carbohydrates

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There are three types of carbohydrates in foods. These are:
            1. Polysaccharides
            2. Disaccharides
            3. Monosaccharides

Polysaccharides = starches, pectins and dietary fibre

Monosaccharides & Disaccharides = sugar

Polysaccharides have different reactions to Monosaccharides & Disaccharides during cooking

Starch molecules are made up of two substances: Amylose and Amylopectin

The ratio of these two substances in a molecule determines the quality of the end product. For example, higher proportions of amylose produces semi-solid, opaque gels whereas higher proportions of amylopectin produce clearer, thicker gels

Gelatinisation

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Definition: The ability of starch to thicken mixtures in the presence of heat

This process takes place regardless of the type of starch

Common thickening agents: Plain wheat flour, cornflour and arrowroot

Two factors affect the qualities of the gel produced. These are the ratio of amylose to amylopectin and the method used to prepare the food

Wheat flour and Cornflour = More amylose, meaning a thick but cloudy gel 



Factors affecting the gelatinisation of starch

The functional properties of carbohydrates are affected by temperature, pH, agitation, and the presence of other ingredients

Temperature: Gelatinisation requires warm moist conditions. Starch is not soluble in cold water and cannot thicken cool liquids. In order for the starch to absorb liquid, the outside of the starch granule needs to be softened by heating

Agitation: Agitation is essential for the creation of a smooth, gelatinised sauce. If agitation is not used the starch granules that are not exposed to liquids burst and trap the liquid around them, preventing it from reaching other starch granules resulting in a lumpy texture

Impact of other ingredients and pH: Sugar and acid have the greatest affect on gelatinisation. Sugar competes with starch for water therefore the starch does not absorb as much water. It also raises the temperature at which the granules start gelatinising, making the mixture more liquid and less likely to remain stable. Acids such as lemon juice should always be added to the mixture after it has thickened and boiled

EXAMPLE: White Sauce

When you think of Gelatinisation think of making a white sauce. A mixture of fat and starch.
First you would make a roux, which is a mixture of melted butter and flour, the continued heat softens the outer shell of the starch granules allowing them to swell up with liquid. As you add the milk and continue to heat the mixture the starch granules burst and the amylose and amylopectin they contain are released, capturing the liquid causing it to thicken and form a gel. Agitation must be used whilst heating to prevent lumps.
The mixture is then further boiled for atleast a minute to ensure all of the starch has reached the desired temperature and all of the starch is gelatinised. A pasty taste will result from any ungelatinised starch. The sauce should look smooth and shiny if it is fully gelatinised.

Dextrinisation

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Definition: The process in which starch breaks down into dextrins

Example: When bread is toasted

Starch can undergo a partial chemical breakdown into shorter glucose chains called dextrins. The process is as follows:


             Starch                >           Dextrins          >          Maltose            >            Glucose
     (polysaccharide)                                                                  (disaccharide)                    (monosaccharide)


Dextrins have a different chemical structure to starch, giving them different characteristics, these are:
     Dextrins dissolve in cold water but starch does not
     Dextrins taste sweet while starch tastes floury
     Dextrins are poor thickening agents while starch gelatinises

Dextrinisation can be caused by many things, these are:
     Exposure of starch granules to dry heat
     The action of acids
     The action of certain enzymes

Dextrinisation is the reason for:
    Bread turning brown during toasting
    Gravies being brown
    Brown crust forming on baked starchy vegetables, such as potatoes

Crystallisation

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Definition: The ability of sugar to dissolve and reform crystals

Crystallisation is used in the making of confectionary such as toffee, brittle, fudge and caramel.

Sugar is dissolved in a liquid  and heated, the liquid evaporates, concentrating the sugar solution. As the solution cools, the sugar re-forms into crystals

Factors affecting crystallisation
 

Temperature:  The hotter the water is the greater the amount of sucrose that can dissolve in it. Lower temperatures produce softer solutions such as fudge, extremely high temperatures produce harder solutions such as brittle

Acid: Adding acids such as cream of tartar or vinegar inhibits the development of large crystals giving the confectionary a smoother texture

Agitation:  Stirring sugar solutions encourages crystals to develop. If a solution is stirred while hot, crystallisation starts too soom, giving the product a grainy texture. Sugar must be stirred and fully dissolved into a liquid before boiling starts. Stirring after boiling point is reached encourages crystals to develop. Different products require agitation at different points of the process

Presence of other ingredients:  Monosaccharides i.e. honey, glucose, acidic ingredients, are included to ensure a smooth consistency. Fat i.e. butter or cream prevents large crystals forming ensuring a smooth texture

Caramelisation

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Definition: Sucrose melts at 186C. Above that temperature acids form and the molten sugar turns a light brown, or caramelises

The breakdown of sucrose occurs as follows:

                               
sucrose + heat     ->    glucose + fructose + water     ->   acids + glucose

Caramelisation occurs without water, as it evaporates before caramelisation takes place

Caramelised sugar adds a distinctive flavour to foods and is often used in commercial products such as milks and cakes



Watch this video on
Caramelisation

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Summary of Properties

TASK

Complete this worksheet to show your understanding of the Functional Properties of Carbohydrates