Why won't my homemade tomato ketchup thicken even though I cooked it for hours?
If tomato puree, paste or ketchup has not thickened after several hours of cooking, the most likely reason is that the product is being cooked in a covered pan. These products must be cooked uncovered to reduce the volume through evaporation. Use fairly high heat. Stir frequently to prevent sticking.
Barbara Willenberg, Nutritional
Sciences, University of Missouri-Columbia
Last update: Tuesday, May 05, 2009