One of the greatest French bistro desserts is tarte au citron, or lemon tart: a rich, tangy curd set in a buttery French tart shell.  In furtherance to ending the tyranny of the lemon in our fair city, I’ve been experimenting with substituting citrus with our local sour cherries.

Background: Classic Fruit Curds

In pastry books there are usually two fruit curd recipes: one for lemon and lime, and another that can be used for almost any other kind of fruit.

Lemon has two traits that let it have its own style of curd: a yellow colour and a very intense acidity.  If you cook lemon juice with enough egg yolks and butter that it sets as a curd when cooled, not only will it have the bright yellow colour we associate with lemons, but the acidity of the lemon juice will cut through the fatty yolks and butter.  If you were to take a lemon curd recipe and substitute, say, blueberry juice for lemon, the fat in the curd would completely overwhelm the weak acidity of the berries, and it would make a tart with an off-putting grey-blue colour.  For this reason there is a second style of curd that is used for basically all types of fruit besides lemons and limes.  To keep a vibrant colour and acidity, the amount of butter and egg yolks must be reduced, which means that the curd must have an additional thickener, usually gelatin and egg whites.

Designing the Sour Cherry Tart

There are three things that I love about our local sour cherries:

1. Acidity, obviously
2. Intense, vibrant colours: eg. Evans cherries are the purest, happiest red, Carmine Jewel cherries are deep purple
3. Distinctive aroma and flavour: eg. Evans cherries have a distinct aroma of almond extract.

An ideal cherry curd would preserve these three characteristics.

In my first experiment with sour cherry tarts I figured that the cherries were sour enough to stand up to the butter-and-yolk barrage of a classic lemon tart.  I simply substituted a strained Evans cherry purée into a classic lemon curd recipe.

The results were this:

• The final curd was a drab, dusty, sad pink.  I had to add food colouring to make it presentable.
• The fat of the curd completely blanketed the natural tartness of the cherries.  The final tart had no discernible acidity.
• Similarly, the more delicate aromas of the cherry (almond) were clobbered and undetectable.  The tart did have a muted, generic cherry flavour.
• The texture was okay.  Very buttery.  A bit stodgy.

For round two I used Carmine Jewel cherries, and instead of using only butter and egg yolks I used whole eggs and gelatin, with only a touch of butter.

Results:

• The colour of the curd was fantastic.  Very close to that of the original cherry.  Honestly it reminded me of Beaujolais Nouveau: Purple with a fuchsia tint.
• While the tart was not fully sour, it did have a pleasant, bright acidity.
• Still none of the great aroma of the cherries

So at the very least I know what style of curd the tart needs to use.  One more iteration and I should have a working recipe.  I’ll keep you posted.