Tomato Paste in the Slow Cooker

tomato paste

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.

the backstory

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!

slow progress

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 36 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.

in summary

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

Tomato Paste in the Slow Cooker

Course: Sauce
Cuisine: American
Keyword: slow cooker, crockpot, tomatoes, preseving, DIY
Prep Time: 1 hour
Cook Time: 1 day 18 hours
Total Time: 1 day 19 hours
Servings: 24 4-oz jars
Make tomato paste at home the easy way: use a slow cooker.
Print Recipe


  • 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 immersion 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.


I like to take some of my tomato paste and freeze it in a silicon ice cube tray. It's so very handy to have a bit of paste to use as needed. A standard ice cube tray's wells hold roughly 2 tablespoons.


  1. Janette

    Thanks Maurita for a helpful Blog Post. I read the comments above with interest also + having seen a typo (had to search to find it again) I thought it would be helpful to show you the one in the quote below… ‘dit’…plus we don’t have anything other than a bench top blender so I searched the one you recommend and think perhaps it was spelt incorrectly too = should it be “immersion blender” ??

    “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. ”

    I am looking forward to trying a slow cooker for Tomato Paste now that I have already used it to finish some Tomato Sauce (a friend’s recipe) and I might even make some Tomato Soup! = I miss the slow combustion stove at our precious farm as it was excellent for dealing with gluts of tomatoes! I like your idea of using chop sticks 😀 Thanks again.

    • Thank you! I’ve fixed the typo, and found a math error: 24+8+4 does NOT equal 30, so I fixed that one too. I hope this will work well for you.

  2. Laura Hansen

    Why do you take the bands off of the jars after the water bath?

  3. Do I need to actually add the citric acid, or will the tomato paste be fine without it?

    • If you plan to can the tomato paste, then yes, you definitely need the citric acid.

  4. Jeannine L Petet

    I’ve been clicking on link after link with a great deal of frustration at the terrible state of decay in our use and understanding of simple English. Tomato sauce and tomato paste are TOMATOES, with the possible addition of citric acid, salt, or sugar for taste and preservation. Yet one after another recipe is really mislabeled pasta or pizza sauce. Good stuff, but not what I wanted.
    So far, very little has worked out, but I’m not quitting yet. My gardens have been devoured not by those lovely but pesky deer but by ravenous groundhogs. I live in an old mobile home in central Missouri, all metal, without air, so summer cooking is out. And I’ve yet to figure out a way to can cuz there’s no way my hot plate cooker will boil more than a saucepan of water. Maybe. But I can dehydrate & freeze. So it finally dawned on me I ought to be able to slow cook sauce without adding to heat inside. Can even set it outside since covered. (Was boiling pasta on deck one night when little tree frog lost footing and plopped into the pot – eww)
    And finally, I found your site! Yahoo! Perfect! And I love the format. Simple. Clean. Few distractions. Little relevant tales without a lot of reports on vacations and blah blah blah that have no bearing on the how or why of the recipe. At least so far. I love following links down tasty or informative rabbit trails, but sometimes I wonder where the recipe went. Thank you. Will try this shortly with a small batch of end of season farmers market romas, and plans to box in all my raised beds in 2017. AND to come back and explore this site more thoroughly!

    • First of all, I do hope that you have great success with your outdoor slow cooker – I wouldn’t want a frog in my pot either!

      The design of my site separates the recipes into their own posts, so you might like simply looking through the ‘recipes’ section. If you’re looking for something specific, the search box is labeled ‘find the good stuff’ and works pretty well.

      And lastly – if you find a misspelling or a typo, please let me know so I can correct it! I do try for accuracy, but sometimes pesky errors do creep in. Thanks!

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I’m Maurita Plouff, and I write about cooking and preserving the local harvest in Southeast Michigan. Any recipe you find here is something I have cooked myself, and enjoyed, and think you might like too. I invite you to try the recipes, and leave comments.

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