Why is ketchup so hard to pour?

Mmmm. Ketchup. What would hot dogs, hamburgers or french fries be without it? Even if you don’t like ketchup, you probably know people who eat it all the time. 

Before the days of squeezable plastic bottles, ketchup typically came in glass bottles (some companies still sell ketchup this way). When you wanted to put some on your french fries you would tip the bottle and wait… and wait… When you finally grew tired of waiting, you would shake the bottle. Or even give it a few good whacks and then all of a sudden there would be ketchup everywhere!

So why doesn’t ketchup flow nicely out of the bottle, like water or juice? It is a liquid… isn’t it? Well, sort of. We’ll get into more about that later.

How do different fluids flow?

The ability to flow is the essential characteristic of a fluid.

Misconception Alert

The terms “liquid” and “fluid” do not mean the same thing. All liquids are fluids, but not all fluids are liquids. Both liquids and gases can flow -- which is what makes them fluids.

Many fluids that you are probably familiar with, like water, gasoline, and cooking oil, flow freely and quickly. But what about when you pour maple syrup or honey? They flow much more slowly. This is because they have a high viscosity. Viscosity is the resistance of a fluid to flowing. The easier a fluid flows, the lower its viscosity. Since honey flows very slowly, it has a high viscosity. 

Two factors can impact the viscosity of a fluid:

1. Temperature
2. Pressure

Sir Isaac Newton (of Newton’s Laws) noticed that when some substances were heated, they had a lower viscosity, and when cooled, they had a higher viscosity. He called these types of fluids Newtonian fluids. Most fluids we deal with on a regular basis are Newtonian fluids. At a given temperature, the viscosity of a Newtonian fluid stays the same, no matter how much pressure is applied to it.

But not all fluids are Newtonian fluids. Some, like mayonnaise, toothpaste, and ketchup, are also affected by pressure. The viscosity of these fluids depends on how hard the force applied to it is (its magnitude) and how long the force is applied (its duration). Fluids that act this way are called non-Newtonian fluids.

What’s so special about ketchup?

Let’s go back to the ketchup bottle. When you open it and turn it upside down, nothing happens -- it doesn’t flow. This is because the ketchup is acting more like a solid than a liquid. Scientists actually call ketchup and fluids like it “soft solids” or “yield stress fluids” because they only flow when the right amount of force is applied. 

Did you know? 

Rheology is the field of physics that studies soft solids, including many common condiments, as well as paint and industrial applications like sewage treatment and mining.

Ketchup is a soft solid because of its ingredients. Ketchup is made from finely ground-up tomatoes, along with sugar, spices, vinegar, and water. If you look closely, you might be able to see all of the small bits of tomato that make up the majority of the ketchup. There are enough tomato bits that the solid parts are all touching each other. This physical connection of the solid parts gives the ketchup the strength to resist flowing.

What’s the best way to get ketchup to flow?

To get it to start ketchup flowing, you can shake or tap the bottle. If you can apply enough force all at once, the ketchup begins to act like a liquid and flows out of the bottle. At this point the ketchup is actually much less viscous than it was before it started flowing! But be warned -- sometimes it can come out quickly and splatter ketchup everywhere.

A less messy method is tap or shake the bottle gently and repeatedly. In this case, a smaller force over a longer period of time will get you to the same result.

Did you know? 

Heinz scientists have figured out that the optimal flow of ketchup is roughly 0.045 kilometres per hour.

But why do both of these methods get the ketchup to flow?

When you tip the bottle, the strength of all the tomato bits sitting together prevent it from flowing. But if you hit the bottle hard enough, some of the tomato bits will shift, which opens up room for more of them to move. With enough force, the ketchup as a whole will begin to flow. Why it works when you hit the bottle gently for a longer period of time is still a bit of a mystery. Scientists and engineers who study soft solids are actually researching this! 

Mmmm...researching ketchup. Sounds like a tasty career!