Week 10: The window butter works in

The week 10 lecture — butter plasticity between 16 and 20 °C, the arithmetic of folds, and why a laminated dough has a temperature ceiling

26 April 2027

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Week 9 dissolved fat into a dough. Lamination does the opposite: the butter goes in as a sheet and has to stay a sheet through a lock-in, three folds, a proof and the first minutes of the bake. That is harder than it sounds, because a sheet of butter inside a sheet of dough is only stable across about four degrees.

Butter is a crystal network. Solid triglycerides form a scaffold that holds liquid oil and water droplets in place, and how much of the fat is crystalline at any moment is a function of temperature. Below roughly 16 °C, too much of it is, and the block is brittle — put a pin to it and it fractures. Above roughly 20 °C, too little is, the scaffold collapses, and the butter flows and gets absorbed by the flour touching it. Between those two numbers it deforms plastically: it takes the shape the pin gives it and holds that shape. A laminated dough needs exactly that, because the butter has to spread at the same rate as the dough around it without breaking and without merging into it.

Fat percentage is why the butter brand matters, which is otherwise the kind of thing that sounds like snobbery. European-style butter runs 82 to 84% fat against the 80% of a standard supermarket block, and the difference is mostly water, held as droplets through the fat. The fat is the part that forms the crystal scaffold, so a block with less of it has a thinner, more interrupted network and a lower temperature at which that network gives way to fracture: the 80% block has started cracking while the 82% block is still bending. Both are butter. Only one of them survives being pounded into a 16 × 16 cm slab and then rolled down to twenty-seven layers three hundredths of a millimetre thick.

The fold count is arithmetic, and it stops paying. Each letter fold triples the layers, so three folds give 27 and four give 81. Layer count is not the goal though; layer integrity is. At 81 layers in a 3.5 mm sheet, each butter layer is thinner than the dough on either side can reliably support, so the layers rupture and merge, and an 81-layer croissant bakes up closer to a bread roll than a 27-layer one does. The rests between folds matter more than a fourth fold would: 30 to 60 minutes in the fridge relaxes gluten that has just been stretched and brings the butter back into its window, and skipping them is the most common way a well-built lock-in falls apart by the third pass.

Everything after that is the temperature ceiling. The proof has to run at 24 to 26 °C — fast enough for the yeast, cold enough that the butter stays where you put it — which is why a general-purpose proofer set to 30 °C for the rest of the kitchen will leak butter onto the tray before the croissants ever reach the oven. Then the ceiling lifts and the constraint inverts: at 200 to 220 °C the 16 to 18% water held in each butter sheet flashes to steam and forces the layers apart, while the gluten sets around the gap. The honeycomb you cut into is a record of which layers were still intact when that happened.

The bench procedure — the formula, the dimensions, the timings — lives on the week 10 session page. This hour is for why each of those numbers has the value it has.

Outline

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