Yeast & fermentation

Yeast is a living thing in your dough, and fermentation is the whole craft: the organism eats the sugars the flour gives up, and what it leaves behind — gas, flavor, and a weakened gluten structure that can finally be shaped — is the bread. Everything in a baguette that is not "baking" happened before the oven.

What's happening

Yeast cells feed on glucose and maltose, releasing carbon dioxide as they go. The gas gets trapped between the gluten strands, and that's the whole "rise": a slow internal inflation. The by-product alcohol and the organic acids are most of what fermented bread tastes of — a dead-yeast dough baked straight away is "fluffy cracker", which is exactly what a no-fermentation dough tastes like. The second half of fermentation is chemical, not microbial: the dough's own enzymes keep cutting the starch and protein into feed, so flavor keeps building even while the yeast itself tires out.

Temperature is the dial on the clock. At the fridge (4 °C / 39 °F), yeast works at a crawl — a day of cold fermentation does what three hours of room temperature would do, but all the flavor and almost none of the gas. At room temperature (about 24 °C / 75 °F), the same dough rises in 45–90 minutes. Past about 35 °C / 95 °F, the yeast ferments so fast it eats through the feed before the flavor has time to develop, and the loaf bakes flat. You can never get the short cut back; you can always wait.

The three dough signals

Bread recipes that say "until puffy" and "until doubled" are giving you two of the three signals; the third is the one bakers actually use, and it needs no clock and no thermometer:

• The poke test. Press the dough down gently with a knuckle. It springs back slowly and mostly — a living fermentation, with gas in it. It springs back instantly and fully — under-proofed, and the gas structure isn't set yet. It stays depressed — over-proofed, the gluten has let go, and the loaf will flatten in the oven.
• The windowpane (gluten, not proof). Stretch a small piece of dough between your fingers until it is translucent light. Strong gluten lets you do it without tearing; weak gluten shreds at 1–2 cm. This is what gluten is for, and it's the test the dough is ready to shape — not to bake.
• The look-up test. Lift the edge of the dough — the interior should show a few large, uneven holes, and the surface should be fine-bubbled and slightly shiny. A surface with one large bubble across it is over-proofed; a surface with no bubbles at all is not.

The moves

• Hydrate the yeast in lukewarm water, not hot water. Above about 50 °C / 122 °F you are killing it, not waking it up — the test is your wrist: it should feel like bath water, and the yeast should be fizzing in 5–10 minutes. If it is not, it is dead — usually because it is old, and no quantity of it will save the batch.
• Mix, rest, then shape — not mix and shape and pray. The rest (20–30 minutes at room temperature) lets the gluten relax and the enzymes do their work; shaping a stiff, newly-mixed dough is what tears it. The rest between shaping and the final proof is the same rest at one-third scale: 15–20 minutes, and it is what lets you press the gas out before the last rise without the dough fighting back.
• Final proof in a covered container, in a warm place, for as long as the poke test says. The clock is the dough, not the timer: 30 minutes in a warm kitchen, 90 minutes in a cold one. When it passes the poke test, it is done; put it in the oven while you can, because the window is the length of a coffee.
• Score, then steam, then heat. The score tells the loaf where to open; the steam holds the crust soft for the first 10–15 minutes so the loaf can fully open (that's the oven spring — the second, short rise, powered by the gas already in the dough and the yeast's last, desperate ferment); then dry heat sets the crust and finishes the crumb. A baguette runs 230 °C / 450 °F in, steam for 10 minutes, then 10 more without it.

Failure modes

• Flat loaf, dense, "baked cracker" flavor. Over-proofed, then the oven: the gas had already left and the gluten couldn't hold what was left. The poke test is the whole fix — when it springs back slowly and mostly, it's ready to shape, not to bake.
• Small, tight, under-risen loaf with a thick dark crust and a raw center. Under-proofed: the yeast was starved, too cold, or too little of it. Give the final proof longer — the dough, not the timer, is the clock.
• Teared, ragged surface after shaping; the loaf splits down the middle instead of along the score. The gluten was too stiff when shaped — it needed the rest after mixing, and the rest between shaping and the final proof, and it did not get either. Rest is not optional in bread; it is the second half of the method.
• Crust that is dark and leathery while the crumb is pale inside. Too much steam, too long, at the wrong temperature for the dough. The steam is what lets the crust expand; past its window it just makes the loaf sweat. 10–15 minutes is the whole allowance.

In practice

Baguette is the pure test: water, flour, salt, yeast — the four-ingredient dough where nothing can hide. Every shortcut the dough is offered (more yeast, higher temperature, less rest) shows up in the crumb, and every rest and cool proof shows up as flavor. Read the doughs article for the gluten side of the same argument — the two are the same dough, seen from the yeast's side and the flour's side.