It will grow mold unless you use a pressure cooker with sealed lids.Speaking of tomato paste, that's what I've decided to make with most of the rest of my tomatoes this year. Seems pretty easy, but it just takes a long time.
Cut up whole tomatoes, cook them for a while and strain out seeds, stems and cores. Then reduce the juice down until you can scoop out a rounded spoonful. I'll reduce it on the stove for a while then transfer it to my Crockpot to finish. It'll be canned in 8oz jelly jars.