Curious about curd
One of the earliest recipes lemon curd goes back to 1844 and suggests that the origin of the name was quite literal – lemon acidulating cream to form curds that could then be separated from the whey through a cheesecloth. Today is made in a similar way to a custard but is something of a paradox in a saucepan – how can something be tart and fresh whilst at the same time be rich and creamy? There is no cream in fruit curds, the luscious texture come from cooked beaten eggs. To understand how they end up smooth and creamy and not like scrambled eggs we need to understand some food science.
Consider the eggs and the chemical reaction that takes place to form a curd. At room temperature eggs are full of protein, lots of strands of it which are tightly curled and separate. Add heat to eggs and the strands relax, unfold and link up with other strands. A process we know as coagulation.
Egg whites start setting at 145°C and become firm at 160°C. This temperature window is pretty hard to hit and is why perfectly scrambled eggs aren’t as easy as they sound. Add sugar and an interesting thing happens, the window opens wider. The sugar isolates the protein strands, moving them further apart from each other and keeping them apart longer. This raises the temperature at which they coagulate. Because sugar acts as a sort of buffer to prevent them from curdling you always you want to make sure that the egg yolks and sugar are well-mixed before adding the juice.
This chemical reaction is the secret behind cooked custards and zabaglione.
Science being science, if you change the ingredients you will change the reaction. Vary the eggs – whole eggs versus yolks – and you’ll change the firmness and richness. Adjust the butter and you’ll either highlight or mask the fruit flavour. Change the sugar and you affect the overall balance of tart and sweet.
Despite all the potential variations, fruit curd is dead simple to make. The key to success is making sure the egg and sugar are well mixed before adding the remaining ingredients. That’s the only way to temper the proteins sufficiently.
When ready, the curd should coat the back of a spoon like a moderately stiff hollandaise that stays separate if you drag your finger through it. It will set more as it cools.
Fruit curd has been made since the late 19th century and can be based on any fruit that the juice can easily be squeezed from. It is very versatile and can be used as a dip, spread or as filling for cakes, tarts and other sweet desserts.