Why Does a Sauce Curdle?


Anyone who insists that oil and water don't mix never made salad dressing. Classics like mayonnaise, hollandaise, cheese and some custard sauces rest on a carefully-achieved balance among theoretically incompatible ingredients. Maintaining the balance becomes particularly challenging when some ingredients contain protein. When they separate, the resulting blotchy mess is called curdling, or breaking. Several techniques prevent curdling or help you rescue a sauce that has broken.

A Delicate Balance

  • When food historians discuss modern molecular gastronomy, they may begin with the creation of mayonnaise. Scientifically, sauces with the potential to curdle are suspensions, mixtures that disrupt chains of existing molecules to form a new substance. In a sugar-and-water solution, sugar and water molecules bond, or dissolve, permanently. Bonding in a suspension is only temporary. Like oil-and-vinegar dressing, suspensions all separate with time. In hollandaise, for example, oil rises, leaving clots of lemon juice-poached egg protein and liquid below. Poor measurement or mixing techniques, haste and temperature changes all prompt rapid deterioration of delicate suspension sauces.

Juggling Eggs

  • Emulsifiers, or substances that help dissimilar ingredients combine, are the key to many suspension sauces. The protein in egg yolk frequently serves as the emulsifier in sauces. In the presence of warmth, or low heat, the long molecular chains that compose proteins uncoil and absorb both acids and fats. The effect of acid on protein molecules is similar to cooking. With too much acid or too much heat, however, proteins recombine in solid clusters. Eggs, for example, scramble. Measuring vinegar carefully and using a double boiler are good ways to control the interactions of food acids and heat on egg protein.

Keeping a Grip

  • Both time and energy help oil and other ingredients combine in a suspension sauce. Be prepared to add both oil and acid very slowly to eggs while whisking vigorously. If you doubt your stamina, rely on your food processor or blender for prolonged, thorough mixing. Transferring lemon juice or oil to small pitchers which are easy to pour from slowly can help you control the pace at which ingredients are added and combined.

Moving Right Along

  • Plan to use suspended sauces soon after they are made, and make only what your recipe requires. Their delicate balance can be disrupted by refrigeration or even a long wait while the rest of dinner cooks. Separation of ingredients is a greater danger than curdling in refrigeration, but a sauce left waiting on a counter can curdle as well.

Cheese, Please

  • Flour-based cheese sauce or the egg-and-cheese-enriched bechamel to top your moussaka present different curdling challenges. Cheese should be thoroughly melted to incorporate into the sauce, but this needs to be done at a low heat to prevent milk proteins from curdling. High heat can scramble added eggs. Extended cooking can also separate sauce ingredients, as flour molecules lose their ability to bond with fats. Proceed with the next step of your recipe as soon as sauce ingredients are thoroughly blended.

You to the Rescue

  • A curdled hollandaise that does not smell or taste of cooked egg can often be rescued by beating in another egg yolk and serving immediately. If it tastes like scrambled eggs, start again. Whisk a fresh egg yolk and a tablespoon of lemon juice or vinegar in a clean bowl, then strain and beat in your previously curdled mayonnaise. Beating in a few drops of boiling water the moment you notice the mayonnaise beginning to separate may be a quicker fix. Strain a curdled cheese sauce and beat it vigorously with a bit of cream. Promise yourself to devote a little more time, energy and precision to your next attempt.

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