Loaf 16: the long cold

This is Loaf 16 of 16, the last one, and the one thing that changed from Loaf 15 is technique: the cold proof goes from 12 hours to 24 hours at 38 to 40°F / 3 to 4°C.

What this loaf teaches is not a longer proof. It is where to stop.

The frozen four

Unchanged from the banana bread you started with, sixteen loaves ago: 400 g flour, 8 g salt (2%), one 9 x 5 inch (23 x 13 cm) loaf pan, and an oven at 375°F / 190°C.

The formula

Loaf 16 is a technique change, so the formula is identical to Loaf 15, gram for gram. Nothing moved. Here it is anyway.

Frozen: 400 g flour · 8 g salt (2%) · 9 × 5 in pan · 375°F / 190°C · Hydration: 85% (340 g water on 400 g flour)

The method

1. Starter to peak, mix, then bulk 3 to 4 hours at 75°F / 24°C to the Loaf 15 gate: bubbled all over, lifted by about a quarter.
2. Pour it into the pan and level the top.
3. Fridge at 38 to 40°F / 3 to 4°C for 24 hours, and check it at 18 hours.
4. Bake from cold at 375°F / 190°C, 65 to 75 minutes, to 205 to 210°F / 96 to 99°C.
5. Cool completely, wrap, and rest a full day at room temperature before slicing.

The check at 18 hours is not optional and it is not nervousness. Read the gate section before you start.

Why this works

A long cold keeps building acid after it has almost stopped building gas. On wheat that is straightforwardly good, which is what Loaf 11 taught. On rye that acid is doing two opposite jobs at once.

Below pH 4.5, the acid holds amylase back. Amylase is the enzyme that caused starch attack and the flying roof back at Loaf 13, so this is the acidity working for you: it is the brake on the failure mode that haunts high-rye bread.

Below pH 4.0, a second enzyme takes over. Pentosanase starts destroying the arabinoxylans, and those pentosans are the only thing holding this loaf up, because rye's secalins never built a gluten network in the first place (American Society of Baking, review of rye constituents and processing). Past that point the loaf loses volume in the proof or in the oven.

So rye's usable window is pH 4.5 to 4.0, and it is narrow (Wordloaf on pH and sourdough). Twenty-four hours cold walks you across most of it.

This is why the course stops at 24 hours and not 48. On a wheat dough, 48 hours cold is a defensible experiment. On 70% rye it walks the dough straight through the bottom of its window, and the reward for the extra day is a flatter, sharper, structurally worse loaf.

Longer stops being better here. That is the last thing this course has to teach you, and it is the most advanced thing in it: knowing that a variable has a ceiling is a harder skill than knowing which direction it improves in. Every loaf before this one had an arrow. More water, more time, more acid, more flavour. This one has a wall, and finding walls is what separates a baker from someone following a recipe.

How you know it's ready

At 24 hours the surface bubbles are small and dense, and the smell is sour and lactic, like yogurt or green apple. That is the loaf you want. Bake it.

Check at 18 hours and use your nose, because the gate here is a smell as much as a look:

• Sour and lactic, yogurt or green apple: on track. Leave it to 24 hours at 38 to 40°F / 3 to 4°C.
• Sharply of vinegar, or of nail polish: it has gone too far. Bake it now, and cut the cold to 18 hours next time.
• A slack top that is collapsing: same verdict. Bake it now, cut the cold to 18 hours next time.

No pH meter required, though this is the one loaf in the whole course where one would actually tell you something. How sour a given starter runs varies enough that your 24 hours and someone else's are not the same 24 hours, which is exactly why the instruction is to go and look.

What goes wrong

The failure mode this loaf introduces is a loaf that shrinks in the oven and tastes acrid.

One cause: the dough went past pH 4.0 in the fridge, where pentosanase dismantles the pentosans holding the crumb up. You proofed the structure to pieces.

The fix is less time. Not more starter, which adds acid faster and gets you there sooner. Not a hotter oven, which cannot rebuild what an enzyme already took apart. Eighteen hours at 38 to 40°F / 3 to 4°C instead of twenty-four, and the teaching point survives intact, because it was never about the number.

When it worked

The darkest, most complex loaf in the course. A pronounced sour that arrives late rather than up front, a dense fine moist crumb, and a keeping quality measured in weeks.

It is sixteen loaves from banana bread, in one pan, at one oven temperature, with no Dutch oven, and at every single step you changed exactly one thing.

Bake it twice successfully before you call it done, then go back to Loaf 9 and change one thing that is not on the list. One good loaf is luck, and two is a skill.