Tomato sauce in glass jar on white background
The Sweeter Ingredient Exchange For Tomato Sauce
Ketchup is much sweeter and more acidic than a standard tomato sauce, but it’s actually a suitable substitute if your recipe only calls for a small amount of sauce.
Ketchup’s tangy and slightly sweet flavor works with many recipes and provides a hint of tomato flavor, much like a spoonful or two of tomato sauce would.
There are even dishes that are actually more traditional when made with ketchup over tomato sauce, such as classic Japanese "Napolitan" spaghetti.
It may not be your mother's marinara, but ketchup's versatility makes it suitable for many cuisines, and its thick texture can also help bind sauces and condiments together.