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The definition of fat can be confusing, because it depends on the context. It can refer to a group of lipids, dietary fat, body fat or just a triglyceride molecule.
This article focuses on the most abundant in food and body fat molecules called triglycerides.
Chemical structure of triglycerides
A triglyceride molecule consists of a glycerol molecule (called glycerol backbone), that has three fatty acids attached to it. It belongs to a subcategory of lipids called glycerides.
The glycerol backbone can connect to one, two or three fatty acids. A glycerol molecule with one fatty acid attached is called monoglyceride, with two – diglyceride and with three – triglyceride.
The following is a simple representation of these 3 types:
Monoglycerides and diglycerides are also lipids. They contain both water insoluble and soluble components (unlike triglycerides that are completely water-insoluble). This feature makes them surfactants, such as emulsifiers.
Triglycerides are the most abundant lipid in our diet and inside our body. They are the focus of this article.
So, what is a triglyceride?
As mentioned before, a triglyceride consists of a glycerol molecule, with three fatty acids attached. A triglyceride can be made up of any combination of fatty acids: saturated and unsaturated (including trans fatty acids).
The naming of fats (saturated or unsaturated) usually indicates the type of fatty acids that are dominant. It doesn’t necessarily mean that the fat is 100% composed of a particular fatty acid type as the following simplified representation of a saturated fat molecule demonstrates.
For instance, saturated fats usually refer to those fats that have a significant proportion of saturated fatty acids, when compared to others. So even saturated fats of animal origin contain some unsaturated fatty acids. (4)
Chemical structure of fatty acids
To understand fats, you need to understand fatty acids. Properties of fats and their health effects in a human body depend greatly on the types of fatty acids they are made up of.
So, what is a fatty acid?
A fatty acid is a molecular compound consisting of carbon (C), hydrogen (H) and oxygen (O) atoms.
A fatty acid consists of a carbon chain with hydrogen atoms attached to the carbon atoms. Fatty acids are divided into two groups: saturated and unsaturated. You can spot the difference between these groups by examining how the carbon atoms are connected with each other (see image below).
The chains may vary in length and usually contain an even number of carbon atoms (mainly due to the way C2 are liked together in a series in biosynthesis reaction). There are over 40 different types of fatty acids.
Most of the fatty acids are connected to glycerol backbone (don’t usually occur in a free form).
Fatty acids are divided into four groups according to the number of carbon atoms:
- Short-chain fatty acids: < 6 carbon atoms
- Medium-chain fatty acids: between 6 and 12 carbon atoms
- Long-chain fatty acids: between 14 and 20 carbon atoms
- Very-long chain fatty acids: > 20 carbon atoms
Most fatty acids have between 14 and 20 carbon atoms.
Saturated fatty acids
The chemical characteristics of saturated fatty acids are:
- They contain only single bonds between the carbon atoms (there are no double bonds).
- The carbon atoms are completely saturated with hydrogen atoms. This means that it is impossible to combine any additional hydrogen atom to a carbon atom.
- The chain has a straight shape (there are no kinks). Therefore, saturated fatty acids can easily align and be more compact than unsaturated fatty acids which have an uneven shape. This ability to compact tightly gives the saturated fat a high melting point and, consequently, a solid state at room temperature.
The image below is a representation of what a saturated fatty acid chain looks like. You can see that every carbon atom has four bonds: two bonds are linked to other carbon atoms to form the chain and the other two are linked to hydrogen atoms.
Saturated fats have been considered “bad fats” for about four decades, but recent evidence shows otherwise.
Unsaturated fatty acids
The following are the chemical characteristics of unsaturated fatty acids:
- Unsaturated fatty acids have at least one double bond between the carbon atoms. They can be either monounsaturated (one double bond) or polyunsaturated (two or more double bonds)
- Each of the two carbons that are connected by a double bond can only have one hydrogen atom attached. These hydrogen atoms are either attached on the same side or on opposite sides of the chain.
The Cis and the Trans configuration of unsaturated fatty acids
If the hydrogen atoms are on the same side of the chain, it is called a “cis” configuration. The one-sided connection of the hydrogen atoms creates an empty space on the other side of the chain, giving its structure a pronounced kink (the chain bends towards the empty space forming a U shape).
These kinks make the shape of the chain uneven. Therefore, it is difficult for other chains to align closely together, resulting in a weaker intramolecular attraction. The presence of these kinks determines the melting point of the fats.
The more kinks, the lower the melting point. This means that polyunsaturated fats have a lower melting point than monounsaturated fats and that polyunsaturated fats with more double bonds, have a lower melting point than those with less double bonds. The “cis” configuration is different from the “trans” configuration of the trans fatty acids (also unsaturated).
In this case, the hydrogen atoms are attached on opposite sides of the chain.
From unsaturated to saturated fatty acids
Hydrogen atoms can be added to the carbon atoms with double bonds, through a chemical process known as hydrogenation. This process is also called saturation, since it “saturates” the fatty acids with hydrogen atoms. This chemical reaction occurs naturally in the stomachs of ruminant animals and in industrial hydrogenation of unsaturated vegetable oils.
The process of full saturation turns unsaturated fatty acids into fully saturated fatty acids, by removing all of the kinks in their structure.
Monounsaturated fatty acids
Monounsaturated fatty acids have only one double bond between the carbon atoms. The hydrogen atoms attached to the carbon atoms with double bond can be either on the same or opposite side of the chain. If on the opposite side, these monounsaturated fatty acids are called trans fatty acid. (see section below).
Polyunsaturated fatty acids
Polyunsaturated fatty acids have two or more double bonds along the chain. The hydrogen atoms attached to the carbon atoms with double bond can be either on the same or opposite side of the chain. If on the opposite side these polyunsaturated fatty acids are called trans fatty acid. (see section below).
Trans fatty acids
The trans fatty acids are unsaturated (either mono or poly-unsaturated) fatty acids that contain one or more carbon double bonds with a “trans” configuration. The “trans” configuration in Latin means “opposite side”.
This refers to the hydrogen atoms that are attached to the carbon atoms with double bonds on opposite sides of the chain. The characteristics of the trans fatty acids chemical structure are:
- Just like in other unsaturated fatty acids, there is at least one double bond and each carbon atom, connected by the double bond, has only one hydrogen atom attached.
- In “trans” configuration (unlike “cis” configuration), the hydrogen atoms are attached on opposite sides of the chain. This opposite-sided connection of the hydrogen atoms does not cause a pronounced U-shaped kink (bend) in the chain structure, as in the “cis” configuration. It rather causes a slight unevenness, making the shape of the trans fatty acid similar to saturated fatty acids.
Dietary trans fatty acids exist in nature from ruminant animals (rTFAs), and are considered as safe for human consumption, but the majority are derived from the industrial hydrogenation process of unsaturated oils. These are called industrial trans fatty acids (iTFAs) and are harmful to our health.
iTFAs and rTFAs differ in their structure, mainly in the distribution/position of the double bonds along the chain. (5)
Industrial trans fats have strong links to many health issues, such as an increased risk of developing heart diseases and other serious medical problems. (read more..)
Hydrogenation of unsaturated fatty acids
The hydrogenation process saturates unsaturated fatty acids with hydrogen atoms – adds hydrogen atoms to the carbon double bonds (see above). The industrial hydrogenation process of unsaturated fatty acids uses hydrogen gas, high temperatures and high pressure. This results in some fatty acids getting damaged, instead of saturating them, by flicking the “cis” to a “trans” double bond configuration. (read more..)
Hydrogenation can be either full or partial.
Full or complete hydrogenation turns all unsaturated fatty acids into saturated (full saturation). There are no double bonds present in fully hydrogenated fats.
How does the complete hydrogenation work?
During the hydrogenation process, hydrogen gas is combined with liquid oil under high temperatures and high pressure. Catalyst metals, such as nickel, are also added to aid in the reaction. Those metals are then almost completely removed (some trace particles are left).
Looking more closely, at a particle level, during this reaction a pair of hydrogen atoms gets attached to the pair of carbon atoms with double bonds, removing the double bond.
The result is a saturated fatty acid, with a straight shape (the kinks disappear) that can pack closely together.
If the hydrogenation process is stopped prematurely, not all fatty acids get fully saturated. Some are left unsaturated in a “cis” configuration and some of the “cis” configuration get damaged, forming a “trans” configuration.
The following image shows two results of the hydrogenation process: fully saturated fatty acid and trans fatty acid.
Damaging fatty acids through hydrogenation – trans fatty acids
The double bonds on the “cis” configuration are the weakest points in the unsaturated fatty acids, making them more vulnerable to chemical reactions. This is where the chain bends towards the empty space that is created by the missing hydrogen atoms.
The hydrogenation process (bombardment with hydrogen gas under high pressure and high temperature) removes one of the hydrogen atoms from the “cis” double bond.
The bond then becomes unstable and quickly attracts another hydrogen atom. This new hydrogen atom may attach to the other side of the chain, forming a “trans” configuration which stabilizes the fatty acid.
This addition removes the sharp kink and the empty space of the “cis” configuration, making the fatty acid’s shape almost straight and similar to a saturated fatty acid chain.
The straighter shape of trans fatty acids allows them to pack closely together and, as saturated fatty acids, increase their melting point. This is why partially hydrogenated oils become solid at room temperature (e.g. shortening or margarine).
Trans fatty acids are, however, an unwanted by-product of this process, since they are extremely harmful for our health. (6)
If the hydrogenation process continues and reaches full saturation, all double bonds, including those with “cis” and “trans” configurations, disappear. In effect, full hydrogenation oils don’t have trans fatty acids and are entirely composed of saturated fatty acids.