Perspectives of micro-mechanical assessment of the apple fruit cuticle

The cuticle is a natural polymeric membrane that covers the surface of aerial organs (including fruit) of terrestrial plants. The cuticle membrane mainly consists of cutins, waxes, and polysaccharides and serves as a protective barrier against water movement, adverse environmental conditions and the invasion of pathogens. Fulfilling its barrier function requires an intact cuticle. During fruit development, the cuticle is stretched and heavily stressed as the fruits grow exponentially in phases. In apple fruit, cutin and wax are synthesized constitutively throughout development. The newly synthesized cutin is deposited on the cuticle’s inner side. This results in a gradient of age and strain in the cuticle from outside (older, more strained) to inside (younger, less strained). Wax is deposited mainly within the cutin network and fixes the elastic strain of the cutin network. Studies indicated that wax concentration among the different layers of the cutin also varies with more wax in the outer cuticular proper and less in the inner cuticle layer. ^{1, 2} As a result, the cuticle of mature apple fruits exhibits a complex micromechanical structure. Characterizing this structure poses challenges that cannot be overcome using conventional tensile testing methods. In this study, we employed a Brillouin scattering setup to investigate the micromechanical structure of the cuticle.