The Food Lab: Better Home Cooking Through Science

principle behind, say, scratching up the surface of potatoes
before roasting them to get them extra-crisp (see here) or
packing your burger extra-loose to give it a crisper exterior
and more browning (see here).
When roasting at 375°F, because the dehydrating and
protein setting is taking place at the same time that the
connective tissue is breaking down, there’s never really a
stage when the skin is relatively soft. It goes from being firm
because of the connective tissue directly to being firm
through dehydration.
On the other hand, after 8 hours in a 250°F oven, the skin
has very little structural integrity—it’s stretchy, soft, and
easily bent. Indeed, if you looked at it under a microscope,
you’d find that the structure of the skin very much
resembled a whole bunch of interconnected balloons just
waiting to be filled. How do you fill those balloons? Let heat
do the work for you.
If you take that slow-cooked pork and bang it into a
preheated 500°F oven, air and steam trapped within the skin
will rapidly expand, causing millions of tiny bubbles to
form. And here’s the key: as the bubbles expand, their walls
stretch out thinner and thinner; eventually they are so thin
that the heat from the oven is able to quite rapidly set them
into a permanent shape that won’t collapse even when the
pork is pulled out of the oven.
In this sense, pig skin is very much like raw pizza dough
going into a hot oven: the high temperature causes gas
expansion, which then gets trapped in a protein matrix that
firms up in the heat of the oven to create a crunchy, crisp
crust.