S12 Rice - Absorption and Diffusion

Topic

Cooking this rice involves three simultaneous, coupled physical processes: water absorption by each grain; starch gelatinization—driven by both temperature and the absorbed water; and the energy balance within the pot, which ultimately determines both the cooking temperature and the amount of water remaining available. If any one of these coupled processes accelerates, the other two also proceed more rapidly, as they all share the pot's temperature as a common variable.

At the start of cooking, water absorption by each grain follows Fick's Second Law; the effective diffusion coefficient of water within the grain rises sharply once the cooking temperature exceeds the gelatinization temperature range specific to that rice variety. Consequently, a grain submerged in cold water absorbs it very slowly, whereas a grain submerged in boiling water hydrates fully in a much shorter time. If the cooking temperature surpasses the gelatinization range sooner, the effective water diffusion coefficient within the grain increases earlier as well, leading to faster completion of the grain's hydration. The gelatinization of the grain's starch—the same structural transformation described in greater detail by another model in this set—is the key transformation in cooking this rice: the starch granules, initially crystalline, absorb water, swell, and eventually rupture, releasing starch components into the water and consequently forming a gel within the grain. This transformation is irreversible and is precisely what defines the rice as cooked; furthermore, it follows first-order kinetics—the same kinetics established by another model in this set—where the rate is highly sensitive to the cooking temperature due to a significant activation energy (a concept also established by that other model). Consequently, a higher cooking temperature causes the rate of starch gelatinization to increase sharply, driven by this significant activation energy.

Peleg’s model—also established elsewhere in this set—empirically describes water absorption by the grain without requiring assumptions about the grain's specific geometric shape: its first parameter controls the rate at which the grain absorbs water at the start of cooking, while its second parameter defines the maximum water absorption capacity reached by the grain by the end of the process. For long-grain rice, this maximum water absorption capacity corresponds, in culinary practice, to a water-to-rice cooking ratio of approximately two to one by volume. The fact that this second parameter of the Peleg model is lower means that the maximum water absorption capacity reached by the grain is consequently higher; as a result, the water-to-rice cooking ratio must also be increased to accommodate this maximum capacity.

Thus, this model—considering the water absorption and starch gelatinization aspects introduced earlier—also explains the traditional rice-cooking technique through an energy balance within the pot: cooking with the lid on reduces water evaporation (a factor accounted for by another model in this set); high heat at the start accelerates the process until the water boils; and lower heat subsequently balances evaporation to the minimum level needed to maintain boiling temperature without wasting available water. Once the water in the pot has been fully absorbed by the rice grains, the temperature inside the pot—no longer held at the boiling point by liquid water—begins to rise again; it is precisely this rise in temperature that signals the rice is ready.

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