E62 Liquid <> Steam - Boiling and Steaming

Topic

Boiling and steaming are the two most common water-based cooking methods, and both transfer heat to the food in different ways: boiling transfers heat to the submerged food via convection and direct contact with the boiling water, whereas steaming utilizes the latent heat of condensation from the steam. Cooking food by steaming rather than boiling means that heat reaches it through a different transfer mechanism.

The Rohsenow correlation describes the heat flux during nucleate boiling: the heat flux increases sharply—far more than proportionally—with the local superheating of the water above its boiling point, such that a small additional rise in the temperature of the pot's bottom produces an even greater increase in heat flux. This pronounced dependency explains the intense bubbling observed when the burner power beneath the pot is increased. Greater local superheating at the bottom of the pot results, according to the Rohsenow correlation, in a significantly higher heat flux during nucleate boiling.

However, the critical heat flux marks the upper limit of nucleate boiling: if the heat flux exceeds this critical value, a continuous vapor film forms around the surface instead of the individual bubbles characteristic of nucleate boiling—a phenomenon known as the Leidenfrost effect. This film thermally insulates the surface, thereby drastically reducing heat transfer to the water. In the kitchen, however, this same Leidenfrost effect rarely occurs on the bottom of a pot, because the critical heat flux required is too high to be reached with an ordinary household burner.

During steaming, the steam at its boiling point condenses on the food—which is cooler than the steam itself—and transfers the same latent heat mentioned earlier for every unit of steam mass that condenses. The condensation coefficient of the steam on the food's surface is very high, making steaming more energy-efficient than boiling for the same temperature difference between the cooking medium and the food. Because this condensation coefficient is high, the latent heat is transferred more rapidly into the interior of the food.

This explains why a pressure cooker—utilizing the heat flux dynamics of nucleate boiling and steam condensation described earlier—raises the pressure on the water above atmospheric levels. Consequently, the water's boiling point rises far above its value at normal atmospheric pressure, in accordance with the Clausius-Clapeyron equation (established in another part of this model). The kinetics of the cooking reactions—such as tissue softening and protein denaturation—depend exponentially on the cooking temperature; thus, these reactions proceed much faster inside the pressure cooker than at the water's normal boiling point. Further increasing the pressure within the cooker raises the boiling point even higher, causing the cooking reactions to proceed even more rapidly.

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