Wall material properties of yeast cells. Part II. Analysis

Abstract

In the preceding paper (Part I) force-deformation data were measured with the compression experiment in conjunction with the initial radial stretch ratio and the initial wall-thickness to cell-radius ratio for baker's yeast (Saccharomyces cerevisiae). In this paper, these data have been analysed with the mechanical model of Smith et al. (Smith, Moxham and Middelberg (1998) Chemical Engineering Science, 53, 3913-3922) with the wall constitutive behaviour defined a priori as incompressible and linear-elastic. This analysis determined the mean Young's modulus (Ē), mean maximum von Mises stress-at-failure (σ̄(VM,f)) and mean maximum von Mises strain-at failure (ε̄(VM,f)) to be Ē = 150 ± 15 MPa, σ̄(VM,f) = 70 ±4 MPa and ε̄(VM,f) = 0.75 ± 0.08, respectively. The mean Young's modulus was not dependent (P ≥ 0.05) on external osmotic pressure (0-0.8 MPa) nor compression rate (1.03-7.68 μm/s) suggesting the incompressible linear-elastic relationship is representative of the actual cell-wall constitutive behaviour. Hydraulic conductivities were also determined and were comparable to other similar cell types (0-2.5 μm/MPa s). The hydraulic conductivity distribution was not dependent on external osmotic pressure (0-0.8 MPa) nor compression rate (1.03-7.68 μm/s) suggesting inclusion of cell-wall permeability in the mechanical model is justified. ε̄(VM,f) was independent of cell diameter and to a first-approximation unaffected (P ≥ 0.01) by external osmotic pressure and compression rate, thus providing a reasonable failure criterion. This criterion states that the cell-wall material will break when the strain exceeds ε̄(VM,f) = 0.7 ± 0.08. Variability in overall cell strength during compression was shown to be primarily due to biological variability in the maximum von Mises strain-at-failure. These data represent the first estimates of cell-wall material properties for yeast and the first fundamental analysis of cell-compression data. They are essential for describing cell-disruption at the fundamental level of fluid-cell interactions in general bioprocesses. They also provide valuable new measurements for yeast-cell physiologists. (C) 2000 Elsevier Science Ltd. All rights reserved.