Fermented Chili Paste (Doubanjiang-Style)

A thick, savory, brick-red paste in the doubanjiang lineage: koji-cultured broad beans and salted chilies ferment and then age for months, where enzymes break the beans down into deep umami and the chilies mellow into a slow, smoldering heat. This is a long, high-salt ferment — patience is the ingredient.

Ingredients

A paste ferments thick and high-salt — here 5% of the chili + bean + water base. The koji-bean component (optional but traditional) brings the enzymes for umami; make it ahead via method-koji or buy fava/soybean koji.

Ingredient	1 Jar (~1 qt / 1 L)	1 Crock (~2 L)
Red chilies (Erjingtiao / Fresno / cayenne), stemmed, chopped	350 g	700 g
Cooked broad (fava) beans, or fava/soy koji	250 g	500 g
Garlic, minced (optional)	20 g	40 g
Filtered water (only if needed to mix)	30 g	60 g
Base weight (chili + beans + garlic + water)	650 g	1300 g
Salt at 5.0% of base	33 g	65 g

Yield: about 600 mL paste per Jar; 1.2 L per Crock.

Calculated values

Salt: 5.0% of the chili + bean + garlic + water weight (paste range 3-6%; long-aged styles run higher). Size it with calc-brine-salinity.
Target finished-paste pH: 3.4-4.2 (meter-verified) after the active ferment and age.
Lacto ferment temperature & time: 18-22 °C / 64-72 °F for 1-4 weeks of active ferment.
Koji incubation (if you make your own): 28-32 °C — see method-koji.
Aging: weeks to many months, cool and dark (or the traditional sun-and-dew cycle).

Readiness window

Days: active ferment 1-4 weeks at 18-22 °C, then age 1-6+ months. The high salt keeps it slow and safe over the long age.
Done-when: the active phase ends when bubbling stops and the paste smells deeply savory and sour, not raw; a meter reads pH 3.4-4.2. The paste is “ready” when the color has darkened to brick-red and the flavor has rounded into umami — months, by taste and aroma.
Arrest / store: refrigerate at 2-4 °C once it tastes finished, or keep aging cool. A film of oil or a clean salt layer on top protects the surface during a long room-temperature age.

Method

Salt and mash

Gloves on. Weigh the chopped chilies, koji or cooked beans, garlic, and any water; sum for the base weight and add 5.0% salt. Pound or pulse to a thick, coarse paste — thicker than a hot sauce.

Pack dense, exclude air

Pack hard into the vessel to drive out air pockets (a common mold site in thick pastes). Smooth the top and cover it with a thin protective layer — a film of neutral oil, or a sprinkle of salt, or a weighted follower under the paste’s own liquor.

Active ferment 1-4 weeks

Hold at 18-22 °C / 64-72 °F. The paste bubbles, darkens, and turns sour-savory. Stir and re-smooth every few days if you’re sun-aging open; burp if sealed.

Confirm the active phase

When bubbling has stopped and the aroma is deep and savory, confirm pH 3.4-4.2 on a calibrated meter.

Age for months

Move somewhere cool and dark (or run the traditional sun-by-day / cover-by-night cycle). Age 1-6+ months, tasting periodically, until the color is brick-dark and the flavor is rounded and umami-rich.

Store

Refrigerate at 2-4 °C under a thin oil layer once it tastes finished. It keeps for many months cold.

Variations / mix-ins

Sichuan style: fold in toasted, ground Sichuan peppercorn and a splash of neutral oil after aging.
Sambal-style (no bean): drop the beans, use 600 g chilies as the base, keep salt at 5%; ferment 1-2 weeks then blend with garlic and a little vinegar — shorter, brighter, no koji.
Lower salt (3.5-4%): faster and tangier, but age it cooler and watch it more closely; do not go below 3%.
Smoky harissa lean: add ground roasted cumin, coriander, and caraway after the active ferment.