Week 1: Lemon Curd

Lemon curd is rich and lemony. It can be used as a dessert topping or a filling on any number of desserts. The  perfect lemon curd recipe is really a personal preference, and many different combinations of the ingredients can work. My ideal lemon curd has a rich texture without being heavy and a strong, tart lemon flavor. Lemon curd typically consists of lemon juice, eggs and/or egg yolks, sugar, butter, and sometimes lemon zest for some extra lemon flavor. While lemon curd has very few ingredients, it is positively packed with science! Over the next few weeks, we will start deconstructing lemon curd further and try to converge on a recipe that checks all the boxes, but first we need a bit more background about the basic science behind lemon curd.

Lemon curd is an emulsion

An emulsion is a suspension of the droplets of one fluid in another fluid, where the two fluids cannot be dissolved in each other. The classic example of an emulsion is a salad dressing made of vinegar and oil. In this case, the water from the vinegar and the oil are the two fluids. When you shake the dressing up, you are breaking the oil into small droplets that are suspended in the water. If you let the salad dressing sit for some time after shaking, the oil droplets coalesce into larger and larger droplets until the oil and water in the dressing are fully separated. In the case of lemon curd, we are trying to mix fat from the butter into the water from the lemon juice.

The eggs in lemon curd stabilize the emulsion

If we want to prevent our emulsion from separating, we need to keep the small, suspended droplets from combining. One way to do this is by adding an emulsifier. Emulsifiers effectively create a barrier between droplets to keep them from recombining. Many molecules are either hydrophilic or hydrophobic. Hydrophilic molecules love water, while hydrophobic molecules repel water. Emulsifiers are amphiphilic, which means that they are a mix of both! They have a part that loves water, and a part that repels it. When you mix an emulsifier into an emulsion, the hydrophilic part moves toward the water, while the hydrophobic part moves away from it. Because of this, the molecules of the emulsifier arrange themselves along the surface of the droplets, creating a barrier between one fluid and the other. This barrier prevents the droplets from coalescing. Another way to stabilize an emulsion is by making it more difficult for droplets to collide through making it harder to move through the medium, which is typically achieved by thickening the liquid that the droplets are suspended in.

Lemon curd uses both of these techniques. The egg yolks contain an emulsifier called lecithin that acts as a barrier between the fat droplets and the water in the lemon curd mixture. The lemon curd is also thickened by heating the eggs until they coagulate, forming a complex network of intertwined proteins that keep the fat droplets from moving.

Acidity and sugar help things along

Lemon curd is already complicated enough, but we have yet to consider the acid in the lemon juice or the sugar. To better understand their roles, we need to understand more about how eggs coagulate. The proteins in eggs are made up of long chains of amino acids that are arranged into very specific shapes. When heat is applied, some of the structural bonds in the proteins break, loosening the structure and allowing these long chains to unfold in a process called denaturation. Once the proteins are denatured, individual molecules on these long chains bond to the molecules on other chains, forming a complex network of bonds that solidify the egg.

The lemon juice in lemon curd is highly acidic, which means that it has many positive hydrogen ions. These hydrogen ions bond with the amino acids in the egg proteins, which neutralizes negatively charged chains and adds positive charge to neutral chains. The positively charged chains then repel, pushing the proteins apart and denaturing the proteins just as with heat. The lemon juice thus aids in the coagulation process, further assisting in creating a stable emulsion.

The sugar in the lemon curd not only adds to the flavor, but it also regulates the coagulation process. When lemon juice is added to the eggs in lemon curd, it can cause premature coagulation. By adding sugar, we add more molecules to the mix, which physically block the protein chains from bonding to each other. We essentially make it so crowded that the denatured protein chains cannot find each other and bond as quickly. This is why many lemon curd recipes call for whisking the sugar into the eggs before adding the lemon juice. Sugar continues to play the role of the regulator as we heat the lemon curd. Without sugar, egg whites begin to set at 140˚ F and are fully set by 143.5˚ F, while yolks begin to set at 147˚ F and are fully set between 167-176˚ F. If we overcook the eggs in a lemon curd, we get eggy flavors and a curdled, gritty texture. However, the sugar continues to separate the protein chains as we heat the lemon curd. This raises the coagulation temperature of the mixture, which slows the coagulation process while ensuring that we can cook the curd to safer temperatures.

Technique is critical

The ingredients in lemon curd all have critical roles to play, but technique is equally important. As mentioned above, many recipes call for whisking the sugar into the eggs before adding the lemon juice to protect the curd from coagulating too soon. Some recipes have you mix everything together at once. I tried this once, but it resulted in an instant scramble, so this step really is important. Another critical step is heating the lemon curd enough to make sure it is thick enough but not overcooked.

Most lemon curd recipes also direct you to whisk the butter in slowly after the eggs, sugar, and juice have been heated. This serves two roles. The first is to slowly add the fat into the mixture to keep its droplet size in the emulsion small. The second is to slowly cool the curd, thickening it as more butter is added to keep the droplets separated. As the lemon curd cools further in the fridge, it will continue to thicken as the droplets of butterfat solidify. The butter thus further contributes to the thickness of a lemon curd.

With this framework in place, we can begin playing with lemon curd recipes next time!