Tomato paste is, in principle, an easy thing: squish the tomatoes, separate the skins and the seeds, and reduce the resulting pulp until it’s very thick. I have already posted a Roasted Tomato Paste recipe – here’s a version using a slow cooker or two, for when you can’t stand the idea of using the oven.
Necessity is the mother of invention. Sometimes you hit a roadblock; something knocks your plans horribly askew. So it was for me this week, when I planned to reduce a half-bushel (25-26 pounds) of tomatoes to thick tomato paste.
We had washed, quartered, squished and strained the entire basket, and put the sauce into two large flat hotel-style pans. The pans went into the oven at 425˚F for what was intended to be 4 to 5 hours of reducing.
But the oven stopped working, and wouldn’t start again. (It turns out the electronic board that controls operation just crashed. Crashed hard. I never crashed a stove before.)
Faced with 10-11 quarts of hot tomato pulp and no working oven, I decided to try to make tomato paste in my two large slow cookers. I filled them quite nearly to the brim and set them on HIGH. I laid two chopsticks across the ends of both these long ovals, and rested the lids on top, so there was about a half-inch gap, which I hoped would provide opportunity for evaporation. After an hour or two, I checked the pots, and found moderate bubbling around the edges, but no active boiling. I decided to let them work. It had to be better than stirring a big pot on the stovetop for hours and hours!
24 hours later, I was pleased that there had been no browning or scorching, and the tomatoes had reduced by nearly half. I was able to combine the pulp into just one pot: my largest 6-quart slow cooker. I set the lid on the chopsticks again, and let it continue.
In another 8 hours, the tomato pulp was reduced almost by half again, and was clearly becoming thick. I didn’t like the rough texture, though, so I took my immersion blender, and blitzed the tomato glop into silky smoothness: progress! I tested it: I dropped a spoonful on a saucer, and tilted the saucer. Darn, clear liquid drained, so it’s still not thick enough!
It was 4 hours later when the tomato paste showed thick enough that a spoonful dropped on a saucer wouldn’t separate at all, and would retain its shape. It took 30 full hours in the slow-cooker on HIGH, with the lid(s) propped up to permit evaporation. I continued as in my original recipe, putting the paste in 4-ounce jars, adding a bit of citric acid, and sealing in a waterbath for 45 minutes.
Good news: you don’t need an oven to make tomato paste. You don’t need to stand over a hot pot and stir so that the tomatoes won’t scorch. You do need patience, though, and a large slow cooker or two. Flat large cookers will work faster than tall ones for this purpose; the more surface area, the better. I wish I had taken photos all throughout this process. Next time, I promise I will.
Result: from a half-bushel of Roma tomatoes, 24 4-ounce jars of slow cooker tomato paste, and the satisfaction of establishing a new easier cooking method to use in the future.
Tomato Paste in the Slow Cooker
- ½ bushel ripe paste (Roma) tomatoes (about 25 pounds)
- citric acid
- Wash and cut the tomatoes. Check each fruit, cut out any blemishes or moldy spots. Cut the tomatoes lengthwise in quarters.
- Put the tomatoes through a strainer – follow the directions for the one you have. I use an attachment for a KitchenAid stand mixer. Extract the most pulp that you can.
- Put the tomato pulp into one or more large slow cookers. Set the cookers on high and cook, covered, for 1-2 hours.
- At the end of that time, check that the tomato pulp is hot and bubbling around the edges. Now prop the lid with chopsticks or wooden spoons, so that there's about a half inch of airspace. This permits evaporation.
- Cook for 24 hours or so. Stir occasionally if you'd like. When you can dit all the pulp into one slow cooker, feel free to do that. If the texture of the tomato pulp is a bit lumpy, whiz it with an immerzion blender to make it smooth.
- Continue cooking with the lid propped open until the tomato paste is thick and uniform. Test it: if a spoonful plopped on a saucer does not separate, and retains its shape, it's done.
- Ladle the tomato paste into prepared 4-oz jars, leaving ½ inch headspace. While I prefer the smaller jars, the National Center for Home Food Preservation says you may can tomato paste in (at largest) 8-oz jars. Add citric acid to each jar: ⅛ teaspoon for 4-oz jars, and ¼ tsp for 8-oz (half pint) jars.
- Place lids and bands on jars. Process in a boiling water bath 45 minutes (the time is the same for 4-oz and 8-oz jars.)
- Let cool, label, remove bands, and store in a dark place.