Mechanical properties of sugar beet Ca-pectate gel usable for cell immobilisation and heavy metal accumulation

After alkaline hydrolysis, pectin extracted from sugar beet pulp is able to form gels that can be used for whole cell entrapment and heavy metal accumulation. The purpose of the present work was to appraise the ability of sugar beet pectin (SBP) gel particles to accommodate the stresses encountered in bioreactors. We investigated the rheological properties (mechanical behavior) of SBP gels shaped as flat membranes and compared them to those of citrus pectin and algal alginate gel membranes. We evaluated the fracture properties of the different gel membranes by constructing stress-strain curves (obtained by uniaxial compression at constant speed). Stress and strain at fracture, Young's modulus, fracture and flow energies were determined. SBP and citrus pectin gel membranes were less resistant to stress than algal alginate membranes, with stresses at fracture of 1.7 x 10(5) N m(-2)(SBP) and 4.83 x 10(5) N m(-2) (citrus pectin) as compared to 2.44 x 10(6) N m(-2) for alginate. A chain flow phenomenon prior to fracture was observed in the case of pectin gel membranes (flow energies: 3500 J m(-3) for SBP; 4400 J m(-3) for citrus pectin, negligible for alginate). These results show that pectin gels are more heterogeneous than those of algal alginate and behave as a viscoelastic material, in opposition with alginate gels that display an elastic behaviour. The relatively low mechanical resistance of SBP gels may impede their long-term utilization in the biological treatment of wastes. (C) 2000 Elsevier Science B.V. All rights reserved.