Week 2: Lemon Curd

Last time, we looked at the science behind lemon curd, and this week we'll focus on getting the right technique before diving into more rigorous tests.

Vanilla or no vanilla?

In looking over numerous recipes for lemon curd, I found that some called for vanilla extract while others did not. The inclusion of vanilla extract seemed odd to me given that lemon is such a strong flavor. Given my suspicions and the fact that vanilla extract is expensive, I decided to test this out before jumping into recipe testing. Vanilla extract has a high alcohol content, which means that it cooks down and evaporates when heated. This is why most stovetop recipes call for adding it last. Since the vanilla extract is added at the end, I made a single batch of a lemon curd recipe that called for vanilla, split it in two, and only added vanilla to half. Upon tasting, my suspicions were confirmed. In the batch with vanilla, the vanilla only served to muddle the clarity lemon flavor, so I will not include vanilla extract in any of my test recipes.

To strain or not to strain?

Additionally, the recipe I used called for leaving the lemon zest in the mixture after cooking it to enhance the lemon flavor versus straining it out. While straining your lemon curd isn't critical, it can catch any over-cooked bits of egg, but if you don’t overcook the curd, you don’t necessarily need this step. However, I found that the lemon zest really distracted from the creamy, silky mouthfeel that you should get with lemon curd, so I will be straining all my lemon curd recipes.

Most recipes that call for straining say to do it at the end once you have added all the butter because straining the mixture straight off the stove can lose some of the heat used to melt the butter. It seemed like it would be easier to strain the lemon curd mixture before thickening it with butter, and I figured that if you strain it very quickly, you can't lose that much heat, so I tried straining the mixture right off the stove, and it was fine. I thus use this method, but if you would prefer to strain at the end, that is fine too. Just don't forget to scrape underneath the strainer when you are done or you will waste a lot of curd!

Making the most of your zest

Lemon zest adds excellent lemon flavor because the outside of the lemon peel contains lemon oil. In a lemon curd, it is essential to getting more lemon flavor without getting more tartness. When zesting, we only want the yellow part of the peel--if you hit white, you've gone too deep and entered the pith, which has an extremely bitter flavor. Zesting is arduous, so we really want to make the most of our lemon zest. One way recipes say to do this is to rub your zest into the sugar. The sugar cuts into the zest, releasing more lemon oil and boosting the flavor. Since we're straining the lemon zest out at the end, this seemed like a good technique to test to see if we could make the most of our lemon zest. I made two batches of the same recipe: one where I just added the zest to the lemon curd and one where I rubbed the zest into the sugar. The difference was noticeable, so I will rub the zest into the sugar in all my tests.

Generalized Technique

With these tests out of the way, I can now outline the generalized technique that I will use to make all my test lemon curds. Eggs and lemons are all different sizes, so to ensure that I am making consistent batches that I can compare variations between, I weigh all my ingredients. The butter should be straight from the fridge because we want it to cool and thicken the curd. I cut my butter into small 1/2" cubes so I can add it in incrementally at the end.

First, I rub the lemon zest into the sugar. Next I add the sugar into the eggs and whisk 1-2 minutes until the eggs are light yellow to make sure that the sugar can play its role of regulator. Then I add the lemon juice, whisk together until incorporated, and turn on the heat to medium low (for me, this is 3 on my 10-tick mark stovetop).

With lemon curd it is always better to go low and slow than to end up with scrambled eggs. Some recipes call for a double boiler to cook the lemon curd, but I use a thick-bottomed pot that heats evenly, and this does just fine without all the hassle of a double boiler. I heat the lemon juice, sugar, and egg mixture stirring continuously with a rubber spatula and making sure to scrape down the corners of the pan often. Knowing when the lemon curd is ready to come off the heat is tricky, and I found that the temperature wasn't always a good measure. I make sure that I am between two temperature goal posts: I want to be above 165˚ F to make sure it is fully cooked and below 180˚ F to ensure that it doesn't overcook and get grainy. Cook time can also vary depending on what your pan and stovetop are like, so it is not a great measure either. The best way to tell when it is done is to coat the back of a spoon with the mixture and run your finger across the surface, creating a line. If the mixture holds the line without running back in, then it is done. This took me some iteration to get right, so don't be discouraged! I kept not taking mine far enough because I was scared of getting scrambled eggs. If your lemon curd is too thin, you probably need to cook it a bit more in the future.

Once the mixture is done, I strain it quickly through a large strainer to remove the zest and begin whisking in the butter. I add 1-2 cubes at a time and wait until they are fully melted before adding more butter. This takes a while, but it is critical to add it slowly to avoid breaking the emulsion. Once all the butter is added, I pour the lemon curd into a container, cover the surface with plastic wrap to avoid forming a skin, cover, and put it in the fridge. It usually takes a few hours to cool in the fridge before it is ready to eat. I can't say how long it can last in the fridge because mine has never lasted more than a few days!

Next week, we will begin testing out recipes!