Week 3: Lemon Curd

Last week we settled on our baseline technique for making lemon curd. This week, let's look at the ingredients. Table 1 shows the quantities of several recipes in percent of the total recipe weight. I have removed any vanilla, salt, and lemon zest as they primarily contribute to flavor but don't have important interactions with the primary ingredients. When I make recipes, I will include zest and salt, but they don't help us understand the differences between these recipes, so I will leave them out of the ingredient tables.

Table 1: Several recipes for lemon curd in percent from [1], [2], [3], [4], [5], [6], [7]. Note that these are rounded to the nearest tenth of a percent, so the total values may not add up to a perfect 100% due to rounding error.

As we can see there is a huge range of variability in ingredients! This really shows how the perfect lemon curd will be a matter of personal taste. Right off the bat we can see two general classes. The first (Recipes 1-3) has lower sugar and more lemon juice, while the second (Recipes 4-6) has lower lemon juice and more sugar. Recipe 7 is somewhere in between the two.

Whole Eggs vs. Yolks

Another thing I see in Table 1 is that there are many different combinations of whole eggs vs. yolks, so the first thing I want to see is what role whole egg vs. yolks play in the texture and flavor. To try this out, I want to take the simplest recipe and play around with the ratio of whole egg vs. yolk. This leaves either Recipe 3 or Recipe 6. Since I used Recipe 2 for the vanilla/no vanilla test (Splits A and B), I want to try a recipe from the other class, which would be Recipe 6. I will make two batches: one with only yolk and the other with the same amount of whole egg.

Table 2: Different mixes of yolk vs. whole egg. Recipes also included the zest of 4 lemons and 1/2 tsp salt.

I think it is safe to say that this first test was disastrous and completely inconclusive. Neither recipe ever thickened, leaving me with syrupy, gooey, sickly, sweet soup. I even tried to remake this a couple of times to make sure it wasn't human error. This recipe has the largest amount of sugar of all the recipes, so my only guess is that there was so much sugar that it blocked the eggs from coagulating at all. Recipe 5 is also similar in the amount of sugar to Recipe 6 and has less lemon juice, so I am going to remove both of these from further tests. If I had a penalty box, I would put Recipe 6 there forever.

The soup du jour

Let's Try Again: Whole Eggs vs. Yolks

After recovering from the horrors of Recipe 6, I am going to jump back to the low sugar, high lemon juice class and redo this test. I will be using Recipe 3, and I will do all whole egg, all yolk, and somewhere in between the two:

Table 3: Different whole egg to yolk ratios. The recipes also include 1/2 tsp salt and the zest of 1/2 lemon.

Split E was the thickest followed by Split F then Split G. Split E felt luxurious and velvety smooth in the mouth, but it had a bit of a metallic flavor from all the yolk and was so rich that the lemon had trouble shining through. Split G was very light and the lemon flavor was the brightest. Split F had some of the richness of Split E and some of the brightness of Split G. Split G also became a bit grainy overnight. It could be that I just overheated it since whites coagulate at a lower temperature than yolks, or the emulsion could have broken a bit since with less yolk, there was less emulsifier. Lemon curds with more whole egg than yolk could be more susceptible to these effects. You also get a richer yellow color with higher yolk content.

The role of the egg yolk is thus two-fold: it makes a thicker, richer texture and mutes some of the lemon's tartness. My first thought on the thickening properties was that this must be backwards because we always hear that egg whites have the most protein in the egg. More protein means more coagulation and thus a thicker curd. However, this begins to make sense once we look at the how much of the yolk and white are made up of protein. The yolk is ~17 g total and has ~2.7 g of protein for a total protein content of ~16%, while a white is ~33 g and has ~3.6 g of protein for a protein content of ~11%. A whole egg is ~50 g total with ~6.3 g of protein for a total protein content of ~13%. While it is true that an egg white has more protein total, gram for gram, the egg yolk has more protein. Thus, more yolk heavy recipes will have a thicker texture.

The role of the yolk is also why there is so much variation in the recipes: purists love the rich texture of an all-yolk lemon curd, but others prefer their curd a bit lighter. We can now look at Table 1 and make some more sense of the recipes. For example, Recipe 2 has more sugar and less lemon juice than Recipe 3 because it uses more whole egg. It needs less lemon juice to get the same lemon flavor and more sugar to counter the tartness. Based on the test today, my perfect recipe would have a mix of whole egg and egg yolk, but I will have to tune the ratio of whole eggs to yolks until it is just right. Next week, I will find my favorite base recipe to work with before I start playing around.