Tomato paste is made by cooking tomatoes to reduce their moisture, producing a sweet, rich concentrate. It is used in many recipes as a thickener, binding agent or sweetener. If you do not have tomato paste for a recipe, you can substitute with tomato sauce. However, you cannot simply add tomato sauce in place of paste. You must allow an extra 30 minutes to turn the sauce into something that can substitute for tomato paste in any recipe.
Preheat a sauce pan with medium heat on a stove top.
Pour 1 cup of tomato sauce, for every 1/2 cup of tomato paste called for in your recipe, into the preheated sauce pan.
Stir the tomato sauce with a wooden spoon until it comes to an even simmer. This prevents it from burning to the bottom of the pan.
Add 1 tbsp. of white sugar for every 1 cup of tomato sauce you used. Stir this for 60 seconds to dissolve all the sugar.
Allow the tomato sauce to simmer for approximately 30 minutes, stirring occasionally. Within 30 minutes, the tomato sauce should boil down to approximately half its original volume and begin to thicken.
Remove the sauce pan from the stove top and allow the thickened tomato sauce to come to room temperature. This may take up to an hour.
Stir the tomato sauce to even out its consistency. Use the thickened sauce just as you would tomato paste in the recipe.