To stop that, you have to “turn off” the enzyme by exposing it to heat. Since it’s hard to heat the tomatoes quickly enough with home equipment, there is a trick to that.
First, take just a few of the tomatoes and cut them into quarters or halves. Place those in the pot and heat while crushing them. Once you’ve brought the mixture to a boil, continue adding the rest of the tomatoes cut into large chunks, crushing them as you add them. Keep them boiling until you’ve added all of the tomatoes, then simmer 5 minutes longer.
Continue with the recipe, following the directions for safely canning the juice, and it should stay together.
About the Author