The answer does not lie with clever engineering, but with carefully designed pastes.
Coloured pastes are loaded into the tube separately but are not stored in compartments. The pastes are designed to have just the right viscosity or thickness so that they don’t mix, acting like solids inside the tube but flowing smoothly when you squeeze or pump it.
They have similar flow characteristics to ketchup, which only gets moving with a good shake or a squeeze. In some toothpastes, a nozzle at the top of the tube has separate holes for different colours, ensuring they emerge as neat stripes.
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Asked by: Tina Chapman, Colchester
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Dr Emma Davies is a science writer and editor with a PhD in food chemistry from the University of Leeds. She writes about all aspects of chemistry, from food and the environment to toxicology and regulatory science.
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