Substrate + duration → target WPS

Salt Compass

What you are fermenting and for how long set a target WPS (Water Phase Salt); the model works out how much salt holds that concentration in your water — as water increases, salt increases with it. A vegetable paste and a fish garum of the same age are not the same number: protein needs salt that acid would otherwise have provided.

New to this? Start here

In a ferment, salt is not seasoning — it is the preservation system. Spoilage organisms live in the water inside your batch, so what protects you is not how salty the food tastes but how concentrated the salt is in that water. Two batches can both be “5% salt” and only one of them be safe: the dry paste has little water to salt, the wet brine has a lot. That concentration is what this tool solves for, and it is why the number it hands you goes up when you add water.

WPS —: water-phase salt — salt ÷ (salt + water). The safety number, and the one the model makes every decision on.
% of total —: salt against the whole batch weight. The everyday recipe number, easy to weigh, but it can stay flat while WPS quietly falls.
Water activity (a_w) —: how much of the water is actually available to microbes. Salt lowers it; that is the entire mechanism.
Hurdles —: a batch is protected if it clears at least one of acid (pH under 4.6), salt (about 10% WPS) or dryness (a_w 0.93). With no acid, salt is carrying it alone.

Total weightSolids + water, grams

Water ratio40% · 400 g water

Substrate — protein indexk = 0.50

Balanced / legume — soy, chickpea (miso, shoyu)

Fermentation duration8 wk · ~2 mo

1 wk~9 mo18 mo

Acid hurdle — will it reach pH < 4.6?

Only a real acidification lets the target drop below the salt-only curve. Salt is the default because it is the hurdle you control.

Ambient temperaturenot set

Above 20 °C the ferment runs faster (Q10 ≈ 2, rate doubles per 10 °C), so the same calendar time is more weeks of progress and the target moves up the curve. Below it nothing changes: cold slows the spoilage organisms that would warn you, but not C. botulinum type E, so it never buys you less salt.

8 wk · ~2 mo target: 10.9% WPS at k = 0.50. Holding that in your current water takes 49 g salt (≈ 4.9% of total). Binding constraint: water-phase (WPS) — more water means more salt.

Salt ratio · model

% of total · = 49 g salt

Type a value to override

Achieved WPS

10.9%

target: 10.9%

0%● achieved · ▏target · ▏10% botulinum28%

✓ Meets target. Achieved WPS 10.9% ≥ target 10.9% (a_w ≈ 0.93) — 0.0 points of room.

Salt to weigh

49 g

automatic

% of total weight

4.9%

recipe metric

Approx. a_w

0.927

clears the 0.93 hurdle

Guide & Tips

Click a question to expand it.

~10% WPS / a_w ≈ 0.93 / pH < 4.6 are the C. botulinum thresholds; no setting drops the target below 3.5% WPS. For anaerobic and animal-based ferments, verify against a trusted authority and a tested recipe.