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Lemon curd is a creamy mixture made from juice (usually lemon, lime or orange), sugar, butter and egg yolks. The ingredients are cooked together until the mixture becomes quite thick. When cool, the lemon (or lime or orange) curd becomes thick enough to spread and is used as a topping for breads and other baked goods. It can also be used as a filling for pre-baked pie or tart crusts. After making, thin any recipe with 1/4 to 1/2 cup simple syrup to the desired consistency to make a tangy dessert sauce.
THE SCIENCE OF LEMON CURD
Lemon juice is a multifunctional ingredient in lemon curd recipes. It flavors, denature proteins, and act as an acidulant. As a source of flavor, lemon juice provides most of the tartness and characteristic lemon flavor. In addition, the citric acid in the lemon juice denatures proteins, providing the typical texture and mouth feel characteristics of the lemon curd (Schmidt, 1983).
By denaturing the egg's protein, the emulsion stability of the lemon curd is increased. As an acidulant, citric acid lowers the pH of the lemon curd. By lowering the pH, it prevents the growth of many microorganisms such as Salmonella species usually found in eggs, a major ingredient in the lemon curd. Finally, citric acid also serves as a sequestering agent, sequestering metal ions present in the product that could potentially accelerate oxidation or browning.
SARAH SAYS: An aluminum pan should not be used to prepare lemon curd (or any acidic curd) because it will react with the egg yolks, turning them chartreuse.
Sugar raises the coagulation point of the egg yolk. It also protects it from premature coagulation during the addition of the lemon juice. If the juice were added directly to the unprotected yolk, the yolk would partially coagulate and, when strained, a large percentage of it would be left behind in the strainer. Be sure to mix the sugar well with the egg yolks before adding the juice.