Week 6: Lemon Curd

Last week, I found the ideal balance between richness and brightness by tuning the ratio of yolks to whole eggs. If you look at the recipes that I started with (below), you can see that as the percentage of eggs goes up, the percentage of butter goes down. This is likely because they both thicken the lemon curd. The eggs are thickening the lemon curd through coagulation, while the butter thickens the lemon curd as it cools and the butterfat solidifies. This week I want to play with the butter to egg ratio of our reference recipe (Split M).

Table 1: Several recipes for lemon curd in percent from [1], [2], [3], [4], [5], [6], [7].

We know that Recipes 7 and 4 had too much butter from our recipe tests in Week 4, so we can put the upper bound on butter at ~24%. The lower bound is ~7.8%, but that was a bit too low, so we can aim a bit higher. We want to keep the total amount of the recipe the same so if we add butter, we will reduce the egg. Doing so gives us:

Table 2: Recipe splits with different butter to egg ratios. Recipes also include 10 g of zest and 1/2 tsp salt.

While cooking the lemon curd variations, I noticed that Split Q had to go to higher temperatures to coagulate while Splits R and S coagulated at lower temperatures than the reference. As we change the butter to egg ratio, we are also changing the sugar to egg ratio. Splits R and S have a lower sugar to egg ratio, so there is less sugar to interfere with coagulation, leading to the lower coagulation temperature. Similarly, the higher sugar to egg ratio in Split Q made it harder for the eggs to coagulate, necessitating a higher coagulation temperature. This is a nice confirmation of how the sugar works as a regulator in a lemon curd recipe.

Split M was thicker than Split Q, which means that butter does not thicken the lemon curd as strongly as the coagulation of the eggs. Following a similar trend, Splits R and S were successively thicker due to their increasing egg content.

From the top left corner going clockwise: Split M, Split Q, Split S, Split R.

Split Q had a silkier texture than Split M, but it dissipated quickly in the mouth, which made it feel less rich. Split R also dissipated quickly but less so than Split Q. Split S was very thick, but did not have any richness. It reminded me of lemon meringue pie. From these tests, it is clear that butter is responsible for the rich, silky texture of the lemon curd. However, you need the right thickness 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. Overall our reference (Split M) had the best texture: it was thick with a rich mouthfeel that lasted. I will keep Split M as my reference next week when I play with the sugar content of the lemon curd.