Download Chemical and functional properties Carbohydrates

Functional and
chemical properties
Carbohydrates
By the end of this lesson students should be able to:
• explain the term gelatinisation
• explain the term dextrinisation
• explain the term caramelisation.
Gelatinisation
The process in which moist heat is applied to starch grains, which swell,
increase in size and then break open, releasing amylose, which thickens
the mixture around boiling point.
Gelatinisation occurs between 75°C and 87°C
Stirring will prevent lumps forming.
When the thick liquid cools it forms a gel which will then set.
The presence of two molecules called amylose and amylopectin in starch
gives it the ability to thicken sauces. They bond with each other when heated
to form a gel:
- Amylose causes sauces to thicken, turn cloudy when cooked and get
even thicker as they cool.
- Amylopectin produces a clear gel when it thickens and has the same
thickness hot or cold.
Different starches contain different amounts of amylose and amylopectin
molecules
Cornflour made from maize cereal, for example, is almost pure starch and is
high in amylose. This means it will form a strong gel. It blends to a smooth
cream when mixed with a cold liquid
Starchy rich food
How they thicken
Wheat flour
Traditionally used to thicken white sauce (bechamel)
Cornflour
Used to thicken gravies, custard and soups
Arrowroot
Used to thicken gravies, soups, stews, sauces
Potatoes
Used to thicken soups and casseroles as they release starch
Tiny pasta shapes
Added to soups e.g. minestrone
Rice
Used to thicken milk puddings e.g. rice pudding
Barley
Added to a soups and casseroles to thicken it
Factors affecting gelatinisation
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Amount of liquid: Small amount of liquid in proportion to starch will produce a thicker
sauce and a large amount of liquid to proportion of starch will produce a runny sauce.
Type of starch: Cornflour is better for thickening than plain wheat flour, as it is pure
starch.
Temperature: gelatinisation requires hot, moist conditions. Starch will not dissolve on
cold water and cannot thicken cool liquids. In order for starch to absorb liquid the outside
of the starch grain need to be softened by heating. At a temperature between 75°C and
87°C the sauce will start to thicken and at boiling point it stops thickening. Sauces
should be gently boiled while stirring for a full 2 minutes to ensure that all the starch
grains have gelatinised.
Stirring: is essential for the creation of a smooth gelatinised sauce. If not stirred the
starch grains will settle at the bottom of the saucepan. Here the grains will absorb some
of the liquid around them, but they will also stop liquid from reaching other grains,
resulting in a lumpy texture.
Sugar: competes with starch for water. This means the starch does not absorb as much
water as it could. Sugar makes the sauce runnier and less likely to be stable. Always add
sugar after gelatinisation in sauce-making.
Acids: such as lemon juice should always be added to the sauce after it has thickened
and boiled, because they can break down the gel. Recipes like lemon meringue pie
should follow this method.
Critical stages when making a starch based
sauces
Dextrinisation
• Dextrinisation occurs when starch is broken down into dextrin by dry
heat, e.g. baking, grilling or toasting. Dextrin adds a sweet taste to baked
products.
• Dextrinisation contributes to the brown/ golden brown colour and flavour
of many foods such as toast, bread, pasties, biscuits, cakes, scones
• When you toast bread until it is very dark in colour, it means that it has
gone past the dextrinisation stage and the oxygen and hydrogen part of
the carbohydrate have been driven out by the heat as water and what is
left behind is the carbon.
Caramelisation
•
Caramelisation is the process of changing the flavour and colour of food products
containing sugar from white to brown when heated. It can give a buttery, toasty or even
a nutty flavour to food.
• Caramelisation is one of the most important types of browning processes in foods.
• It leads to a desirable golden brown colour and an attractive flavour in baked goods and
drinks.
Process: When sugar is heated to high temperatures (at least 160 -180°C), it first melts,
then forms a syrup and boils. Water is released from the sugar as steam when it is heated.
The colour of the sugar gradually changes from colourless and clear to golden brown to
dark brown and eventually black and bitter when it is overheated. Sugar begins to break
down to pure carbon at about 200°C.
When cooled the syrup becomes brittle and hard.
Crème brulée is a dessert that has
a hard sugar topping. This is
achieved by using a blow torch or
a very hot grill to caramelise the
sugar.
Examples:
- Fudge, toffee, jam, honeycomb, brandy snaps
- Crème brulee, Crème caramel, spun sugar
- Surface of biscuits, bread, pastry
- Vegetables such as onions when fried for extended period of time e.g. French onion
soup