This recipe takes everything you love about store-bought ketchup and packs in even more tomato flavor—get your burgers and fries ready!
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Add all ingredients to 12-inch nonstick skillet. Use rubber spatula to stir to combine.
Bring tomato mixture to simmer over medium-high heat (small bubbles should break often across surface of mixture). Reduce heat to medium and simmer, stirring occasionally and scraping bottom of skillet, about 20 minutes.
Ask an adult to carefully transfer tomatoes and liquid to blender jar. Place lid on top of blender and hold lid firmly in place with folded dish towel. Turn on blender and process mixture until smooth, about 1 minute. Stop blender.
Set fine-mesh strainer over medium bowl. Pour tomato mixture into fine-mesh strainer. Use rubber spatula to stir and press on mixture to push liquid through strainer into bowl. Discard solids left in strainer.
In your refrigerator, you’ll probably find ketchup in an easy‑to‑squeeze plastic bottle. But it wasn’t always so simple to add a squirt of ketchup to your burger. For most of the 1800s and 1900s, ketchup was served in glass bottles. And if you’ve ever tried to get ketchup out of a glass bottle, you’ll know that it’s really hard! (We recommend asking your grown-ups about this.)
When you turn the glass bottle upside down, the ketchup stays stubbornly in place. But if you give the bottle a good whack, all of a sudden you’ve got a ketchup landslide on your plate. It’s hard to get ketchup out of a glass bottle because ketchup is a non-Newtonian fluid. Newtonian fluids (named after famed scientist Sir Isaac Newton), such as water and oil, pour in a steady, even way from their containers.
Non-Newtonian fluids don’t behave as nicely. Some of them, such as ketchup, mayonnaise, and toothpaste, won’t start flowing unless you apply a force to them—like that whack on the bottle. The added force changes ketchup’s texture from thick to thin. On a microscopic level, bottled ketchup is held together by tiny pectin molecules from the tomatoes and tiny xanthan gum molecules. When you whack the ketchup bottle, the xanthan gum molecules lose their grip on each other, and the ketchup is free to flow.