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Food TechnologyStructures of Food

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Learning objectivesL

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To learn how to make different kinds of sauces.

To understand about how different structures are formed.

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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.

What are sauces used for?

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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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.

How are sauces made using starch?

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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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.

Using fruit and vegetables to make sauces

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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:

Tomato sauce on pizza

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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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.

How are eggs used to make sauces?

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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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:

What are 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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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).

How is jelly made?

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Pectin is a complex carbohydrate which exists in the cell walls of fruit and vegetables. It is used as a gelling agent in jam.

How is jam made?

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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A foam is formed when a gas is mixed into a liquid or solid.

How is a foam formed?

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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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:

What are 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 pointsK

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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.