Nanoindentation study on apple tissue and isolated cells by atomic force microscopy, image and fractal analysis

• Chemical isolation procedure of apple cells caused severe microstructural changes.
• Young's modulus of the tissue was higher than for isolated cells and showed a wider variability.
• The variability of the Young modulus distribution was characterized by fractal analysis.
• Texture parameters of the AFM images can be linked with nanomechanical properties.

Atomic force microscopy (AFM) was used to evaluate the mechanical properties of apple tissue (Malus domestica var. Golden Delicious) and isolated cells. Young's modulus (E) obtained for the tissue was 0.86 ± 0.81 MPa, while for isolated cells an average value of 0.63 ± 0.42 MPa was found. The fractal dimension of the distribution of E (FDE) was estimated to be 1.81 for tissue and 2.47 for isolated cells, which indicates more anisotropic structures in single cell topography in comparison with the tissue. Finally, an image texture analysis was performed and different values were obtained; for roughness Ra = 49.70 ± 23 and 18.00 ± 3, entropy Ent = 5.26 ± 0.66 and 5.70 ± 0.23 and for the fractal dimension of the texture image FDT = 2.55 ± 0.02 and 2.62 ± 0.03, for tissue and isolated cells respectively.Industrial relevanceAFM is rarely used to evaluate the structural and mechanical properties of food materials. AFM studies could provide insight on food properties. For example, the nanoindentation of cells by AFM could be used to follow-up changes to the mechanical properties that occur during the processing and storage of fruits with industrial relevance such as apple. Study of the nanostructure and nanomechanical properties of vegetable cells could lead to a more in-depth understanding of the relationship structure–functionality in foodstuffs. This could be applied to address several issues, such as extending the storage life of climacteric fruits, verifying the structural modifications of the tissues when they are chemically treated and having a better control of mechanical and structural properties. Currently, fractal and image analysis is gaining attention in the food science industry. Their application allows one to obtain quantitative information from nanoindentation data and AFM images which can be related to structural modifications which occur during ripening and processing of climacteric fruits. This methodology could be extended to other food materials.