Is it true that when they float to the surface of the cooking water they are done?
ÉCHAUDÉS ARE VERY OLD PREPARATIONS : As early as 1651 Nicolas de Bonnefons mentions small pieces of dough that have been “scalded” in boiling water. There are many recipes, but from the oldest échaudés to potato gnocchi and gnocchi à la parisienne the principle is the same: One begins with a dough composed of starch, egg, and water. In the case of potato gnocchi, extra starch is contributed by the granules found in the potatoes (which are cooked, peeled, and mashed). Other recipes include parmesan cheese and milk. The dough is kneaded with a spatula, then placed on a floured baking sheet and rolled into cylinders, which are cut up into small sections and pressed against a fork to give them their classic shape. Next comes the cooking. The échaudés or gnocchi are put into a saucepan containing salted water or broth, and when they rise to the surface they are said to be cooked. Is this maxim trustworthy? If so, why? The question is not trivial, for many preparations call for the same type of ingredients and the same type of cooking: Alsatian spätzle, various central European dumplings, and so on.
If we put some échaudés or gnocchi in boiling water we discover that they fall to the bottom of the saucepan. Then they gradually swell and become lighter. At first they follow the convection currents in the saucepan, continually coming back to the bottom. But after a while—thirty seconds or so—they begin to float. It is therefore accurate to say that échaudés and gnocchi rise to the surface.
Are they actually cooked at this point, as the old saying has it? If you taste them, you will find that they are edible. But are they really done?
Denser, on Average
Let us analyze the problem for classic gnocchi, which are made of potato, flour, and egg. This dough preserves its form in the course of cooking because the egg coagulates on contact with the boiling water, the temperature being higher than 68°C (154°F). The potato has been cooked beforehand, with the result that the starch granules that fill its cells have swelled up. As for the flour, it is composed of starch granules and gluten; the latter is made of proteins, which with the kneading of the dough form a network that embeds the starch granules in the cellular structure of the potato. The starch contained in flour, like that of the potato, is insoluble when cold, but in hot water the suddenly porous starch granules absorb the water molecules.
This mechanism explains the swelling of gnocchi during cooking. And because the density of the starch is greater than that of water (when flour is mixed with water it falls to the bottom of the container), the total density of the gnocchi diminishes, approaching—without ever quite reaching—that of the water. Why, then, do the gnocchi wind up floating on the surface? It must be that some substance whose density is lower than that of water, either air or steam, is incorporated with it in the form of bubbles. What sort of experiment would determine which one it is?
If we cook some gnocchi in water from which the dissolved air has been expelled by heating the water for a long time (the bubbles that form at the bottom of the pan in the initial stage of heating and then rise to the surface of the water are air bubbles, not steam bubbles), we find that the gnocchi nonetheless rise. So it is not air that makes them do this.
The only other possibility is that water vapor forms bubbles that cling to the gnocchi and cause it to rise. Remove one of the gnocchi that have come to the surface of the degassed water, gently roll it on a cutting board in order to puncture any steam bubbles, and then put it back into the water, and it sinks again. Once its surface crevices have again been filled with microscopic steam bubbles, it will rise to the top once more.
If you want to see this with your own eyes, cook some cauliflower florets. Their surface is so irregular that it efficiently traps the steam; a glistening gaseous layer can be seen to cover the florets just when they begin to float. Pat the surface and you will release the steam, causing the florets once again to descend to the bottom.
A Qualified Maxim
Our investigation of échaudés and gnocchi is not yet finished, for we still do not know whether the fact that they float means that they are cooked. When is an échaudé cooked? When the egg has coagulated? When the starch has stiffened? To answer this question, let’s form échaudés of various sizes—half a centimeter and 10 centimeters in diameter, for example. Let’s then cook them together and measure the internal temperature when they rise to the surface of the water. We find that the temperature of the small échaudé is much higher than that of the large échaudé. This proves that buoyancy is not a reliable sign that échaudés are cooked or, at least, if one judges according to this criterion, that échaudés of different sizes will be cooked to different degrees.
Moreover, the temperature of the largest échaudé sometimes turns out to be lower than that at which starch stiffens and egg coagulates. In other words, the simple fact of buoyancy is insufficient. If one aspires to a standard of precision worthy of the great chefs, it is necessary to prolong cooking beyond the point at which they come back up or to draw up a table that gives cooking temperatures as a function of an échaudé’s size.