How to choose wines and cheeses so that the fondue never flops.
DOES THE TRUE CHEESE FONDUE come from Savoy in France, or the Valais in Switzerland, or the canton of Fribourg? How many types of cheese should be used? One? Two? Four? Connoisseurs passionately disagree. Wars have been started for less. Physical chemistry may not permanently settle such disputes, but it should at least enable lovers of the dish to reach agreement over why, despite its simplicity, the fondue sometimes flops. Athony Blake, director of food sciences and technologies for the Firmenich Group in Geneva, has discovered a surefire way to prevent it from turning into a solid mass lying at the bottom of the pot beneath a greasy liquid.
A fondue is no more than cheese heated with wine. The combination of water (from the wine) and water-insoluble fat (from the cheese) means that the successful fondue is necessarily an emulsion, a dispersion of microscopic droplets of fat in water solution. The fondue therefore is a cousin to béarnaise and hollandaise sauces, which are also obtained by the fusion and dispersion of a fatty substance (in this case butter) in an aqueous phase or zone (from vinegar and egg yolks).
In a béarnaise sauce, the fat droplets are coated by tensioactive molecules found in the egg yolk, in such a way that the water-soluble (hydrophilic) part of these molecules is exposed to the water and the water-insoluble (hydrophobic) part to the fat. The surface-active molecules that cover the fatty droplets in a fondue are known as casein proteins, which are already present in the milk, itself an emulsion, and which combine to form aggregates called micelles. These aggregates are made up of several types of casein, bound together by calcium (especially phosphate) salts. One of the caseins, the kappa-casein, typically lies outside the micelles and ensures their mutual repulsion (because of the negative electrical charge they bear). This repulsion is important for the stability of the milk, for it prevents the coalescence of the fatty droplets covered by the micelles.
In cheesemaking, the rennet that is added to the milk contains an enzyme that detaches a part of the kappa-casein, triggering the aggregation of micelles into a gel in which the fatty matter is trapped. Cheese therefore seems an unlikely candidate for reviving an emulsion in the fondue, having been formed from a milky emulsion that has deliberately been ruined. It nonetheless lends itself to this purpose because it has been aged and mixed with wine.
Aging and Viscosity
Connoisseurs of fondue know that the success of the dish has to do particularly with proper cheese selection. Questions of flavor come into play as well, but well-ripened cheeses are best suited to the preparation of fondues because, in the course of aging, enzymes called peptidases have broken up the casein and the other proteins into small fragments that are more readily dispersed in the water solution. These casein fragments then emulsify the fatty droplets and increase the viscosity of the aqueous phase (which is why a Camembert fondue, for example, will always turn out well).
This increase in viscosity is analogous to the heretical practice of thickening a fondue by adding flour or any other ingredient containing starch, such as potatoes. Swelling up in the warm aqueous solution, the starch granules increase its viscosity and limit the motion of the fatty droplets, which thus are kept separate from one another. In this way the emulsion—which is to say, the fondue—is stabilized.
To Doctor or Not to Doctor
Connoisseurs challenge this practice on the ground that it changes the taste of the dish, insisting instead on the skillful combination of cheeses and wines. They select very dry wines—indeed, wines that are excessively acidic and, if possible, very fruity. Why are these properties useful? Athony Blake has shown that such wines have high concentrations of tartaric, malic, and citric acids. Malate, tartrate, and especially citrate ions are very good at chelating (or sequestering) calcium ions. The acidic and fruity wines experts prefer help separate the casein micelles and release their constituent proteins, which stabilize the emulsion by coating the fatty droplets.
Chemists have devised ways to tweak the classic recipe for fondue, for example by adding bicarbonate of soda, which neutralizes the acids and encourages the formation of calcium-chelating ions. Another option, if one suspects that the wine contains too little tartaric, malic, or citric acid, is to add some; the best choice is citric acid, in its salt form, in a proportion of 1–2%. Do this and you can be sure your fondue will be a success.