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The understanding of thickening and cooking of starchy foods.

Gelatinisation is key to learning when making sauces. The application of gelatinisation covers many dishes and cuisines.



Gelatinisation is best known as the process of thickening when making sauces, soups and custards. In sweet and savoury dishes a texture change is possible due to thickening.  To create a sauce to coat, pour or bind, a basic Velouté, Béchamel or plain white roux sauce is chosen. Thickening is a process requiring skill and understanding.
What exactly happens when gelatinisation takes place?
Starch is the ingredient or component part that permits thickening when heat is applied. Starch is composed of two molecular types, amlyose which has a long straight chain formation and amylopectin, which has a branching structure.  Starch has densely packed molecules so it is not soluble in cold water.  A solution of starch will form but will separate out on standing.
Hot water has more energy and at around 60?C it begins to break open the starch molecules.  As the temperature rises so this action is intensified until all the starch molecules are forced apart and begin to swell. As they burst they release further smaller starch molecules that draw more liquid into their network. The starch grains absorption of liquid increases the viscosity of the sauce, thickening it gradually over a range of temperature.  At 96?C no further starch gelatinisation takes place and the process is complete.  The ratio of starch to liquid is critical to the end performance of the sauce.  Pouring sauce needs less starch than coating sauces.  Gelatinisation functions as temperature increases to thicken a liquid and also when temperature subsequently decreases.  As a sauce cools to eating temperature so it begins to ‘set’ and firm up.  In scientific terms the sol becomes a colloidal gel at around 38?C.
The type of starch used to thicken influences the end result of gelatinisation.  If wheat flour is selected as for a roux sauce, there is starch and also protein present in the wheat grains.  This results in a cloudy, opaque sauce.  When cornflour is used there is no protein present and the resulting sauce has a gloss. 
Special flour for sauce making is given a treatment called pre-gelatinisation.  This is when the starch is steamed and then dried again which means it can be added to hot liquids without forming lumps.
Once a sauce has been prepared the colloidal infrastructure will begin to collapse and shrink after a few days storage. This is known as retrogradation.  Fluids begin to leak from the structure and the gel deteriorates.
That is not the whole story of gelatinisation either, gelatinisation is a process that enables starch to become digestible.  Take for example the potato, in its raw state a potato is not digestible but when gelatinised the potato takes on characteristics we know and love.  Fluffy, white mash or baked potato centres and the soft insides of chips and roast.  Due to gelatinisation the densely packed starch in a potato is broken down and softened releasing vitamins and energy.  
Gelatinisation is a process occuring during the cooking of many traditional starchy foods and starch based desserts.  It is the way the starch becomes soft and edible.  Dishes such as porridge,pasta, rice pudding, sticky rice, and savoury rice all rely on gelatinisation.  
See how rice grains swell as they absorb hot water during cooking.

Try making the Chocolate sauce recipe on this website to understand the role of gelatinisation in saucemaking.  The method of making the sauce is called the 'blending' method because the cornflour used to thicken the sauce is firstly blended with cold milk before being added to the hot milk.

Gelatinisation is worth learning about if you are to understand cookery related to starchy ingredients.



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