A darn good question. Especially good when you consider that to the untrained eye lemon curd closely resembles any number of lesser flour- or cornstarch-thickened lemon fillings, fillings that form the basis of many (especially commercial) lemon tarts, lemon-filled doughnuts, fruit pizzas, cakes…yadda and yadda. Not to say that these types of preparations don’t have value. Starch-thickened lemon fillings can go where many a curd cannot. Into the oven for example. Gently spooned onto a delicate butter cake a good lemon curd is the best evidence I know of for the existence of a benevolent creator. Used as a cookie or bar filling though, it separates into a clumpy, runny, greasy mess in a matter of minutes. Yet therein lies a clue to lemon curd’s odd name.
For “separating” is also known as curdling, when an emulsion (tiny drops of liquid fat suspended in a water…or the reverse) breaks into its constituent parts. Often this happens when long and languid protein molecules (used as “emulsifiers”, or substances that get between the fat drops to keep them from combining with one another) constrict into knots. The cause of the process is typically heat, and left unchecked it ends with ugly clumps of protein and fat suspended in a watery liquid. This can be a very bad thing if you’re making a sauce or a cake batter, but a very good thing if you happen to be making cheese. Or lemon curd, which many Brits actually call “lemon cheese” and is thickened via a process of carefully controlled curdling. Obviously to get curdling you need fat, which lemon curd has in spades if you look at this week’s recipe. That’s another thing that distinguishes it from simple starch-thickened fillings which, lean and useful though they may be, are nowhere near as yummy.