Why Does Ketchup Flow More Easily When Squeezed?

Turn a ketchup bottle upside down and the sauce may sit there almost like a solid. Squeeze it, and it can suddenly flow. The bottle has not simply “shaken it loose”; ketchup responds to force differently from water.

Water is approximately Newtonian: at a fixed temperature, its viscosity does not change much with the rate of stirring. Ketchup is a complex suspension containing fine solids, pectin and thickening ingredients that form a temporary internal structure. A weak force may not produce sustained flow at all; the applied stress must first exceed a yield stress. Once flow begins, stronger shear helps particles and structures align with the flow, lowering the apparent viscosity. This is shear thinning.

Squeezing therefore does two things. It pushes the stress past the threshold for flow and creates strong shear through the narrow outlet, making the sauce easier to move. When squeezing stops, structure gradually rebuilds, so the ketchup can hold its shape on a chip. Formulation, temperature and resting time all affect the result. Calling every ketchup “thixotropic” goes too far: thixotropy specifically means a reversible change in viscosity with the duration of shearing, which cannot be established merely by watching sauce flow after one squeeze.

https://pmc.ncbi.nlm.nih.gov/articles/PMC10048648/