Lemon curd is a paradox in a saucepan: It's rich and creamy. It's tart and fresh.
Those are the characteristics that make it delicious; they're also what make it seem impossible to make. Think about it: What happens when you pour lemon juice into milk? How would you make that taste good?
The answer lies in a particularly basic and very useful bit of food chemistry.
Fruit curds are creamy despite having little or no cream in them. Instead, the luscious texture is supplied by cooked beaten eggs. But fruit curds are smooth, not like scrambled eggs at all. How does this work?
First, you need to understand a little egg chemistry. Eggs are full of strands of protein, which at room temperature are tightly curled and separate. As they heat up, the protein strands relax and unfold. As they unfold, they bump into other protein strands and link up, capturing the liquid that is present in the egg. This is called coagulation.
Typically, with pure egg, the white begins to set between 145 and 150 degrees and is firm at 160 degrees. By 180 degrees, the protein strands have tightened to the point that all the liquid is wrung out. (Egg proteins are not alone in this behavior; that's what happens when you overcook a chicken breast, too.)
That 145-to-160-degree window is pretty hard to hit, especially over a live flame, which is precisely why perfectly scrambled eggs are such a miracle. Add sugar, however, and an interesting thing happens. The window opens wider. The sugar isolates the protein strands, moving them farther from each other and keeping them apart longer. This raises the temperature at which they coagulate.
Whether they knew this or not, cooks have been taking advantage of this chemical reaction for centuries. The phenomenon is the secret behind both cooked custards and zabaglione, the Italian dessert consisting of egg yolks, sugar and, traditionally, Marsala that is beaten and cooked until it is a slightly foamy cream.
In fact, if you take a zabaglione, replace the liqueur with lemon juice and add butter, you have a lemon curd.
But there are lemon curds and there are lemon curds. To come up with the version I liked best, I first analyzed a dozen recipes from different cookbooks. Though the techniques are basically the same, the proportions of ingredients vary greatly.
Some recipes, for instance, call for whole eggs, some call only for yolks and still others call for a combination of the two. I made curds with pure yolks and with an equal volume of whole eggs and compared them.
Though they work equally well, there are major differences. Made with yolks, the curd is stiffer and the flavor richer and more custard-like. With whole eggs, the color is lighter, the set is softer and the flavor is more intensely fruity. I ended up compromising, using a couple of whole eggs and adding two more yolks to give it a slightly smoother flavor and slightly firmer set.
The amount of sugar in the recipes varies even more, ranging from half a cup to a full cup. A recipe made with half a cup of sugar is well balanced. I tried three-fourths of a cup and found that it is too simply sweet, particularly in the aftertaste. I never added as much as a full cup.
Butter amounts can vary from two tablespoons to a full cup. I found that the amount of butter is a much more subtle distinction than the amount of sugar. Curd made with only a little butter is more tart and fruit-like. The more butter you add, the more complex the flavors become but also the more the fruit flavor is masked.
I prefer curd made with about six tablespoons of butter. I can certainly see the arguments in favor of a lesser amount, though, particularly in desserts for which you want a cleaner, fresher flavor.
Curious about curd, I made it with different citrus fruits, too. Though lemon is the exemplar (and Meyer lemon even better), lime is very nice, with pointed acidity and a slightly grassy, herbal quality. Orange is softly acidic and delicately floral. I tried blood oranges, too, and that was the only wash-out. When cooked, the vivid red color fades to a bruised purple and the intriguing berry quality becomes nasty and artificial-tasting.
Most interestingly, I tried to make curd with plain water to see the effect of liquid with no acidity and wound up with sweet, watery scrambled eggs. Acidity lowers the coagulation temperature and obviously plays a very important role in tempering the action of the sugar.
Still, the most amazing thing about curd isn't its many variations but the one thing all versions have in common: They are dead simple to make.
It's easy to do in a single small saucepan. In fact, as delicate as curds may seem, you don't even need to use a double boiler. You can use a common pastry chef's trick: Bring the mixture quickly to a boil over high heat and then simply strain out any bits of curdled egg.