E23 Absorption - Rice and Pasta

Topic

Rice, pasta, or legumes absorb water during cooking through a combination of internal diffusion, starch gelatinization, and the relaxation of their polymer structure; this process is analogous to the molecular diffusion described by Fick's Second Law in another model within this set, except that here, the effective diffusion coefficient of water within the grain is not constant—it varies with both the concentration of already absorbed water and the cooking temperature. A higher effective diffusion coefficient results in faster water penetration into the grain's interior.

The Peleg model—a widely used empirical, two-parameter model in food science—describes water absorption by the grain without requiring assumptions about the grain's specific geometric shape. The first parameter governs the initial rate of water absorption at the start of cooking; a low value indicates rapid initial hydration. The second parameter defines the grain's maximum water absorption capacity by the end of the cooking process. Consequently, a lower first parameter results in faster initial hydration, whereas a lower second parameter corresponds to a higher maximum water absorption capacity. The gelatinization of the starch within the grain—occurring within a temperature range specific to the grain variety—transforms the starch's crystalline structure into an amorphous one; this transformation drastically increases the effective water diffusion coefficient, allowing water to rapidly penetrate the grain's interior. Ultimately, this structural phase change in the starch is the central physicochemical phenomenon in the cooking of the rice. As the starch gelatinization progresses further toward this amorphous structure, the effective water diffusion coefficient rises, causing water to enter the grain's interior more quickly.

Higher cooking temperatures significantly accelerate water absorption by the grain, in accordance with the Arrhenius relationship applied to the effective diffusion coefficient; for instance, a grain cooked at a full boil hydrates completely in far less time than it would at a more moderate cooking temperature. The Avrami model captures a characteristic sigmoid curve for this hydration process: hydration proceeds slowly at the start of cooking—while starch gelatinization is just beginning to nucleate within the grain—accelerates during the intermediate cooking phase, and slows down again toward the end as the grain approaches its maximum water absorption capacity. Given the grain's starch gelatinization and the sigmoid hydration curve involved, cooking it *al dente* entails deliberately halting hydration before maximum water absorption is reached; this leaves a central core that is less gelatinized than the rest of the grain—which, ultimately, ends up absorbing several times its own weight in water. Stopping the cooking process earlier—further away from the point of maximum water absorption—results in a central core that is less gelatinized and firmer to the bite.

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