Oil, commonly found in cooking oils or industrial lubricants, is a long-chain organic compound composed primarily of triglycerides (fats). Unlike water, oil molecules do not undergo traditional evaporation or “drying out.” When oil appears to change form, such as hardening or developing an off-odor, it is not due to a physical phase change. Instead, these perceived changes result from complex chemical reactions that transform the oil molecules themselves, driven by exposure to oxygen, heat, and light.
Why Oil Does Not “Dry Out” Like Water
The concept of a liquid “drying out” refers to evaporation, a physical process where molecules escape the liquid phase and enter the gas phase. Volatility relates directly to a substance’s molecular weight and the strength of intermolecular forces. Water molecules are small (about 18 grams per mole) and held by weak hydrogen bonds, allowing them to easily transition into vapor at room temperature.
Oils, conversely, are composed of very large triglyceride molecules, often hundreds of times heavier than water. These long carbon chains create strong intermolecular forces that bind the molecules tightly. Consequently, oil has a low vapor pressure and an extremely high boiling point. Since oil molecules cannot easily overcome these forces to escape into the air, oil is considered non-volatile and does not evaporate in the conventional sense.
The Primary Chemical Change: Oxidation
The fundamental process driving nearly all oil degradation is autoxidation, a chemical chain reaction involving atmospheric oxygen. This process targets the double bonds within the oil’s unsaturated fatty acid chains, making adjacent carbon-hydrogen bonds vulnerable to oxygen attack.
Oxidation is a free radical chain reaction with three stages: initiation, propagation, and termination. Initiation occurs when an external factor (such as heat, light, or trace metals like iron or copper) abstracts a hydrogen atom from the fatty acid chain, creating a highly reactive free radical.
The propagation stage begins when the alkyl radical reacts rapidly with oxygen to form a peroxyl radical. This new radical then abstracts a hydrogen atom from another stable oil molecule, forming a new alkyl radical and creating a hydroperoxide molecule. Hydroperoxides are the initial, unstable, and odorless products of oxidation.
The reaction continues until two free radicals combine during termination to form stable, non-radical products. Catalysts, especially heat and light, significantly reduce the activation energy required, causing oil exposed to these conditions to degrade faster.
Two Distinct Fates of Oil: Rancidity and Polymerization
The hydroperoxides formed during initial oxidation are unstable intermediates that quickly decompose. This decomposition leads to two distinct outcomes of oil degradation: rancidity and polymerization. The outcome depends heavily on the oil’s fatty acid composition and its degree of unsaturation.
Rancidity (Spoiling)
Rancidity typically affects edible oils classified as non-drying or semi-drying, such as olive or canola oil, which contain fewer double bonds. The hydroperoxides in these oils cleave into a complex mixture of smaller, volatile compounds. This scission primarily yields short-chain aldehydes and ketones, which cause the unpleasant odors and flavors associated with spoiled fats.
This process, known as oxidative rancidity, is the most common form of oil spoilage. Consuming significantly rancid oils can be detrimental to health due to the accumulation of oxidized products. The appearance of an off-taste indicates that the oil’s chemical structure has been compromised.
Polymerization (“Drying”)
Polymerization affects highly unsaturated oils, often called drying oils, such as linseed, tung, or poppy seed oil. These oils contain many polyunsaturated fatty acids with multiple double bonds, making them highly susceptible to oxidative cross-linking. After hydroperoxide formation, free radicals react with neighboring fatty acid chains, forming new chemical bonds between them.
This process creates a three-dimensional polymer network, converting the liquid oil into a solid, tough, and elastic film. This solidification is often mistakenly perceived as “drying out,” but it is a chemical curing process driven by oxygen incorporation. Polymerization is desirable in industrial applications like paints and wood finishes because it forms a durable barrier. The oil’s degree of unsaturation determines the speed of this hardening process.
How to Slow Down Oil Degradation
Since oil degradation is driven by a free radical chain reaction, effective preservation methods inhibit the initiation and propagation steps. Storing oil in opaque containers blocks light, which acts as a catalyst for free radical formation. Keeping oil cool, such as in a refrigerator, reduces the rate of all chemical reactions, including oxidation.
Limiting exposure to air is also necessary, as oxygen is a required reactant in the propagation stage. Tightly sealing containers minimizes available oxygen headspace and prevents constant replenishment. The addition of antioxidants, whether natural (like tocopherols) or synthetic, provides chemical protection. Antioxidants function as chain-breakers by reacting with free radicals, converting them into stable, non-reactive forms, and terminating the destructive chain reaction.