What is Gelatinisation?
Gelatinisation is best known as thickening for sauces, soups and custards. In sweet and savoury dishes 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.
Starch is the ingredient 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, as seen in these photos.
Hot water has more energy and at around 60⁰C 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 96⁰C 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. A pouring sauce needs less starch than a coating sauce. Gelatinisation functions as the sauce temperature increases to thicken a liquid, and also when temperature subsequently decreases after cooking and on storage. 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 38⁰C.
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, called sauce 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 and allows some fluids to escape. This is known as syneresis.
Cooked potato is an example of gelatinised starch. 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 used in many of the traditional starches and starch-based desserts. It is the way the starch becomes soft and edible. Dishes such as porridge, rice pudding, sticky rice, and savoury rice, pasta cooking all rely on gelatinisation.
Starchy rice grains swell as they absorb water during cooking as do pasta shapes. Noticing their increased size is one way of telling if the starch is cooked enough.
Gelatinisation is helpful in making starchy ingredients digestible and in enabling liquids to be thickened to make delicious sauces.