On the pouring of ketchup

It’s not easy to find the perfect topic for a mid-summer science column, when people are more interested in getting to the lake, swimming in the pool, or barbecuing in the backyard than–

Wait, wait. “Barbecuing in the backyard…”

I think I’ve got it–the perfect summer science topic!

Thanks to Robert Allgeyer, whose definitive paper on the topic can be found online, I can offer you sage scientific advice regarding the best way to get ketchup from a traditional glass narrow-mouthed bottle.

After all, who hasn’t struggled to get that red elixir of tomatoey goodness out of a bottle and onto the fries where it belongs?

As Allgeyer points out (and as I remember pointing out back when I worked at the Saskatchewan Science Centre), ketchup exhibits thixotropic tendencies.

A thixotropic substance is not one that is thick, though that does seem to apply to ketchup. Rather, it’s a solid that loses its solidity when agitated. At the Science Centre we created a thixotropic mixture of corn starch and water. You can stir it and pour it, but if left alone it looks solid. You can also hit a puddle of it in a plate with the palm of your hand without it splashing: instead, it becomes even more solid. Very weird, but a lot of fun to play with (only, don’t try the slapping a puddle with the palm of your hand thing with ketchup. Or if you do, don’t send your dry-cleaning bill to me.)

The problem with the narrow-mouthed glass bottle is that, in order for ketchup to flow out of it, air must flow in. Air pressure acts like a thumb pushing the ketchup into the bottle. Once air gets into the bottle, air pressure is no longer a factor, because it’s the same inside and out.

Because ketchup in its undisturbed state is very solid, and both holds its shape and clings to the sides of the bottle, gravity alone is unable to pull the ketchup from the bottle and through the air pressure plug at the bottle’s neck.

This leads to tapping. Most people, having failed to get any ketchup out by turning the bottle completely upside down, bang the bottle’s bottom. If a little air has gotten into the bottle already, this may liquefy and dislodge a small amount of ketchup. But usually the bottle mouth promptly plugs up again and you’re no better off.

You need to somehow move the ketchup to one side so air can get into the bottle. The answer should be obvious, since we do it with other liquids all the time: hold the bottle on its side, not completely upside down. (Nobody turns pop bottles completely upside down to get the pop out, after all!)

Gravity then pulls the ketchup down toward the bottom of the neck…but because the ketchup is still essentially solid, gravity isn’t usually enough. You need to somehow increase the ketchup’s weight so it is more likely to drop, opening the bottle’s mouth and allowing air to enter.

To increase something’s weight, you accelerate or decelerate it. Most people try to do this by tapping the bottle’s bottom again, but this is counterproductive: it actually drives the ketchup deeper into the bottle, just like you’re driven back into your seat if your car is rear-ended.

Tapping the top of the neck doesn’t work either, since it forces the ketchup up, not down, making it harder, not easier, for air to enter.

No, the correct method is to tap the bottom of the bottle’s neck. This accelerates the bottle neck upward. The ketchup has inertia–it doesn’t want to be moved from its resting state–and so it resists the upward movement, breaking loose of the bottle glass and pressing against the bottom of the neck. With luck, an opening is created, air rushes in, and the ketchup begins to pour.

As Allgeyer notes, “it would be possible to print instructions on the side of the ketchup bottle, but you wouldn’t read them if they were there, now would you?”

Of course, now that you’ve read this perfect summer science column, you don’t need them anyway.

Permanent link to this article: https://edwardwillett.com/2007/07/on-the-pouring-of-ketchup/

1 comment

    • Ian H. on July 17, 2007 at 5:03 pm
    • Reply

    Wow… all I can respond with is Ogden Nash:

    Shake and shake the ketchup bottle/
    First comes none, and then a lottle.

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