Food Technology
Structures of Food
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Learning objectives
Learning objectives
To learn how to make different kinds of sauces.
To understand about how different structures are formed.
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What are sauces used for?
Without sauces a lot of food would be boring. Imagine roast lamb
without mint sauce, roast pork without apple sauce, chips without
tomato sauce, apple pie without custard and macaroni cheese
without the cheese sauce.
Sauces add flavour, colour and
texture to a food product. A
definition of a sauce could be
‘liquid seasoning for food’. They
cannot be too much of a liquid,
however, as they need to stay on
the plate! They need to be
thickened in some way.
Some sauces are thickened by adding starch to a liquid, others
by adding egg or some with fruit and vegetables as in a purée.
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How are sauces made using starch?
When starch is heated in a liquid, it goes through the process of
gelatinization – the starch absorbs the liquid and thickens the
liquid, making a sauce.
In a liquid the walls of the starch
granules become soft. As the liquid
is heated, the granules become so
soft that they start to absorb the
liquid (the liquid passes through the
cell walls of the starch granules).
With more heating the starch
granules absorb even more liquid
and swell to such a big size that
they eventually burst. This thickens
the sauce.
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How does gelatinization work?
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Different methods of gelatinization
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Thickness of gelatinized sauces
Different thicknesses of sauces can
be made depending on the amount
of starch used – the greater the
amount of starch relative to the
liquid, the thicker the sauce.
Some additional ingredients can
also be added which will affect the
thickness of a gelatinized sauce.
Acidic foods such as tomatoes or
lemons affect the structure of the
starch cells and reduce the
thickness of the sauce. Sugar
behaves in the same way.
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Making sauces using starch
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Using fruit and vegetables to make sauces
A fruit or vegetable sauce is called a purée or a coulis and can
be made from raw or cooked fruit and vegetables. In a fruit or
vegetable sauce, the structure of the fruit or vegetable is
broken up and the cell walls collapse. This is done by cooking,
sieving or using a food processor. It can be just one fruit or
vegetable or a mixture.
Raspberry coulis is an example of an uncooked single fruit
sauce – fresh raspberries are pressed through a sieve to make
a seedless sauce.
Red pepper purée is an example of a cooked mixed vegetable
sauce – peppers, onion and a little deseeded tomato are
cooked and then made into a sauce with a food processor.
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Tomato sauce on pizza
The tomato sauces in pizzas tend to vary and manufacturing
companies which produce pizzas spend a lot of time
experimenting with ingredients to develop their ideal sauce. One
way to make a good sauce is as follows:
Sweat one diced onion in a
tablespoon of oil for 10 minutes.
Add one 450g tin of tomatoes, one
clove of garlic, one tablespoon of
tomato purée and ½ teaspoon of
mixed herbs to the onions and
simmer for 20 minutes.
Put in a food processor and ‘whizz’
on high for 2 minutes.
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How are eggs used to make sauces?
The protein in egg coagulates when heated and thickens to
make a sauce.
In real custard, eggs are heated together with sugar and milk or
cream; in hollandaise sauce eggs are heated together with butter.
The more egg which is
used, the thicker the
sauce. A small amount of
egg is used to thicken
some soups and a large
amount of egg is used to
thicken the custard in
custard tarts and
savoury quiches.
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Sauces quiz
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What are colloidal structures?
Colloidal structures are formed when one substance is
suspended in another substance but the molecules of each are
too big to combine into a solution. There are different types of
colloidal structures:
sols – a solid suspended in a liquid, i.e. flour in water
gels – a solid suspended in a solid, i.e. when a sol cools
(blancmange is a good example)
foams – a gas suspended in a liquid or solid
emulsions – a liquid suspended in a liquid or solid.
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How is jelly made?
Gelatine is a protein extracted from animal collagen and is
used in ordinary cartons of jelly as the gelling agent. When the
protein (gelatine) is heated, it melts and becomes dispersed in
the liquid. When cooled the protein unwinds to form a network
which traps the liquid and forms a gel. Products made from
gelatine are unsuitable for vegetarians.
Carageen is a carbohydrate extracted from a red seaweed on
the coast of Ireland and agar is a carbohydrate extracted from
algae off the coast of Japan. Both of these can be eaten by
vegetarians and are used in quick gel (jelly which sets in an
hour).
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How is jam made?
Pectin is a complex carbohydrate which exists in the cell walls of
fruit and vegetables. It is used as a gelling agent in jam.
When making jam, the fruit is crushed and cooked at a high
temperature. This releases the pectin (a gum-like substance)
from the cell walls of the fruit. As it is released, it traps the sugar,
water and fruit to form a gel.
Different fruits have different
pectin contents. Strawberries
and blackberries have low
pectin contents and often
have apples added to the jam
as they have a high pectin
content.
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How is a foam formed?
A foam is formed when a gas is mixed into a liquid or solid.
For example, as raw egg or
cream is whisked, air bubbles
(gas particles) are added to the
mixture. The mechanical action
of the whisking makes the
protein in the egg or cream
unfold to form a network. The
network traps the air and forms
a foam.
Such a foam is unstable unless it is cooked (as in meringues,
cakes or bread) or a gel is added to set it (as in a fruit mousse).
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What are emulsions?
Emulsions are mixtures of liquids which do not usually mix
together such as oil and water. There are solid emulsions (for
example, butter or margarine) and liquid emulsions (for example
mayonnaise).
There are two types of liquid emulsions:
unstable emulsions (vinagrette or french dressing) where the
emulsion is temporary and made by shaking two liquids
together
stable emulsions (mayonnaise) where an emulsifier is used to
keep the two liquids mixed up. In mayonnaise, egg yolk is the
emulsifier – it lowers the surface tension between the two
liquids so that they combine and form a stable emulsion.
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Colloidal structures quiz
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Key points
Key points
When starch is heated in a liquid, it gelatinizes to make a
sauce.
Fruit and vegetable sauces can be made by cooking,
sieving or using a food processor to break down the
cell walls of the fruit or vegetables.
Eggs can be used to make sauces since the protein in
the egg coagulates when heated.
Colloidal structures are formed when one substance is
suspended in another substance but the molecules of
each are too big to combine into a solution.
There are different types of colloidal structures: sols,
gels, foams and emulsions.
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