Answer: Shearing Forces

Tomato ketchup isn’t just a delicious and widely used condiment, it’s a veritable science experiment in a bottle. In order to create a topping that can be applied to food without running all over the place, various thickening agents are added to the ketchup, including a tiny bit of xanthan gum. The addition of the xanthan gum gives the ketchup a really novel and curious property: it acts as a pseudoplastic with a shear thinning property. In other words, when it is at rest it is resistant to moving, but when the right amount of shearing force is applied, it changes the viscosity of the ketchup and it suddenly moves very quickly.

In fact, this is exactly why Heinz Ketchup bottles have a small circle with a 57 inside it located on the neck of the bottle. It’s not just a branding trick, that logo was placed in the glass bottle mold at the optimum place to tap the bottle with the back of a knife or other piece of silverware to apply the most efficient shearing force to the ketchup and trigger movement. After the ketchup flows out of the bottle and is no longer exposed to the shearing force, it thickens again to its original viscosity.