High-pressure homogenizers are often used in tomato juice production to fragment tomato cells, to increase viscosity. Compared to emulsification, little is known about the mechanism of cell breakup in general and of vegetable cell breakup in particular. The objective of this contribution is to extend the mechanistic understanding of vegetable cell breakup in homogenizers, using a research methodology previously proven fruitful for emulsification. Four mechanistic suggestions for cell breakup are identified in the emulsification and cell breakup literature (turbulent inertial, turbulent viscous, laminar shear and physical interactions). Theoretical expressions predicting how cell fragment size should scale with homogenizing pressure, serum viscosity and flowrate are derived and compared to experiments. It is concluded that none of these commonly suggested mechanisms (or combinations thereof) comply with the data, suggesting that an altogether different mechanism controls cell breakup in homogenizers. Results are used to compare drop and cell breakup in homogenizers.