Prepare Your Tomatoes: Wash the tomatoes thoroughly to remove any dirt. Using a knife remove the cores and roughly chop them. Add them to your stockpot.
Simmer & Reduce: Bring the mixture to a gentle simmer while occasionally stirring it to prevent sticking on the bottom. Cook for 1-2 hours allowing the excess liquid to evaporate.
Blend Tomatoes: Allow the tomatoes to cool about 15 minutes. Add the tomatoes into the blender and blend on the low setting for 30 seconds to puree. Continue this process until all tomatoes have been blended. Add the blended tomatoes back into the stockpot.
Season Sauce: To the sauce add the vinegar, sugar, cinnamon, salt, pepper, celery seed, mustard powder, garlic powder, and chopped onion. Stir until the seasonings are thoroughly mixed. Allow to cook over medium heat for 30 minutes.
Strain Sauce: Place the strainer over a large bowl and begin straining the sauce. You will be separating the juice. Do NOT throw away the sauce/meat of the tomatoes reserve both the juice and the strained sauce. The strained juice is what we are going to be using to make the ketchup. Set the sauce aside in a separate bowl. The sauce can be used for so many things! I usually use the sauce to make homemade marinara. If you need a marinara recipe you can get my recipe here.
Thicken Juice: Add the juice back to your stockpot and you can either allow the tomato juice to continue to cook down even further. This can take several hours depending on the type of tomatoes you used. If you don't want to take the time to cook the juice down to thicken you can add in ThermFlo. Over medium heat bring the juice to a boil and then simmer. Do NOT pour the ThermFlo directly into the hot juice. Instead remove about 1/3 cup of the tomato juice and allow it to cool to room temperature. While the bowl of juice is cooling turn the Once the juice is cooled add 1/3 cup of ThermFlo to the bowl of cooled juice. Whisk the ThermFlo until it is fully incorporated. Slowly Whisk the ThermFlo mixture into the stockpot of juice until the mixture thickens. This should take less than one minute.
Fill Jars: Using a canning funnel, carefully ladle the hot ketchup into clean mason jars, leaving about an inch of headspace at the top. This will allow for proper sealing and expansion during processing. To each jar add ¼ tsp of citric acid if your using quarts or ⅛ if your using pints. You can also use bottled lemon juice if you do not have citric acid. (You will have to adjust the measurement for lemon juice.)
Remove Air Bubbles: Using a debubbler remove any bubbles.
Wipe Rim, Add Lids, and Rings: Using a clean kitchen rag and vinegar wipe the rims of each jar to remove any food or water that may be on the rims that could prevent the jars from sealing. Using a magnetic lid wand place your clean lids on each jar and add a ring to each jar.
Process Jars: Place your filled jars into your water bath canner and process for 40 minutes for quarts or 35 minutes for pints once the water is at a rolling boil. Remove jars and place on a clean kitchen towel and let cool. Check each lid for a proper seal after 24 hours.