Stress-strain relationships for gellan gels in tension, compression and torsion

Abstract

Formulated foods often contain small amounts of polymeric ingredients that interact to form three-dimensional networks which stabilize structure and provide desirable textural quality. One goal of food engineers is to understand how gels are formed and to be able to predict their mechanical properties. We have studied calcium-crosslinked gels of gellan, a polysaccharide recently introduced to the food industry, in order to understand their stress-strain behavior in tension, compression and torsion. This information will be applied when designing stability and texture for a variety of food systems. Gels were prepared by dispersing gellan polymer in 90 °C water, adding calcium chloride and cooling in molds to form the test specimens. Four levels of polymer, ranging from 0.6 to 1.8% w/v, were used with seven calcium concentrations between 1.5 and 60 mM. Molds of differing construction provided specimen shapes that were cylindrical with enlarged ends for tensile tests, cylindrical for compression tests and capstan-shaped for torsion tests. Specimens for tension and torsion testing were modified with plastic adapters to facilitate attachment in mechanical testing machines; whereas compression was carried out between lubricated parallel Teflon plates. Gel properties were influenced strongly by the content and combination of polysaccharide and ion. Shear stress relationships in small deformations were almost identical in the three testing modes for a given gellan and calcium concentration. In large deformations, gels were more rigid in tension than in compression and torsion, but they failed at the same maximum shear stress regardless of testing mode. Stress-strain responses were analyzed using constitutive equations based on energy functions. Mooney-Rivlin constitutive equations were applicable for stress-strain relationships for small strains; however, equations based on three-term energy functions fitted the data more accurately at larger strains.