In this lesson, we will learn about plant
oils – where they come from, how to extract them, and their generic structure. Plant oils are obtained from plants. These oils are found in all parts of a plant
– seeds, nuts, fruit, leaves, flowers, bark, and even roots. However, these oils must extracted before
they can be consumed or used. There are a few key methods to extract plant
oils. Mechanical extraction Distillation Steam distillation Olive oil is obtained from olives by mechanical
extraction. This method is also known as cold pressing. The olives are crushed and pressed to squeeze
all the oil out. In the past, these heavy machines were operated
by humans. Nowadays, most mechanical extraction is done
in a factory by machines. Coconut oil is also obtained using this method. Note that both of these oils are essential
cooking ingredients in certain parts of the world. Some plant oils, such as sunflower oil and
soy oil, are obtained by distillation. These oils are first dissolved in a solvent,
and the solvent is removed by distillation. Any impurities are also removed during this
process. This method is used for oils that are more
difficult to extract from their respective plants. Steam distillation is used to extract floral
oils, such as rose and lavender oils. Steam is passed through the flower petals. The oils are then able to evaporate. Since the oil droplets are very small, they
can form an emulsion with water. The oil can then be separated by distillation. We will now look at the general structure
of plant oils. The structure can be broken down into two
parts: The glycerol “head” which is hydrophilic,
or water loving. It is made of three carbons. These three carbons are each attached to three
fatty acid “tails” which are hydrophobic, or water-fearing. The length of the tail can vary from oil to
oil – it can be anywhere from 4 carbons long to 28 carbons long. These “tails” can be saturated or unsaturated. The term saturated means that there are only
C-C single bonds. Saturated plant oils are solids at room temperature,
and are also known as vegetable fats. An example of a saturated plant oil is palm
oil from oil palm trees. The term unsaturated means that there are
C=C double bonds somewhere within the structure of the fatty acid tail. A monounsaturated oil has only one C=C double
bond. A polyunsaturated oil has more than one C=C
double bond. Most unsaturated oils are liquids at room
temperature. These unsaturated oils can be hydrogenated
through a process called hydrogenation. This hardens the oil and is a key step in
the production of margarine. In the laboratory, one can differentiate between
a saturated and unsaturated oil by testing with bromine water. An unsaturated oil will decolourise bromine
water, whereas a saturated oil would not. Final Summary In summary, plant oils can be obtained by
mechanical extraction, distillation, or steam distillation. All plant oils have the same generic structure
– a hydrophilic glycerol head, and three hydrophobic fatty acid tails. The fatty acid tail can be saturated if there
are only C-C single bonds, or unsaturated if there are C=C double bonds present.