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How Ketchup Works

Ketchup Physics
Ketchup is what scientists call a non-Newtonian fluid. That makes it hard to get out of the bottle. Kroeger/Gross/Getty Images
Ketchup is what scientists call a non-Newtonian fluid. That makes it hard to get out of the bottle. Kroeger/Gross/Getty Images

So, Americans fell in love with tomato-based ketchup. But there was just one problem: Ketchup didn't flow easily out of a bottle. You had to thump the bottom of the bottle to get it pouring, or stick in a knife or straw. That's because of ketchup's physics.

Ketchup is something scientists call a non-Newtonian fluid. Shampoo, mayonnaise and toothpaste are other examples of non-Newtonian fluids. Newtonian fluids — like water and oil— have a single viscosity. Thus, they always flow in the same manner. But non-Newtonian fluids, like ketchup, have more than one viscosity, or thickness, and these viscosities change depending on external forces [source: Adams].

When you thump the bottom of a ketchup bottle, the external force you're providing decreases— or thins out — the condiment's viscosity so it can more easily flow out. But if you hit the bottle too hard, the ketchup will shoot out, probably spewing more than you wanted.

For years, scientists tried to solve this dilemma. One idea was ketchup packets, which debuted in 1968. But they never became that popular, except at fast-food chains. In 1983, plastic squeeze bottles were unveiled. They released ketchup faster than glass bottles, but they tended to make funny, farting noises when squeezed. The plastic bottles also created "ketchup juice," the unappetizing, watery squirts that come out of a bottle when it's nearly empty (the industry term is serum.) Sadly, squeeze bottles still didn't give you that much control over where and how much ketchup shot out [sources: Adams, Poon].

The problem was solved in 1991 when the owner of a precision-molding shop named Paul Brown created a new silicone valve for liquids in plastic containers. This valve had right-angled slits cut into it that opened when you squeezed the bottle, allowing liquids to flow out neatly. But the slits also closed back up when you stopped squeezing, sealing the fluid back inside. The design was sold to shampoo companies and sippy-cup makers, and, years later, to ketchup companies.

In 2002, Heinz and Hunt's (its main competitor) introduced this new valve when they debuted upside-down bottles. The ketchup caps for these new bottles features a new grooved section that trapped ketchup juice and mixed it back in with the ketchup [source: Greve].

There still is one problem remaining with ketchup bottles, however. You can't squeeze all the fluid out of the bottle. (This also happens with other non-Newtonian fluids such as shampoo and toothpaste.) Luckily, this petty issue should soon be solved as well. Kripa Varanasi, a researcher at the Massachusetts Institute of Technology, recently invented LiquiGlide, a product that makes a surface slippery. If ketchup manufacturers coat the insides of their bottles with LiquiGlide, the consumers will be able to get every last drop out of them [source: Chandler].

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