Why does pastry blistered?

Answer 1

This can very well depend on the type of pastry, however, the basics on this is a little logical. Pastry dough can contain a small amount of leavening agents (usually very small amounts), however when heated this produces carbon dioxide which is released, which in turn causes the pastry to "rise". What happens when the oven is set at to high a temperature the release of the gas can not match the speed in which the crust of the pastry is hardening. Instead the crust hardens and becomes less porous and eventually traps the gas inside and the pastry then "blisters" as a result of the gas being trapped.

Think of a balloon, if you blow up a regular balloon the air is trapped inside correct. Now before blowing up the balloon poke a very small hole in the opposite end, as you blow into the balloon some air will escape, however the balloon itself will still expand and eventually lose all the air, in baking the "blister" doesn't "go down" because it is hardened into this shape. Now take the same balloon with no air and poke a huge hole in it. When you try to blow this balloon up the air will escape with equal speed and the balloon will not change it's shape.