Pastry
Blind baking exists because filling and pastry cook at different rates
A soggy base is a timing problem, and every part of the standard procedure is aimed at getting the shell ahead of what goes into it.
By Tomas Bergqvist4 min read
Two materials with incompatible schedules
A tart is an assembly of a thin dry pastry and a wet filling, and they need very different treatment. The pastry needs enough sustained heat to drive out moisture, set its structure and brown; a custard filling needs gentle heat and will curdle if it goes much past the point where egg proteins coagulate. Bake them together from raw and one of them will be wrong. Usually it is the pastry, and usually the symptom is a base that is pale and damp while the filling is done.
Blind baking resolves the conflict by separating the schedules. The shell is baked alone until it is set and coloured, then the filling goes in for whatever it needs, which may be a short bake or none at all. This is not a refinement; for a tart with a wet or delicate filling it is the only order of operations that works.
Where the sogginess actually comes from
Moisture migrates from wet to dry. A raw filling in contact with raw pastry sends water into the starch and protein of the dough, and once the dough is wet it cannot get above the boiling point of water at that interface, so it will not brown or crisp there — the base stays at 100°C while the top of the tart happily goes past it. What sets is a pale, dense layer instead of a crisp one.
A partly baked shell resists this because its starch has already gelatinised and set and much of its water has gone. Baking it fully, until it is properly coloured rather than merely dry to the touch, matters more than most people credit. Colour indicates that browning reactions have run, which in turn indicates that the surface got genuinely hot and dry, and that surface is the barrier the filling has to get through.
Why the weights and the paper are there
An empty shell in a hot oven does two unwanted things. The base puffs, because trapped steam and air lift it away from the tin, and the sides slump, because the fat softens before the structure sets and gravity does the rest. Filling the lined case with something heavy prevents both, holding the pastry against the tin until it has set enough to stand on its own.
Docking — pricking the base with a fork — addresses the same puffing problem by giving steam an escape route, and it works well for a pastry that will hold a dry or set filling. It is a poor idea when a liquid filling will be poured in later, since the holes are exactly where it will leak. That is one reason weights are preferred for custard tarts, and why the weights are removed for the last portion of the bake so the base can dry and colour properly.
Sealing the surface is the second line of defence
Even a fully blind-baked shell will soften eventually under a wet filling, so a barrier helps. Brushing the hot baked shell with beaten egg and returning it to the oven for a minute or two sets a thin coagulated film across the surface and into any small cracks, which both waterproofs it and repairs the leaks that a fragile pastry usually has. It is a small step with an effect out of proportion to the trouble.
Other barriers work on the same principle. A thin layer of melted chocolate on a cooled shell sets into a genuinely waterproof fat layer, which is why so many fruit and cream tarts have one. Ground almonds or crumbs scattered on the base absorb moisture before it reaches the pastry. Jam brushed under a fruit filling does something similar. None of these makes an unbaked shell work; they buy time for a properly baked one.
Judging the two stages
The usual approach is to bake with weights until the sides are set and just beginning to colour, then remove the weights and bake on until the base is dry and evenly golden. How long each stage takes depends on the pastry, the tin and the oven, and a metal tin conducts heat into the base far better than a ceramic dish, which is why ceramic tart dishes are more prone to a pale bottom.
The second stage is the one people cut short. A shell that looks done from above is frequently still pale in the centre of the base, and that centre is precisely where the filling sits deepest. Lifting the tin and looking underneath is worth doing. So is accepting that a shell destined for an unbaked filling should be taken further than one that will go back into the oven — it has no second chance to dry out.
Common questions
What can I use instead of ceramic baking beans?
Dried beans or rice work perfectly well and can be reused many times for this purpose, though they will not be much good for eating afterwards. Sugar is used by some bakers and conducts heat well; it toasts lightly and can be kept for baking later. The important property is weight and the ability to fill the corners.
Can I skip blind baking for a fruit pie?
Often yes, because a covered fruit pie is baked long and hot enough for the base to set, and the filling is thick rather than liquid. The risk rises with juicy fruit. Thickening the filling and starting the pie on a preheated tray in the lower part of the oven both help.
Why did my shell crack?
Usually a dough that was too dry or too cold when rolled, or one that shrank against the tin as it baked. Small cracks are repairable: patch them with raw dough or seal them with egg wash and return the shell to the oven briefly before the filling goes anywhere near it.
Editor, The Last Bite
Tomas writes the explanatory pieces on baking basics, chocolate & sugar, frozen and thinks most subjects are more interesting once you know how they work.
