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Fermented tomato ketchup

Wild fermented foods can be a great way to support the health of your gut microbiome – especially if your microbiome analysis results have come back showing low levels of Lactobacilli species. Medical research now shows that the beneficial bacteria found in probiotics and fermented foods don’t ‘reseed’ our gut as was originally hoped, although the beneficial bacteria may stay in the gut for a few weeks after ingestion. But we do know they do good on the way through, and that a percentage of lactic acid-producing bacteria in the microbiome is associated with health.

When you wild ferment vegetables, naturally occurring beneficial bacteria produce the lactic acid that preserves the vegetables. Many of these bacteria are found on the veg from the soil they grew in. Organic farming takes care of soil fertility – and therefore the soil bacteria – so fresh vegetables from the organic stalls at your local farmers' market are likely to have the greatest diversity of bacteria to enlist in your fermenting and to support your gut health.

At this time of year, the tomatoes are amazing, so I gathered some gorgeous, ripe, heritage tomatoes from the farmer’s market and made a fermented tomato ketchup. The fermentation took just three weeks, and resulted in a delicious, probiotic condiment to add to meals.

Here’s how I made it.

I decided I wanted to experiment with a new recipe: this time including some carrot and turnip to add some depth to the flavour. I also added some fresh sage. After fermenting, when it was time to blend and bottle, I added some spices for depth and sweetness. I did three different flavours: cinnamon & smoked paprika; paprika & chilli flakes; cinnamon & nutmeg. They are all utterly delicious!

CHOP Tomatoes, 1 carrot (grated), 1/2 turnip (grated), 1/2 red onion, 1 clove garlic, 2 sprigs fresh sage.

SALT Aim for a 2–3% salt, eg 15–22g in a 750ml jar (roughly 3–4tsp), depending on taste. Some people use higher salt concentrations as the sugaryness of tomatoes favours yeast fermentation (ie tomato wine) but I stopped at what tasted good in the bowl.

PACK Massaging in the salt made it very soupy so I ladled it into jars. Gently press the salted ingredients into the jar so that the juices create a brine. Make sure all veg are below the brine (using a weight if you have one).

WAIT Leave it to ferment beneath the brine for 2–3 weeks, until bubbling has stopped – indicating the lactic acid has built up enough to preserve the ketchup.


BLEND Pour off the briney tomato juice – retain in case you need to add some back to get your desired consistency, and for various taste adventures in the kitchen, or cocktail bar! Blend.

STRAIN (optional) Push through sieve, to remove seeds if you want. If you'd like it thicker, strain through butter muslin. I didn't do this, this time.

BOTTLE Bottle and store in the fridge.

Three jars of fermented tomato ketchup
I used all the ingredients in the first photo. This gave me 2x750ml jars of ferment that yielded around 1L ketchup after straining and blending (3x350ml jars pictured).

References and further reading

Viola Sampson BSc RCST is a Microbiome Analyst practising in London, UK and online. For consultations, please book online.

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