and Essential Oils: Key Differences in Definition, Composition, Properties, and Extraction
Humans derive their primary nourishment through the mouth and digestive system, with fixed oils forming an important part of this internal nutrition, providing energy and essential fatty acids vital for cell building and supporting vital functions. Conversely, the concept of nourishment is not limited to what enters the body; it also encompasses what is used externally on the skin, complexion, and hair, which can be termed functional external nourishment.
In this context, specialized fixed oils act as nourishing and moisturizing carriers, while essential or volatile oils serve as concentrated active ingredients, providing sensory and functional effects such as revitalization, protection, and support for radiance, renewal, and glow. From this integrated perspective, understanding the relationship between fixed and essential oils becomes a scientific and practical necessity to ensure optimal and safe use, whether in food or body care.
This meaning is perhaps best exemplified by the noble prophetic guidance concerning olive oil: «Eat the oil and anoint yourselves with it, for it is from a blessed tree.»; this prophetic advice combined two complementary paths: internal nourishment by consuming the oil, and external care by anointing with it, profoundly indicating that oil can be both internal sustenance for the body and a means of external care, preservation, and radiance.
Natural oils are among the most important plant products that humans have used for centuries in food, medicine, personal care, perfumery, and industry. However, confusion between the two main types of oils—fixed oils and volatile or essential oils—remains common, even among many experienced users. This confusion is not limited to linguistic or commercial aspects; it can lead to errors in use and potentially to health complications, especially when concentrated essential oils are used directly or without proper consideration.
Hence the importance of this guide, which clarifies the fundamental differences between the two types in a precise scientific and easily understandable practical manner. This first part will focus on the scientific basis: definition, chemical composition, physical and sensory properties, scientific explanations for the differences, and then extraction and production methods. The second part will cover uses, safety, storage, the integration of fixed and essential oils, and carrier oils and safe blends.
Fixed oils are fatty substances that are liquid at room temperature or body temperature, non-volatile, retain their state for long periods, and leave a distinct oily residue on the skin, surfaces, or clothing. They are characterized as true oils chemically because they are primarily composed of triglycerides, which are three fatty acid molecules linked to one glycerol molecule.
Common fatty acids in fixed oils include oleic acid, which is the predominant fatty acid in olive oil, linoleic acid, and stearic acid. These oils also contain important secondary components, such as tocopherols (a form of vitamin E), polyphenols, sterols, and phospholipids.
The molecular weight of fixed oils is relatively high, reaching approximately 800–900 grams/mole, which explains their non-volatility under normal conditions. The strong ester bonds between glycerol and fatty acids require very high thermal energy to break, which is why these oils do not evaporate at room temperature but remain stable and leave a distinct oily residue.
As for essential or volatile oils, they are highly concentrated, fragrant liquids that evaporate easily. They are typically extracted from plants, often through distillation or pressing, and are composed of complex, lightweight organic compounds. Chemically, they are not fatty oils and do not consist of triglycerides, but rather of volatile compounds such as monoterpenes, sesquiterpenes, aldehydes, ketones, alcohols, and esters.
Monoterpenes, such as limonene in citrus and pinene in pine, are among the most important components in many essential oils. Sesquiterpenes like caryophyllene, aldehydes like citral, ketones like camphor, alcohols like linalool found in lavender, and esters like linalyl acetate are also present.
The chemical reason for the volatility of these oils is their low molecular weight, typically ranging between 150 and 250 grams/mole, in addition to the weaker bonds between their molecules compared to fixed oils. Therefore, they easily transition from a liquid state to vapor, even at normal temperatures.
Fixed oils differ significantly from essential oils in terms of texture, aroma, volatility, and interaction with air. Fixed oils typically have a viscous, oily texture and leave a greasy residue, whereas essential oils are lighter in consistency, highly aromatic, and evaporate quickly.
Why Do Essential Oils Evaporate Quickly?
Essential oils are lightweight, and therefore their molecules possess greater kinetic energy at room temperature. According to the physical understanding of molecular motion, the lighter a molecule is, the higher the probability it will gain enough energy to escape from the liquid and transition into a gaseous state. This is why lavender essential oil evaporates quickly, while olive oil remains a stable liquid on the skin or surface.
Why Do Fixed Oils Leave a Greasy Residue?
The large, heavy molecules in fixed oils remain in a liquid state for a long time. When applied to skin or clothing, they do not evaporate easily; instead, they spread slowly and leave a noticeable greasy residue. This is why oils like olive oil, sweet almond oil, and jojoba oil are suitable for massage and skincare, as they stay on the skin long enough to act as a moisturizing and nourishing medium.
Why Are Essential Oils So Strongly Scented?
Essential oil molecules disperse easily in the air, quickly reaching the olfactory receptors in the nose, even when using very small amounts or high dilutions. This is why we perceive the scent of lavender, peppermint, or lemon oil as soon as the container is opened, whereas the aroma of fixed oils is often faint or limited.
This presentation clarifies that the difference between fixed oils and essential oils is not merely a difference in name or aroma, but a profound distinction in chemistry, physics, behavior, and extraction. Fixed oils are large, heavy fatty molecules that provide energy, nourish the skin, and remain on the surface for a long time. Essential oils, on the other hand, are light, volatile compounds that disperse quickly in the air, carrying the plant's aroma and sensory properties.
Thus, Part One has laid the scientific foundation for understanding the differences between the two types. As for Part Two it moves from understanding to application, discussing practical uses in food, skin and hair care, and aromatherapy. It then clarifies safety and storage issues, safe dilution ratios, the concept of carrier oils, and examples of balanced blends.
