Fermented Tomato Ketchup
The end of August means lots of tomatoes for me, and it’s also usually the time of year that I need to travel for a conference*. This is not ideal for making sure they get used. I had almost 5kg of various types of tomatoes on my counter the weekend before the conference, so there’s only one thing I could think of doing:
Throw them in vacuum bags to deal with later!
2025-08-23: Processing Tomatoes
I had a vague notion that I would probably make ketchup from at least some of it, but I didn’t have much time to plan anything complicated, so the process was simple: chop, bag, add smashed garlic, salt, and shio koji*.
I split the tomatoes into 3 groups based on the type of tomato in order to fit into vacuum bags that were already pretty long, roughly 11×20 inches. Tomatoes were roughly chopped, mostly just to expose the insides and help compact in the bag. To each bag I added a smashed clove of garlic (including skin), 2% salt, and 4% ship koji.
Bag 1: beefsteak tomatoes (Cherokee Purple, Italian Heirloom)
- 1440g beefsteak tomatoes
- 8g garlic
- 29g salt
- 58g ship koji
Bag 2: paste tomatoes (Ten Fingers of Naples, Piennolo del Vesuvio, Korean Long)
- 1912g paste tomatoes
- 10g garlic
- 38g salt
- 76g ship koji
Bag 3: cherry tomatoes (Honey Drop, Cherry Ember)
- 1445g cherry tomatoes
- 6g garlic
- 29g salt
- 58g shio koji
These were set in aluminum trays in the basement as I flew off to another part of the country for almost a week.
2025-09-05: Deflating & Cooking
It’s been about 2 weeks and the bags have inflated to the point where at least one has popped.
I deflated and resealed the beefsteak and plum bags, but the cherry bag had popped and I figured I might as well test my approach to ketchupification.
While each had liquefied considerably, each bag did retain some unique aroma and flavor characteristics. The beefsteaks were savory, the plums full-bodied, and the cherries fruity. Of course all smell a bit like pickle soup now as well, particularly owing to the garlic. I’m glad I didn’t add more than one clove to each bag, otherwise it would have likely been overpowering.
As I said, I did not plan ahead much, though I did add a bit less salt than usual knowing that I’d probably be concentrating it later. That means I did also plan to cook the tomatoes, as I would any time I’m making sauce from whole fresh tomatoes. In order to mill them well, and also to extract as much flavor as possible, I cook them with the skin and seeds all together until broken down.
Once cooked down to the point that the liquid did not quickly cover the bottom of the pot when pushed to the side, I put it through my hand mill and then continued cooking the pulp down until I was satisfied with the consistency (and frankly tired of cooking).
By this time it was late and I was tired, so I put the cooked pulp in a jar in the fridge and figured I’d finish it the next day.
2025-09-13: Ketchupification
And so, a day week later, I was finally ready to finish the ketchup.
I had about 380g of thickened tomato pulp. It tasted good but not quite like ketchup, still more like pickle paste. Also, the texture was still a bit pulpy, and I wanted something smoother.
Keen-eyed readers might recognize the vinegar bottle from the Tomato Vinegar Experiment (3 Ways) logbook, this is the version made by adding soju and it has darkened significantly in 4 years.
I put the pulp in my blender and added sugar and vinegar a tablespoon at a time until it tasted right, which ended up being 2 tablespoons (30g) each. Not too sweet, nice kick, a fruity, complex flavor that is still recognizable as ketchup, and most important of all: the pH tested at 3.45, well below the 4.6 considered safe. The blender also got the consistency nice and smooth. I don’t have an empty ketchup bottle, but this mayo squeeze bottle should do the trick.
I might make ketchup with the other 2 bags unless I think of something else more creative, and will update this logbook if so. If you’ve got ideas for what else to use fermented tomatoes for, let me know in the comments!
