Noadswood Science, 2011

To know how plant oils are useful in cooking

Wednesday, April 19, 2023

The fatty acids in some vegetable oils are saturated – how many bonds do they have between their carbon atoms?

Saturated oils have single bonds between their carbon atoms – they tend to be solid at room temperature (e.g. lard)

The fatty acids in some vegetable oils are unsaturated – how many bonds do they have between their carbon atoms?

Unsaturated oils have double bonds between some of their carbon atoms – they tend to be liquid at room temperature and can be divided into two categories: -• Monounsaturated fats have one double bond in each fatty

acid• Polyunsaturated fats have many double bonds

During hydrogenation, vegetable oils are hardened by reacting them with hydrogen gas at about 60ºC (this increases their melting point)

A nickel catalyst is used to speed up the reaction – the double bonds are converted to single bonds by the hydrogenation

This causes unsaturated fats to be made into saturated fats…

Saturated vegetable oils are solid at room temperature, and have a higher melting point than unsaturated oils

This makes them suitable for making margarine, or for commercial use in the making of cakes and pastry

Complete the cooking in oil and water experiment

The temperature that a liquid boils at depends on the size of the forces between its molecules – the bigger these forces the higher the liquid’s boiling point

The molecules in vegetable oils are much bigger than water molecules (so their boiling point is much higher)

Cooking food causes permanent changes to occur to the food – cooking in vegetable oils causes different reactions to the food as the temperature is so much higher (often the food cooks more quickly, turns a different colour on the outside and becomes crisper)

Also cooking in oil can cause the food to absorb some of that oil meaning the energy content of the food is much higher (one reason why fried food can be bad for you)!

Vegetable oils are important nutrients and provide a lot of energy.

Vegetable oils are also used as fuels for vehicles (some of this biodiesel is made from waste cooking oil and rapeseed oil with benefits as these fuels are carbon neutral)

It can be questioned how ethical it is to use food crops in this way, instead of using them for feed when famine is still a global problem…

Vegetable oils do not dissolve in water – if a mixture of oil and water is shaken, then left to stand, eventually a layer of oil will form on the surface of the water

Emulsifiers can be added to the oil and water, causing an emulsion to form (a mixture of the two)

Emulsions are more viscous than oil or water on their own, and contain tiny droplets of one of the liquids spread through the other liquid

Oil droplet

Water

Emulsifier molecule –

hydrophilic head and hydroscopic

tail

Examples of oil droplets in water: -• Egg yolk• Milk• Ice cream• Salad cream• Mayonnaise

Examples of water droplets in oil: -• Margarine• Butter• Skin cream• Moisturising lotion

Partially hydrogenated vegetable oils may contain trans fats

These are thought to cause health problems such as heart disease in humans, and food manufacturers are being encouraged to reduce the amount of them in our food…