The Difference Between Oil and Fat?
Both are subcategories of a group of natural substances called “lipids.” Lipids are substances soluble in organic solvents like ether and chloroform but insoluble in water. Oils and fats are lipids whose basic structural unit is triglycerides (3 fatty acids + 1 glycerol alcohol). Typically, if they are liquid at room temperature, they are called Oils, and if they appear solid, they are called Fats.
Why are Oils Liquid, While Fats are Solid?
We know that the structural unit of both groups is the same (triglycerides). So why, at the same temperature (room temperature), are oils liquid and fats solid? Or in other words, why is the melting point of fats higher than that of oils? This means we need more heat to bring fats into a liquid state (Example: Sheep tallow: 42°C). The explanation for this phenomenon lies in the triglycerides that make up fats and oils.
In fats, inherently, the fatty acids making up the triglycerides are linear and straight, and they are arranged this way within the triglyceride. This allows triglycerides, like uniform and identical bricks, to stack together in an orderly fashion. Consequently, triglycerides are closer together, have more contact surface area, bond together more easily, and form a regular network. This causes them to be solid at room temperature. When high temperatures near their melting point are applied, the bonds between triglycerides weaken, the kinetic energy of the triglycerides increases, they break out of their orderly network, slide over each other, and take on a liquid appearance.
However, in oils, which are liquid even at room temperature, the presence of one or more double bonds in the carbon chain of a fatty acid causes that fatty acid to bend. The presence of a double bond and the bending of one or more fatty acids in a triglyceride means the triglycerides do not have that uniform and identical shape and cannot stack well together or bond effectively. Therefore, instead of forming a network, triglycerides slide over each other and move irregularly because their shape prevents proper stacking. Thus, we observe a liquid state in them.
Furthermore, a fatty acid that has a double bond in its carbon chain is called an unsaturated fatty acid. Conversely, a fatty acid that has no double bonds and is linear is called a saturated fatty acid (saturated with respect to hydrogen).
Type of Oil and Oxidation
In the final part of the introduction, we must mention that the saturated/unsaturated nature of the fatty acids in oils and fats, and consequently their liquid or solid state, is an inherent and primary characteristic specific to that particular oil or fat.
For example, fat (beef tallow) contains 43% saturated fatty acids and 57% unsaturated fatty acids. Whereas olive oil contains 15% saturated fatty acids and 85% unsaturated fatty acids, and corn oil contains 13% saturated fatty acids and 87% unsaturated fatty acids.
The type of unsaturated fatty acids varies between different species (corn, olive, canola, etc.), and this is also considered a characteristic of an oil. Most vegetable oils have high percentages of unsaturated fatty acids.
Oils that have any amount of unsaturated fatty acids are sensitive to oxidation. Therefore, vegetable oils, which are liquid and unsaturated, are highly sensitive and prone to oxidation. During oxidation, under the influence of air, light, heat, metals, and moisture, oxygen attaches to the double bond site in the carbon chain of the unsaturated fatty acid. The breakdown of the resulting compounds produces substances that, besides creating bad odor and taste in the oil, have proven serious health detriments, including carcinogenicity.
To mitigate such effects, compounds called “antioxidants” are added to oils. Common antioxidants include BHA, BHT, TBHQ, citric acid, and ascorbic acid.
