Nano-mechanical properties of starch and gluten biopolymers from atomic force microscopy

• Scratching with AFM tip was used to screen polymer resistance.
• A tribological model was developed to determine polymer hardness.
• The methodology is used to reveal hardness contrasts in biomaterials at nanoscale.
• Wheat starch was found to display higher hardness and shear strength than gluten.

An original method based on atomic force microscopy (AFM) in contact mode was developed to abrade progressively the surface of tablets made of starch or gluten polymers isolated from wheat. The volume of the material removed by the tip was estimated from the analysis of successive topographic images of the surface, and the shear force was measured by keeping a constant normal force. Our data together with a simple tribological model provide clear evidence for a higher hardness and shear strength of starch compared to gluten. Gluten appears to have mechanical properties close to soft materials, such as talc, whereas starch displays higher hardness close to calcite. Our results are in a better agreement with structural properties of gluten (complex protein network) and starch (granular and semi-cristalline structure) than earlier studies by micro-indentation. This work shows that the AFM scratching method is relevant for the characterization of any polymer surface, in particular in application to materials made of different polymers at the nano-scale.

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