Rhubarb crumble is best made with careful selection of the ingredients, particularly of the rhubarb itself. Signs of freshness should always be examined, as should the type of rhubarb being bought. Preparation of the ingredients is another important matter; for example, rhubarb leaves should not be included in the filling. Moderation in cooking should also be practiced, as the rhubarb shouldn't be overcooked, and the crust not over-mixed. Paying special attention to the baking and serving of the rhubarb crumble will ensure an enjoyable dessert.
As a rule of thumb, fresh rhubarb has leaves that are of a yellowish color; any signs of blackening mean that the rhubarb is of inferior quality. The conditions under which the rhubarb is grown also play a major role—hothouse-grown rhubarb is widely considered as the best variety for rhubarb crumble, as the bright red coloring indicates higher sugar content, making it less tart and more ideal for desserts. Hothouse-grown rhubarb is also more tender than the field-grown variety.
Preparing the rhubarb is another important matter. It is essential that the leaves are completely removed before the plant is chopped, as rhubarb leaves contain a toxic substance called oxalic acid, which adds an undesirable sourness to the dish. Once the leaves are completely stripped, the flesh of the plant should be chopped into cubes measuring about 1 to 2 inches (about 2.5 to 5 centimeters) in width. This allows for a perfectly-sized bite once the rhubarb crumble is cooked.
The rhubarb itself shouldn't be cooked before baking. Stewing the flesh before it goes into the oven can make it too tender, resulting in a jam-like consistency in the filling of the rhubarb crumble. This is especially true if hothouse-grown rhubarb is used. The heat of the oven, combined with the moisture from the other ingredients, should be more than enough to soften the rhubarb without losing too much of a bite.
The crust of the rhubarb crumble, on the other hand, requires its own special rule: to avoid over-mixing. If the crust mixture is blended too well, it'll turn out more like a dough than a proper crumble crust. The best way to guarantee a chunky, slightly crisp crust is to mix the crust ingredients with the fingers, making sure to stop when the texture is similar to bread crumbs. From there, the crust should be carefully watched while baking, which should end when a light golden-brown coloring is achieved. The rhubarb crumble will be best enjoyed fresh out of the oven with custard or ice cream to accentuate the flavors.