Four ways a filling sets, and this is the fourth
This site keeps returning to the same question in different clothes: what actually makes a filling hold together. By now it has specified three answers. A jam sets on pectin meeting acid and sugar. A pastry cream sets on starch gelatinising as it cooks. A curd sets on egg protein coagulating.
A chocolate ganache does none of those. It sets on fat, on the cocoa butter in the chocolate recrystallising as the mixture cools. There is no pectin, no starch and, in a plain ganache, no egg doing structural work.
Which inverts the usual troubleshooting. Every other filling here is fixed by more heat or more time on the heat. A ganache that has not set needs cooling, and heating it further does the opposite of helping.
The dough underneath
A ganache goes into the shell already liquid and sets there, which makes the base’s water resistance less critical than it is under raw fruit, though not irrelevant, because a ganache is largely cream. The dough this site records as strong enough to hold a liquid filling without leaking is pâte sucrée, bound with egg, and it is described as tight, crisp, cookie-like.
A chocolate tart also conventionally uses a fully blind-baked shell rather than a partly baked one, and the reason is in the blind-baking specification: a fully baked case is the one for a filling that never bakes. A ganache never goes back in the oven, so the shell has to be finished before it arrives. The three doughs compared on sugar, butter and binding liquid are at pâte sucrée and pâte sablée.
What the blood orange brings, and what it risks
Orange is in the register: pH 3.69–4.34, low acid, high pectin, set group 2, and 9.35 g of sugar per 100 g in 86.75 g of water. High pectin makes it a good candidate for a marmalade or a set compote, which is the preserving use.
In a chocolate tart it is doing something else and carries one risk worth naming. Fruit brings water, and water in a ganache is what causes it to split, as the emulsion of fat and liquid breaks and the filling turns grainy. Which is why blood orange in this context is normally confined to zest, a reduced juice, or segments placed on the set surface rather than stirred through. It is the same class of problem as acid in a starch-set cream, described at crème pâtissière: the addition and the setting agent interact, and the order matters.
The curd case, where the fruit is the filling rather than an addition to it, is at lemon squares.
Questions about chocolate tarts
What makes a chocolate tart filling set?
Fat. A ganache sets as the cocoa butter in the chocolate recrystallises on cooling, not on pectin, starch or egg like the other fillings specified on this site.
My ganache has not set. Should I reheat it?
No. It is the only filling here that sets by cooling rather than by heating, so more heat is the opposite of the fix. Chilling is what sets it.
Why does adding orange juice make a ganache grainy?
Water. A ganache is an emulsion of fat and liquid, and extra water can break it. That is why citrus in a chocolate tart is usually zest, a reduction, or segments on the set surface rather than juice stirred through.
Should the tart shell be fully or partly baked?
Fully. The blind-baking specification gives the fully baked case as the one for a filling that never bakes, and a ganache never returns to the oven.