Master Recipe: Lemon Curd

Master Lemon Curd Recipe:

Quantity: Makes 1 2/3 cups of lemon curd

Time: ~40 minutes + ~2+ hours of cooling

• 20 minutes to measure
• 20 minutes to make
• ~2+ hours to cool

Ingredients:

All ingredients are given in grams to ensure a consistent result. I've included volume measures for quantities that are difficult to weigh out on some scales or that come pre-measured for convenience.

• 147 g strained lemon juice (usually ~4 lemons is enough)
• 98 g butter (preferably a European-style butter with >82% butterfat)
• 50 g of whole egg (~1 Grade A Large egg)*
• 51 g of egg yolk (~3 Grade A Large egg yolks)*
• 130 g granulated sugar
• 1/2 tsp. diamond crystal kosher salt**
• 10 g lemon zest (~4 lemons)***

* You don't need to measure the weight of your egg/yolks as they are usually close enough. However, if you get a batch of eggs where the yolks look really small, you might want to measure the yolk weight just in case you need another yolk to get to 51 g. After using on order 100 eggs in my tests, I've found that a Grade A Large egg yolk can range from ~10-20 g, which is quite a bit of variation! 

**If you are using fine table salt, use 1/4 tsp. instead as the crystals are much smaller.

*** If you can get more than 10 g of lemon zest from your lemons, feel free to throw it in. It can only make it more lemony and delicious.

You can convert these all into volume measures using this chart, but the recipe will be much less consistent.

Directions:

1. Cut the butter into 1/2 inch cubes and put it into the refrigerator until needed.
2. Pinch the lemon zest and sugar between your fingers, and rub the lemon zest into the sugar and salt until it is well incorporated and the sugar looks sandy.
3. Whisk the sugar, salt, and zest into the eggs for 1-2 minutes until they are a lighter shade of yellow and smooth.

   

   The whisked sugar and eggs should be smooth and light yellow

   
   
4. Next whisk in the lemon juice until fully incorporated.
5. Pour the mixture into a thick bottomed saucepan and apply medium low heat. (Mine is usually on the 4-5 tick mark range on my 10-tick mark dial.)
6. Stir constantly with a whisk or rubber spatula, making sure to scrape the bottom, sides, and corners of the pan.
7. As the mixture begins to thicken, the bubbles and foam on the top of the surface will begin to dissipate. 
   
8. The mixture is done when you can 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. Temperature is not the best gauge, but you want the final temperature to be between 165˚ F and 180˚ F. At 165˚ F, the temperature is high enough to kill harmful bacteria. At temperatures above 180˚ F, the eggs overcook, giving the curd a grainy texture. If you find at the end that your curd is a bit thin, you likely need to cook it a bit longer in the future.
   
   
   
9. Strain the mixture quickly through large strainer to remove the zest. Do this step as quickly as possible to minimize the heat loss. Don't forget to scrape underneath the strainer when you are done or you will waste a lot of curd!
10. Immediately begin whisking in the butter. Add 1-2 cubes at a time, whisking continuously until they are fully incorporated before adding more butter. 
11. Once all the butter is added, pour the lemon curd into a container, cover the surface with plastic wrap to avoid forming a skin, cover, and refrigerate.
12. Refrigerate for a few hours before using.

Flavor Variations:

There are tons of flavor variations you can make with this recipe by substituting out the lemon zest and juice for those of other fruits. The easiest fruits to substitute in are tart, acidic fruits. This means that other citrus fruits would work well: limes, grapefruit, pomello, oranges, and more. There are two main things to keep in mind when substituting other citrus. The first is that if the fruit is sweeter (e.g. meyer lemons), you will want to reduce the sugar to avoid making the curd too sweet. The second is that there are several citrus fruits that have very bitter zest. Key limes and grapefruits both have particularly bitter zest. In these cases, it is best to omit the zest and only use the juice for flavor. Passion fruit can also make a delicious curd. For less acidic fruits, you may need to add some extra egg or gelatin to help stabilize the curd.

Lemon Curd Takeaways:

It has been a seven week long process (1, 2, 3, 4, 5, 6, 7) with over 24 batches of lemon curd! My freezer has more than 85 egg whites in it, and I've used so many lemons that at one point, our grocery shopper (thanks, COVID-19) texted back in horror "To be clear you want 8 pounds of lemons?!" to which I had to sheepishly reply, "Yes, that's correct."

Totally normal amounts of lemon!

Let's review my main takeaways:

Vanilla muddles the lemon flavor. There is no need to add vanilla extract to lemon curd. It is expensive and doesn't do this recipe any favors.

82% fat European butter adds extra richness and flavor. The butter is responsible for the rich, silky texture of the lemon curd. Butter does not thicken the lemon curd as much as the eggs. To get the ideal richness, you need the right thickness from the eggs combined with butter to keep the lemon curd from dissipating too quickly. Butter thus only increases the rich, silky mouthfeel up to the point where the lemon curd starts losing thickness. The better your butter tastes, the better your curd will taste. It is worth the the added expense here.

Zest adds lemon flavor without sourness. This is because the zest contains lemon oil. Be careful to not zest too deep as the white part of the peel (the pith) is extremely bitter. You can make the most of your zest by rubbing it into the sugar. The sugar is abrasive so cuts into the zest, releasing more of the lemon oil.

Eggs make the lemon curd a stable 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. 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 keep the suspended droplets from recombining in two ways. 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, preventing them from recombining. 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. 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.

Lemon juice provides flavor, but also helps denature the egg proteins. 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.

Sugar provides sweetness, but it also acts as a regulator in the coagulation process. The sugar molecules physically block the protein chains from the eggs from bonding to each other. Whisking the eggs with the sugar before adding the lemon juice keeps the eggs from coagulating prematurely. When heated, the sugar raises the coagulation temperature so that we can cook the mixture to high enough temperatures to kill bacteria. Too much sugar and the curd becomes a bit loose, too little sugar and the curd can coagulate prematurely.

Whisking the butter in slowly after the eggs, sugar, and juice have been heated is critical to forming a stable emulsion. By slowly whisking the fat into the mixture, we are keeping its droplet size in the emulsion small. Additionally, adding the butter slowly cools the curd, thickening it as more butter is added to keep the droplets separated.

Straining the zest out makes for a smoother lemon curd. While some recipes call to leave the lemon zest in for additional flavor, I prefer to strain it out to get a creamy, silky mouthfeel that isn't disrupted by gritty chunks of zest. This can also catch any over-cooked bits of egg. If you quickly strain the mixture before adding the butter, it is thinner and easier to strain, and you will still have enough heat to melt the butter. You can also strain it after adding the butter if you prefer.

The balance of whole eggs to egg yolks controls the richness and brightness of the curd. Egg yolks give the curd a thicker, richer texture, but they mute some of the lemon's tartness. Whole eggs give a very light texture with a very bright flavor. By finding the right balance between the two, you can get the best of both worlds.

Avoid using plain aluminum utensils and saucepans. The aluminum can react with the acidic lemon juice, causing a metallic flavor and a discolored curd. A stainless steel saucepan with an aluminum core is fine because the surface in contact with your curd is stainless steel.