Up first is the Classic Lemon Tart from Baking Illustrated. The recipe is also available in the Holiday 2007 Cook's Illustrated and is featured over at Leite's Culinaria where the filling recipe can be downloaded.
The January DB challenge had a cornstarch-assisted filling, which I usually avoid. I wanted to use lemon curd instead, which meant I would not be making a pie. A deep dish lemon curd pie would be strange and a tart would be more appropriate.
A proper tart is thin, maybe an inch tall at the most, and it is at the same time homey and chic. Its shell, a pre-baked pâte sablée, must be fit to share the stage with a modest amount of filling:
"Curd" is not quite the correct term for lemon... well, curd, because it is smooth and silken, the opposite of curd. It is probably more of a gel.
When eggs are heated, their proteins unravel. Proteins are made up of chains of amino acids (polypeptides) that usually fold and cluster onto itself. These clusters become linear when heat is introduced. When they become linear, water is able to arrange around them and this is helped by some of the amino acids which bear a charge. This net effect is an increase in order, which is not a favoured state - it needs energy input (like the heat of the stove) to retain its form.
When the energy is taken out the polypeptide strands re-coil to an extent and squeeze out the water that was so neatly arranged around them - this is curdling, the bad kind that you wouldn't want in lemon curd.
What the lemon juice does is it decreases the pH of the environment so those amino acids, which are charged depending on the pH, retain their charges and continue to repel each other and have a much harder time re-coiling. Pectin, a type of dietary fibre from the zest and bits of pulp, aids gelatinization. The result is smooth lemon curd (after straining, of course).
The Baking Illustrated crew, keen on testing these principles, was able to get similar results using rice vinegar. Vinegar curd, anyone?
Finish the gel in the oven after pouring it in the pastry shell, and you get a sunny lemon tart.
Next week, how a good lemon tart becomes even better - thanks to Dorie Greenspan and Pierre Hermé. Here's a sneak peek:
- Recipes: the filling is from Baking Illustrated as featured in Leite's Culinaria and the sweet tart pastry shell (pâte sablée) is from Baking: from My Home to Yours by Dorie Greenspan as featured in Serious Eats.
- Unlike a traditional pie crust, pâte sablée can stand on its own as a sugar cookie. In fact, cookies actually descended from pâte sablée. Re-roll the scraps - it can be handled a lot without detriment - and cut out shapes for cookies.
- Don't be surprised that the Leite link is a recipe for lemon bars. It uses the same curd as the tart. It has just the right balance of pucker and sweet, and it sets up nicely without becoming gelatinous.
- Instead of whisking the curd as it cooks, which whips too much air into the mixture, stir continuously with a wooden spoon or silicone spatula instead. The incorporated air will expand in the oven and may cause an overflow. Also, whisking makes the mixture lose heat, so it will take you longer to reach the desired temperature of 170 °F.
- To avoid an "off" taste, make the curd using a non-reactive pan (and of course, non-reactive bowls, spoons).
- Pre-bake the shell and make the filling. Pour the filling into the still warm shell and return both in the 375 °F oven for 10 to 15 minutes. You want the lemon tart to jiggle slightly in the centre when shaken. Cool and allow to set at room temperature, about an hour.
- Updated March 16, 2008: After the lengthy discussion of the biochemistry of lemon curds, I forgot to mention how much I actually loved how it tasted. The staff of Cooks Illustrated managed to produce a curd that is bracingly lemony and not too much at that. Its smooth and rounded pucker is balanced by the bite of the shell which offers a most welcome sweet reprieve.