The article analyzes the deformation mechanism under uniaxial compression of cylindrical samples cut from the pulp of potato tuber. In its typical S-shaped stress-strain diagram, three characteristic zones of different types of dominant deformation can be distinguished. Clarified the boundary between the second and third zones. The rationale for changing the type of deformations in each zone by adding a new type to the previous one is presented. Grounded elastic deformation in the first zone, which is associated with stretching of the cell membranes. In the second zone, the destruction of individual cells distributed in the sample volume occurs and in the third zone — the predominant gradual unification of these destructions. An explanation is given of the characteristic brittle fracture of the sample with significant residual deformations at the end of the third zone. The erroneous determination of the deformations of the second and third zones as plastic (yield) was noted. In this regard, a new term was introduced — modulus of rigidity Z. The change in the modulus of stiffness Z with increasing irreversible deformations indicates structural changes in the flesh of the sample due to its gradually collapsing cell structure. The reason for straightening diagrams for samples from fresh, hard potato tubers is explained. Taking into account the type of deformations by zones allows us to reasonably proceed to the creation of criteria for quantitative assessments of the degree of flaccidity of potato tubers.