The Perplexing Physics of Ketchup

We Americans love out condiments. And summer is the season of condiments, with a seemingly unending parade of opportunities to grill, picnic, and sit on the patio with happy hour fries and a beer. And when it comes to condiments, ketchup is king. In the U.S., the average person consumes a whopping 71 pounds of it each year!

Which is kind of amazing when you think of how difficult it can be to get it out of the bottle. Granted, there are now newfangled squeezy bottles and upside-down bottles, but have you ever wound up somewhere with the old-fashioned glass models and felt like a ketchup Goldilocks? Either nothing comes out or you shake too enthusiastically and it all comes out at once, leaving your food swimming in a pool of red. What’s the deal with that?

Well, it turns out, from a physics perspective, ketchup is pretty weird. In fact, it’s one of the more complicated substances out there. And that’s because it can’t seem to make up its mind about whether it’s a solid or a liquid. Ketchup – made of particles of pulverized tomatoes, suspended in water, vinegar, sugar and spices – is what’s called a non-Newtonian fluid. What that means, is that its thickness changes depending on the amount of force applied. Push hard and the molecules get knocked around, making it 1,000 times thinner (remember that pool of red on your fries?). But don’t push hard enough and ketchup behaves like a solid, stubbornly clinging to the walls of the bottle. So what’s a hungry condiment-lover to do?

Ketchup-pouring pros swear by the following: keep the lid on and give the bottle a few good shakes to “wake up” the particles. Then uncap, and slowly shake for a steady stream. Scientists are still trying to figure out just why this works, but you won’t have to wait for them to enjoy a perfect pour this summer.

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