The elastic modulus is proportional to the initial slope of the force/distance curve. But the factor of proportionality depends on the shape of your sample. Are you squashing a cylinder or pressing a piston into a dish?

Squashing a cylinder is easier, as the stress is the force divided by the sample's cross section and the stress is distance compressed divided by initial height. This is a naive version. There are various corrections (Hencky strain, effect of lubrication at the piston/gel interface, and maybe others), as always in rheology :-).

For the piston into dish geometry, I can dig out an old reference in Journal of Texture Studies if you need it. It's by an Australian, if my goldfish memory serves me well. Tell me.

Thanks a lot for the response Alan Parker .

I conducted the indentation experiment using Texture Analyser. In the test, a spherical base geometry is indented to the gel at the speed of 0.15 mm/min. The loading and unloading Force vs Displacement data were noted.

I see. A sphere indenter is more complicated than a flat piston, as the contact area changes at the same time as the strain increases. I am not aware of an analysis of this geometry. There may well be one, but I have been out of the area for many years.