Plant oils chemical composition
Plant oils and vegetable fats are made of “triglycerides” which derive from the combination of one glycerol unit and three units of fatty acids.
The difference between a fat and an oil is merely accidental and depends on the temperature of the mixture. If the mixture is solid at normal temperatures it is considered as a fat, if it is liquid is considered as oil. This distinction is just made for convenience because all oils are solified at low temperatures and all fats melt at high temperatures.
This is the general chemical formula for the triglycerides: three lines stand for the fatty acid chains. These can be the same acid or different types (simple or mixed triglycerides). The typical features of drying oils is the ability to react with air oxygen and polymerizing for a heat effect; this reaction is caused by the double bonds of the chains (position 9,12 and 15) of the acid fats. This unsaturation distinguishes the drying form non- drying oils. The unsaturation degree is defined through the iodine value. The reaction of the unsaturated triglycerides with air oxygen is very complex and not completely clarified. The process is accelerated by many catalysers, light and heat and is hindered by antioxidants such as phenols, oximes and amines.
Fats and oils are mixtures of different fatty acids in different proportions. Therefore, the saturation degree of different fats and oils depends on their content of different fatty acids as we can see in the table.
SAFA-Saturated Fatty Acids
Meat from animals, butter, whole milk and some tropical plant oils, like palm and coconut are the main sources of vegetable fats. They majority of this fats are solid at ambient temperature and they contribute to increase the level of LDL cholesterol with potential serious consequences on our health. They need to be consumed as part of a healthy diet.
MUFA – Mono Unsaturated Fatty Acid
Most of the animal and vegetable fats contain mono unsaturated fats but in variable quantities. They are normally in liquid state at ambient temperature, but can solidify if it gets colder. These are the best fats for the diet because they help reducing the LDL cholesterol and increasing the HDL cholesterol in blood.
Good sources of mono unsaturated fats are olive, canola and peanuts oil and most of the nuts. The olive oil has the highest percentage (about 77% of mono unsaturated fats than any other edible oil.
PUFA – Poly Unsaturated Fatty Acids
The main sources of Polyunsaturated fats are seeds, nuts, vegetables and grains. Polyunsaturated fats are mainly liquid at ambient temperature even if they are refrigerated. They reduce the overall cholesterol level as well as the HDL one. The daily requirements of these fats must be part of a healthy diet.
Double bonds can have CIS or TRANS configurations. In the above described structure, they are shown as CIS, which is the most frequent in natural acids, even if non- invariable.
Unsaturated fats configured in cis create a fold. These folds hinder the solid compactness of molecules and their solidification at ambient temperature. For this reason, butter is solid at ambient temperature whereas oils are liquid under the same conditions because they contain a lot of cis unsaturated fats. In other words: the presence of double cis bonds lowers the lipid melting point.
In nature there is a higher percentage of cis than trans fatty acids which derive most of all from some artificial treatments.
The three main origins of TFA (trans fat acids) are:
- The bacterial transformation of unsaturated fats in the rumen of ruminant animals;
- Industrial hydrogenation, for example in the margarine production hydrogen atoms are added to saturate the carbons of the double bond. In this way we get solid triglycerides with saturated acid fats starting from liquid unsaturated lipids;
- Deodorization during the heating and the frying of oils at elevated temperatures.
The impact of the TFAs on the CHD (coronaric heart disease) is higher than the SAFA’s one. Not only the TFAs increase the LDL cholesterol as the SAFAs, but reduce also the HDL one and increases the blood level of triglycerides.
A triglyceride is formed when the 3 hydroxyl groups (OH) of a single hydrogen molecule of glycerol react with the carboxylic group of 3 fatty acids (COOH-) forming the ether bonds.
The three fatty acids included in the triglyceride structure can/can’t be the same even in different triglycerides since there are many possible variations. The length of the fatty acid chains of triglycerides can vary, but normally it is 16, 18 and 20 carbons long.
The de novo synthesis of fatty acids is mostly similar in plants and animals and occurs in the chloroplasts of photosynthetic cells of superior plants and in the cytosol of animal cells through the concerted action of two enzymes: acetilCoA carboxylase and the acid fat synthetase.
This is a complex multienzyme fat that catalyses a repeated sequence of 4 steps through which the acyl chain is added with 2 carbon atoms in the carboxylic extremity in every passage through the cycle. This four-steps process is the same in every organism.
The palmitic acid is the most common saturated acid fat in the lipids of plants and animals, but generally it is not present in vast quantities since it can access in different metabolic ways.
For this reason, it is the forerunner of the stearic acid and can be desaturated by inserting a double bond inside the fatty acid chain giving rise to the palmitoleic acid, the forerunner of all omega 7 fat acids in a reaction catalysed from the enzyme 9-desaturase.
This enzyme is ubiquitarian in the animal and vegetable kingdoms and is the most active in the lipid metabolism of the mammals tissues. Moreover, it is the same that catalyses the stearic acid desaturation into oleic acid.
The Δ9-desaturase adds a double bond between position 9 and 10 of the fatty acid. This position is numbered starting from the carbossilic end of the molecule: if the substrate is made of palmitic acid the double bond will appear between position 7 and 8 of the chain (in this case positions are numbered starting from the methyl extremity) producing palmitoleic acid.
On the other hand, if the substrate is made of stearic acid the double bond will appear in position 9 and 10 of the chain and oleic acid will be produced as a result.
The following table summarizes the typical concentration of fatty acids (their common names and their iodic value) for some selected oils and fats. Fatty acids are carboxylic acids with chain lengths between 6 and 24 carbon atoms.
The composition of fatty acid is expressed as a weight percentage of the total amount of measured fatty acids. The glycerol weight is negligible. The shorten form of the fatty acids is made of two numbers: the first is for the carbon atoms and the second stands for the amount of the double bonds in the fatty acid chain. Many natural fatty acids have common names given when they were isolated and characterized for the first time.
The iodine value indicates the unsaturation degree of oil but doesn’t distinguish between mono unsaturated acid fats (for example oleic) and polyunsaturated acid fats (for example linoleic acid), nor between couples of double cis and trans. This value in natural fats and oils can vary into a specific range.
Fatty acids can be classified according to their saturation, therefore according to the amount of double bonds:
- SAFA don’t have double bonds (for example Myristic, Lauric, Palmitic and Stearic). All fatty acids with one or more double bonds are classified as unsaturated;
- MUFA don’t have a double bond (like Erucic, Oleic, Palmitoleic, Myristoleic and Gadoleic)
- PUFA have two or more double bonds (for example linoleic linoleic, arachidonic acid).
The position of the double bond can vary along the carbon chain and can be indicated in different ways.
Oils and fats are basically mixtures of different acid fats in different proportions, therefore the saturation degree of the different fats and oils depends on their content of different fatty acids, as we can see in the table above.