Reverse Sourdough Scheduler

What Is Reverse Sourdough Scheduling?

Most sourdough recipes are forward-scheduled. They tell you: feed your starter, wait until it doubles, mix the dough, wait for bulk to finish, shape, cold proof, bake. The output time is whenever the chain happens to land. For a hobbyist with no fixed schedule this works. For a working baker, parent, or anyone with a calendar, it does not.

Reverse sourdough scheduling inverts this. The input is the target bake time. The output is a chain of timestamps stepping backwards: when to start cold retard, when to shape, when to begin bulk fermentation, when to mix dough, when to build the levain, when to refresh the starter from cold storage. Every leg of the chain is computed from physical and biological parameters: ambient temperature, dough hydration, flour type, starter activity.

This is what S.D TIMER does. The user enters a target bake time and a small set of variables; the calculator returns the complete reverse timeline. Below is the full mathematical and biological basis for how it works.

Why Forward Scheduling Fails Working Bakers

The fundamental reason forward scheduling fails outside hobby contexts is that fermentation timing is highly variable and the variability compounds across stages. A starter at 24°C might peak in 5 hours; the same starter at 19°C peaks in 10-12 hours. If the baker mixes dough whenever the starter peaks, the bake time depends on at least four sequential biological processes (starter, levain, bulk, proof), each with ±20-30% variance. The cumulative uncertainty is large enough that a single forward-scheduled bake can land anywhere within an 8-hour window.

Reverse scheduling absorbs this variability into the schedule itself. Each backward step is computed using the same temperature data the baker provides upfront. Since temperature is the dominant variable for fermentation kinetics, fixing it at the start collapses the uncertainty.

The Math: Working Backwards from Bake Time

The calculator runs five backward calculations. Each takes the previous timestamp and subtracts the duration required for the preceding leg.

Step 1: Bake Time → Cold Retard Start

If the recipe uses cold retardation (the most common pattern for working bakers), the algorithm subtracts the cold proof duration — typically 8 to 16 hours at 4°C. Cold retard is the most forgiving leg in the chain because the temperature is fixed by the refrigerator and fermentation runs at roughly 1/8 to 1/12 the rate at room temperature. A 12-hour fridge proof at 4°C corresponds to roughly 1.0-1.5 hours at 24°C, so small timing errors are absorbed.

Step 2: Cold Retard Start → Shaping

Shaping happens immediately before cold retard begins. The calculator subtracts a small bench rest (15-30 minutes) for the shaped loaf to settle before transferring to the refrigerator.

Step 3: Shaping → Bulk Fermentation Start

This is the largest variable leg. Bulk fermentation duration depends on dough temperature, starter inoculation percentage, and target volume increase. The calculator uses the Q10 temperature coefficient model: every 5.5°C change in dough temperature roughly halves or doubles bulk time (Gänzle 2014, Food Microbiology; Hartman et al. 2011, Journal of Cereal Science). At 24°C with 20% inoculation a typical bulk runs 4-5 hours; at 27°C it runs 3-3.5 hours; at 21°C it runs 6-7 hours.

Flour type modifies this. Whole wheat ferments roughly 20-30% faster than white flour due to higher amylase activity and bran microbial load. Rye ferments faster still — typical bulk times for rye-heavy doughs are 60-70% of equivalent white doughs at the same temperature. The calculator applies a flour-specific multiplier to the base time.

Step 4: Bulk Start → Mixing (with Active Levain)

Bulk begins immediately after final mix. The calculator does not subtract additional time here unless an autolyse step is specified. Autolyse — flour and water resting before salt and levain are added — is typically 30-60 minutes and is a separate subtracted leg if the user enables it.

Step 5: Mixing → Levain Peak (when to build the levain)

The levain must be at peak ripeness when added to the dough. Peak time depends on the inoculation ratio and temperature. A 1:1:1 levain (equal parts ripe starter, flour, water) at 24°C peaks in 4-6 hours. A 1:5:5 levain at 24°C peaks in 10-14 hours. The calculator uses these ratios as user inputs and computes the peak time accordingly.

Step 6: Levain Build → Starter Refresh from Cold Storage

If the starter was in the fridge, it needs to be refreshed and brought back to active state before the levain build. This typically requires 1-2 feeding cycles (8-12 hours each at room temperature) to reactivate the microbial population to peak activity. The calculator schedules this refresh to land just before the levain build starts.

Worked Example: Saturday 7:00 AM Bake

The baker wants bread out of the oven at 7:00 AM Saturday. Their kitchen is 24°C. They use 80% hydration white bread flour and a 100% hydration starter that doubles in 5 hours at 24°C. Cold retard target: 12 hours.

The full chain is two days. The baker performs 4 actions: refresh starter Thursday evening, build levain Friday morning, mix and bulk Friday afternoon, shape and refrigerate Friday evening. Saturday morning the dough goes straight from fridge to oven.

Worked Example: Weekday Evening Bake (Bake Tonight)

Same baker, same kitchen, but wants bread out of the oven at 18:30 on a weekday. No cold retard. Target: same-day bake.

Same-day bakes require an active starter. If the starter has been in the fridge more than a few days, add another 12-24 hours of refresh cycles before the levain build.

Variables That Shift the Schedule

Ambient Temperature

Largest single variable. Q10 ≈ 2-2.5 for sourdough microflora means moving from 24°C to 28°C halves fermentation times; moving to 20°C doubles them. Kitchen temperature varies seasonally and even hour-to-hour. The calculator asks for current ambient; for highest accuracy, measure dough temperature directly with a probe thermometer.

Starter Inoculation Percentage

The amount of ripe starter relative to flour weight in the final dough. 20% inoculation is typical for white-flour sourdough. Lower inoculation (10-15%) extends bulk time and produces more acid (more sour). Higher inoculation (25-30%) shortens bulk time and produces a milder loaf. The calculator scales bulk duration inversely with inoculation.

Target Hydration

Higher hydration doughs ferment slightly faster because water mobility increases substrate diffusion. The effect is moderate — a 75% hydration dough ferments roughly 10% faster than a 65% hydration dough at the same temperature, all else equal.

Flour Type

White bread flour is the baseline. Whole wheat: 20-30% faster (more amylase, more bran microbes). Rye: 30-40% faster (high amylase, high pentosan content). Spelt: similar to whole wheat. Each flour has a specific multiplier in the calculator's model.

FAQ

How accurate is the reverse scheduler? Within ±15% of actual time when temperature input is accurate to ±1°C and starter activity is known. Without temperature data, predictions can be off by 50% or more.

Does it work for enriched doughs (challah, brioche)? Partially. The calculator targets lean sourdough. Enriched doughs ferment 30-50% slower due to fat and sugar effects on yeast osmotic pressure. The user should manually extend bulk time if applying it to enriched recipes.

What if my schedule changes mid-bake? Cold retard absorbs ±2-3 hours of schedule shift without quality loss. Bulk fermentation has ±30 minutes of forgiveness at room temperature. Beyond these windows the dough is over- or under-fermented.

Can I save my schedule? The web app supports calendar export and browser-saved presets. Each schedule is recomputed on each visit so changes in temperature or hydration are reflected immediately.