Understanding rice gelatinisation to cook the perfect risotto (Introduction)

The process of cooking risotto is sometimes confusing and several procedures have been proposed to be the key to cook the real risotto, which requires ‘al dente’ rice grains and the adequate creamy texture.

If we want to prepare the ultimate risotto recipe we will have to answer some questions: what type of rice should we use?, should we stir while cooking the rice? , what is the cooking time?

Starch plays a vital role in achieving the desired consistency. Starch is a polysaccharide mainly composed by amylose, fundamentally linear, and amylopectin, highly branched. It presents a semi-crystalline structure. In general, amylopectin is the dominant crystalline component, while the amorphous regions mainly consist of amylose and non-crystalline amylopectin.The ratio of both varies depending on the rice genotype.

To use starch as a thickener in cooking we first need to disperse uniformly the starch. Then, while we hydrate the granules they will absorb water and swell.  Finally, we require to heat the mixture so that the starch undergoes gelatinisation.

Gelatinisation is the technical term that defines the irreversible transition from an ordered semi-crystallized state to a disordered and amorphous state due to the heating of hydrated starch particles in suspension. Initially, the swelling of the granules increases the viscosity of the fluid but if the expansion is large enough it can induce the disruption of the granules which leach amylose. That leads to the formation of a 3-dimensional network of amylose where the granule remnants are embedded. This structure contributes to the starch paste properties.  Regarding starch semi-crystalline nature, gelatinisation is defined by the glass transition of the amorphous component and the melting of the crystalline region. This process can be understood from a thermodynamics perspective as a phase transition of a two components system (starch+water), while swelling can be modelled using diffusion equations with moving boundaries.

 

How to cook the perfect risotto is an excellent example of how physics can be useful in understanding or improving cooking techniques, and of how cooking can make physics even more appetizing!

As part of an MSc project we will perform some experiments to measure amylose content and viscosity as well as do some modelling of gelatinisation to bring some light to this issue.

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