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What Makes Ketchup Different From Other Fluids?

Last Modified Date: January 20, 2024

When Henry J. Heinz started selling ketchup (well, "catsup") in 1876, he put his tomato concoction in a glass bottle to show how unadulterated it was. Maybe he didn’t realize how difficult it would be to get the stuff out.

Technically, ketchup is a “soft solid,” or a “non-Newtonian fluid,” which basically means that it will only move when the right amount of force is used – in contrast to liquids such as water that flow at a constant rate.

Ketchup is a non-Newtonian fluid that behaves like a solid; its viscosity changes depending on how much force is applied.

Ketchup is a suspension of pulverized tomato solids in a liquid, and the continuous network of ingredients (which also includes distilled vinegar, brown sugar, salt, and spices) gives it the strength to resist motion. Shaking or tapping the bottom will often persuade the sauce to come out, but it’s also easy to end up with more than you bargained for on your burger or fries. That's because once you've applied enough force, the viscosity changes and it becomes much thinner. Thankfully, manufacturers often sell ketchup in plastic squeeze bottles these days.

The physics of ketchup:

Heinz scientists have determined that the optimal flow of ketchup is roughly 0.028 miles per hour (0.045 km/h). The variable involved is how much you whack the bottle.

Other non-Newtonian fluids include toothpaste, paint, mud, industrial products like cements and mortars, and Silly Putty.

Isaac Newton’s law of viscosity states that a fluid flows at a speed proportional to the force applied, where the constant of proportionality is the viscosity. Because ketchup doesn't obey this law, it's known as a non-Newtonian fluid.