Analysis of a classic dish shows how to avoid the greasiness of deep frying.
SOUFFLÉED POTATOES LOOK LIKE SMALL, crispy golden balloons. They are said to have been discovered on August 25, 1837, during the dedication of the railroad line linking Paris and Saint Germain-en-Laye. The menu for the official luncheon was to include fried slices of potato, but when the train had trouble climbing the last hill the chef was forced to interrupt the frying; once the guests were finally seated, he immersed the slices once again in very hot oil in order to make them crispy. They puffed up.
Since then cooks have differed over the proper way to make this difficult masterpiece of classic French cuisine. Physicochemical analysis has recently illuminated the mechanisms that cause the potatoes to puff up and revealed how to limit the absorption of oil by the potatoes during frying.
Cookbooks do not say why the recipes they give for souffléed potatoes should work. It has long been claimed that this dish and the ideal thickness of the sliced potatoes were studied by the French chemist Michel-Eugène Chevreul (1786–1889), a pioneer in the chemistry of fats. The story is plausible, given the importance of heated fat in this dish, but I have found no trace of any such investigation in the works of Chevreul. Four years after Chevreul’s death, however, chef Auguste Colombié noted in his Éléments culinaires à l’usage des demoiselles (1893), “Thanks to the good offices of M. Decaux, the gracious and learned laboratory assistant of the late Chevreul, who kindly furnished me with the necessary thermometers, I was able to make three scientific experiments on the puffing up of potatoes, Wednesday 14 April 1884, at the warehouse showroom of the Compagnie Parisienne du Gaz.” There follow several pages in which Colombié presents the results of his experiments, with no reference to Chevreul. It therefore seems probable that historians of cookery have identified Colombié with Decaux and Decaux in turn with Chevreul.
The Technology of Soufflage
How should souffléed potatoes be prepared? Most traditional recipes recommend cutting the potatoes lengthwise into slices between 3 and 6 millimeters (1/8 and 1/4 inches) thick. The slices are washed, dried, and then cooked in oil that has been heated to a temperature of 80°C (176°F). Once the slices have risen to the surface, after six or seven minutes, they are removed from the oil and allowed to cool before being put back and cooked a second time, only now at a higher temperature. The authors of these recipes attribute success to the thickness of the slices, the length of time between the two immersions, or the temperature of the oil in each case.
Which is the relevant parameter? Why do the potatoes puff up? How can this puffing up be optimized? In testing the classic recipes one needs to keep two things in mind: that potato cells contain granules of starch, which swell when the cellular water is heated, forming a purée, and that because a potato is a thermally isolating material, its center is slow to cook. If the oil in the first round of frying is too hot, an excessively thick and rigid crust forms before the center is cooked, and the potato will not puff up.
Water Vapor Repels Oil
Next, if we weigh the fried slices, we find that the oil does not replace the water eliminated by heating, as was long assumed. Given a surface of 100 square centimeters (or roughly 15 square inches), about 80 cubic centimeters (almost 5 cubic inches) of steam manages to escape per second. In other words, the pressure of the steam keeps the oil from seeping in. Besides, if the slices quickly rise to the surface, this is because the water has been replaced by steam and not by oil (a potato is composed of 78% water and 17% starch, which is denser than both water and oil).
The behavior of steam bubbles provides the key to the phenomenon of soufflage. In order for the slices to puff up, steam must suddenly be generated, deforming the crust, whose dried-out cells create a steam-resistant compartment within each slice. When vaporization is slow, small trains of bubbles trickle out through openings in the crust, and the pressure of the steam is insufficient to cause the slices to expand, hence the need for hotter oil during the second round of frying.
Puffing up also requires that the compartments formed during the first round of frying be impermeable. The centers of the slices continue to cook during the interval between the first and second rounds, and water is redistributed through the dried-out areas. As the temperature falls the crust probably becomes detached from the center as well. The second round of frying then causes the residual water in each slice to evaporate, triggering expansion because the steam has a hard time escaping through openings in the compartment walls.
This explains why the thickness must be carefully controlled: If the slices are too thin, one does not obtain a crust with an intermediate layer of puréed starch granules, and the quantity of steam generated therefore is insufficient; if the slices are too thick, more time is needed for the center to cook and an overly thick crust forms on the outside, hindering the expansion. It also becomes clear why the greatest care must be taken in handling the potato slices. For if the thin crust is pierced, large vapor bubbles are suddenly able to escape through the openings, and the pressure is no longer sufficient to cause the slices to puff up.
Finally, how can the amount of oil absorbed by the puffed potatoes be minimized? Sam Saguy at the University of Jerusalem has shown that the oil is present mainly on the surface of the sliced potato, in quantities that increase with the rugosity of the surface and repeated use of the same oil: The more uneven the surface, the more oil that adheres to it (because of an increase in tensioactive molecules that results from repeated heating, hence the foam produced by old oil). It is a good idea, then, to fry potatoes in clean oil, to use as sharp a knife as possible, and to wipe off any oil coating the surface of the cooked potato slices so that when the water inside cools and condenses it is not absorbed.