To thicken or not to thicken canned tomato products
Every year, home food preservers want to know how to thicken salsa or tomato soup with flour, cornstarch, rice, pasta or cream before canning. Never, under any circumstances, add a thickening product before canning. These thickening products will change the acidity level of your tomatoes and could also create a thick product that does not allow good heat penetration. As a result, adding thickeners before canning may result in an unsafe product.
Thicken tomato products with flour, cream, cornstarch, etc. just before serving.
To produce a thicker tomato product, try using Italian plum-style or paste tomatoes vs. large slicing tomatoes. Thicken salsas by adding tomato paste or by draining off some of the liquid after you chop the tomatoes. Also, thicken tomato products with flour, cream, cornstarch, etc. just before serving. If you do thicken your tomato soup mixture, it can be frozen successfully but never canned!