Rheologica Acta, Vol.59, No.4, 201-208, 2020
Determination of poro-viscoelastic properties of vegetal tissues as a function of moisture content by means of stress relaxation tests
Because of their great economic and social impact, coffee plants have become the center of interest in different fields of study, from biology to medical sciences. In this work, the determination of the poro-viscoelastic behavior of coffee leaves as a function of moisture content is presented. Destructive stress relaxation tests in tension and indentation were carried out, which showed that the rheological response of the leaf samples is mainly governed by the viscoelastic properties. Finite element poro-elastic models, in which the intrinsic viscoelasticity of the solid phase is modeled using Prony series, were also implemented in order to emulate the observed experimental behavior. Results allowed us to state that as the moisture content is diminished, the stiffness of the vegetable tissue decreases and its viscoelastic properties are increased. This is attributed to the loss of turgor pressure (mechanical stability) and the formation of empty spaces when leaf samples lose water. The moduli of elasticity obtained from the tension tests were greater (18-36 MPa) than those measured for the samples subjected to indentation (2-5 MPa). The equivalent viscoelastic parameter was between 0.36 and 0.53 for tension and between 0.76 and 0.90 for indentation.