Proteins change the behavior of starch in water.
WHY DO MASHED POTATOES MADE WITH MILK stick less than ones made with water? By showing that proteins modify the thickening and gelatinization of starch, Jacques Lefebvre and Jean-Louis Doublier at the Institut National de la Recherche Agronomique station in Nantes have at last provided an explanation of this venerable piece of culinary knowledge and useful indications for the use of flour in sauces.
Flour and potatoes have in common the fact that they contain a great deal of starch, in the form of granules that contain two sorts of molecules: amylose, which is composed of a linear chain of hundreds of glucose groups, and amylopectin, a polymer that is similar to amylose but has instead a ramified, or branched, structure. In each granule of starch these two types of molecule exhibit a crystalline form.
Whereas the starch granules in flour are exposed to the surrounding atmosphere by the milling of grains of wheat, the starch in potatoes inhabits a watery environment enclosed by their cell membranes. Starch does not dissolve, for amylopectin is highly insoluble in water, and amylose is soluble only in fairly hot water, at temperatures higher than 55°C (131°F).
When one cooks potatoes by putting them in a hot fluid (air, water, or oil) the heat is drawn to the center by conduction, triggering the same sort of expansion as the one that occurs when, in preparing an espagnole, a white sauce, or a béchamel, one pours a boiling liquid (water, milk, or broth) into flour: The water molecules dissolve the amylose molecules and alter the structure of the starch granules, disorganizing the amylopectin crystals and causing the granules to swell. During these transformations the starch granules soften, and dissolving the bulky amylose molecules makes the solution flow less easily. Thus a sauce thickens when flour is added to it, both because the swollen granules get in the way of one another and because the solution in which these granules are dispersed becomes viscous.
If cooks do not always understand the molecular details of the thickening of sauces, they nonetheless know that sauces must be kept hot. For example, white sauces form a gelatinous mass when they cool because a gel forms when the neighboring amylose molecules in the water solution combine with one another, forming a network that traps the water, swollen starch granules, and the various dissolved compounds.
Doublier and Lefebvre showed that the phenomena of swelling up and gelatinization are modified when proteins such as casein are present outside the swollen granules. Caseins, found in milk, aggregate into structures called micelles, which are dispersed in the cellular water and coat droplets of fatty matter. As emulsifying agents they are widely used in the food processing industry to make ice creams, dairy products, custards, and so on.
These proteins reduce the quantities of amylose that leak out of the starch granules and also limit the extent to which they swell up. Caseins subsequently bring about a separation in the water phase: Protein-enriched water droplets separate from the rest of the sauce, which is then enriched by amylose in a continuous phase. This increase in amylose concentration favors its gelatinization.
The Proteins of Mashed Potatoes
How do these mechanisms operate in the making of mashed potatoes? When one cooks potatoes the starch granules are not fully expanded because there is not enough intracellular water to be absorbed by the granules. Mashing the potatoes with a bit of reserved cooking water hastens the incorporation of amylose into solution and the swelling up of the starch granules, with the result that the whole thing forms a sticky mass. On the contrary, mashing them with milk, which contains caseins, limits the swelling of the starch and therefore yields a smoother, more pleasing consistency. This phenomenon also must be taken into account when one thickens sauces with flour, for the gelatin of the sauce stock has the same effect on the flour as milk proteins.
In studying variations in viscosity as a function of the rate of flow, the Nantes researchers provided another hint that cooks ought to find useful. Although perfectly expanded starch granules are thixotropic, which is to say that they are deformed when subjected to flow and therefore form a less viscous solution in the mouth than at rest in the saucepan, sauces thickened with flour, in which proteins limit the degree of expansion, preserve an almost constant level of viscosity: The few amylose molecules that leak out of the starch granules into solution are aligned with the direction of flow, while the unswollen granules are deformed to a correspondingly minor degree.