Lacto-Fermented Ketchup

Tomato paste loosened with water, salted, and seeded with a live brine starter sours over a few days into a tangy, complex ketchup — the lactic acid does what a long simmer of vinegar can’t, giving depth and brightness with less added sugar.

Ingredients

Tomato paste is low in fermentable sugar, so this ferment needs a live starter (brine from an active ferment, or whey) to get going. Salt is 2% of the tomato + water base. Vinegar, sweetener, and spices are balanced in after fermenting.

Ingredient	1 Jar (~1 qt / 1 L)	1 Crock (~2 L)
Tomato paste (double-concentrate)	350 g	700 g
Filtered water	250 g	500 g
Tomato + water base	600 g	1200 g
Salt at 2.0% of base	12 g	24 g
Live brine starter (sauerkraut/pickle brine) or whey	30 g	60 g
Cider vinegar (5%), added after ferment	30-45 g	60-90 g
Honey or maple syrup, added after ferment	30-45 g	60-90 g
Spices: onion powder, allspice, clove, mustard powder	to taste	to taste

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

Calculated values

Salt: 2.0% of the tomato-paste + water weight. Use calc-dry-salt or calc-brine-salinity for grams.
Target finished-ferment pH: 3.4-4.0 (meter-verified). Tomato is moderately acidic; the live starter and the ferment together drive it down.
Ferment temperature & time: 18-22 °C / 64-72 °F for 3-7 days.

Readiness window

Days: 3-7 days at 18-22 °C. With a live starter you’ll see surface bubbles and smell a winey tang by day 2-3.
Done-when: pleasantly sour with a clean tomato aroma and visible bubbling, and a meter reads pH 3.4-4.0. No starter and no bubbles by day 3 means the ferment stalled — discard rather than risk it.
Arrest: balance cold with vinegar and sweetener, then refrigerate at 2-4 °C. Keeps 3-6 months cold.

Method

Mix the base

Whisk the tomato paste with the water to a pourable consistency, then stir in the 2% salt and the live brine starter or whey. The starter is not optional here — tomato paste ferments reluctantly on its own.

Pack with minimal headspace

Spoon into the jar, pressing out air pockets and leaving 2-3 cm headspace. A thick puree is prone to surface mold, so keep the top smooth and consider a weighted follower or a tight burped lid.

Ferment 3-7 days

Hold at 18-22 °C / 64-72 °F out of the sun. Burp daily — it produces gas. Stir down any surface film and re-smooth.

Confirm the ferment

When it tastes cleanly sour and shows bubbling, confirm pH 3.4-4.0 on a calibrated meter.

Balance cold

Stir in vinegar, sweetener, and spices to taste. Adding vinegar lowers pH (safe). If you reduce it for a thicker ketchup, warm it gently — a hard boil drives off the lactic brightness you fermented for.

Refrigerate

Move to 2-4 °C. Flavor rounds out over the first few days.

Variations / mix-ins

Smoky BBQ base: add smoked paprika and a touch of molasses after fermenting; reduce gently.
Spiced (Heinz-leaning): lean on allspice, clove, and a pinch of cinnamon plus onion and garlic powder.
Low-sugar: cut the sweetener in half and lean on the lactic tang — this is where fermented ketchup shines over the bottled kind.
Fresh-tomato version: ferment 600 g cooked, reduced fresh tomato puree at 2% salt with the starter; expect a thinner result you’ll reduce more.