Finishing off from last time
Fatty Acids
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The long fatty acid chains stop vegetable oils
dissolving in water
The fatty acids in some vegetable oils are saturated,
and only have single bonds between their carbon
atoms
Saturated oils tend to be solid at room temperature,
and are sometimes called vegetable fats instead of
oils
Lard is an example of a saturated oil
Fatty Acids
 The
fatty acids in some vegetable oils
are unsaturated, and have double
bonds between some of their carbon
atoms
 Unsaturated
oils tend to be liquid at
room temperature, and are useful for
frying food
Unsaturation
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The carbon-carbon double bonds in unsaturated
oils can be detected using the elements bromine or
iodine – these elements react with the double bonds
in the oils, and the more double bonds there are, the
more bromine or iodine is used up
Unsaturated fats can be tested for using a simple
test with bromine water – bromine water is a dilute
solution of bromine, which is normally orangebrown in colour which becomes colourless when
shaken with an alkene, or with unsaturated fats
When shaken with alkanes or saturated fats, its
colour remains the same
Unsaturation
Quick Practical: Use Bromine water to test for saturation of the
products on the side
Thursday, March 24, 2016
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To know how plant oils are useful in cooking
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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
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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…
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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
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Complete the cooking in oil and water
experiment
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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)!
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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…
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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
Emulsifier molecule
– hydrophilic head
and hydroscopic tail
Water
Oil
droplet
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Examples of oil droplets in water: • Egg yolk
• Milk
• Ice cream
• Salad cream
• Mayonnaise
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Examples of water droplets in oil: • Margarine
• Butter
• Skin cream
• Moisturising lotion
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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…