Analysis of the elastic modulus of agar gel by indentation

This study used the approach taken by Bourne to obtain the compressive and shear components for an agar gel. But in addition, the low strain regime for the same experiments was used to evaluate the gel's elastic modulus using the theory of elasticity. Compression testing, using lubricated teflon platens and a travelling microscope, was used to independently measure the gel's elastic modulus (52 kNm(-2)) and Poisson's ratio (0.32). The compressive and shear coefficients compared favourably with analyses performed by other researchers, and indicated that shear dominated in the failure of the gel under the probe. Indentation measurements of reaction force (P) and deformation (h) were analysed with the theoretical relationship for frictionless flat cylinder indentation. A good linear fit (r(2) = 0.991) was obtained for P/h versus indenter radius, indicating the validity of the analysis. The value of the elastic modulus derived from indentation was 79 kNm(-2), an overestimate of 52%. The larger value suggests that additional factors to those considered in the theoretical model influence the indentation process, such as the presence of pores in the gel, frictional forces during indentation, and elastic mismatch between indenter and gel. Similarity in results for the elastic modulus determined from indentation compared to compression, indicates that a simple testing method can yield a more comprehensive evaluation of food textural quality.