Characterizing the tissue of apple air-dried and osmo-air-dried rings by X-CT and OCT and relationship with ring crispness and fruit maturity at harvest measured by TRS

• The apple maturity at harvest is assessed by TRS.
• Microstructural changes in apple rings are found by X-CT and OCT.
• AED coupled to texture analysis reveals the mechanical properties of apple rings.
• Apple maturity and pre-drying osmosis affect microstructure of air-dried rings.
• The changes in microstructure found by XCT and OCT are related to ring crispness.

Air-dried apple rings were prepared from ‘Golden Delicious’ apples selected at harvest as less mature and more mature according to the absorption coefficient measured at 670 nm by time-resolved reflectance spectroscopy (TRS), stored in air for 5 months, and subjected to air-drying with (OSMO) and without (noOSMO) osmodehydration pre-treatment (60% sucrose syrup). Selected rings were submitted to microstructural analysis by X-ray computed tomography (X-CT), to subsurface structure analysis by optical coherence tomography (OCT) and to texture and sound emission analysis by bending–snapping test. Higher crispness index, higher number of sound events and higher average sound pressure level (SPL) characterized the OSMO rings. Total porosity was related to SPLav < 60, tissue and pore anisotropy to SPLav > 60, pore fragmentation index to fracturability and specific surface area to the work required to snap the ring. A differentiation of the drying treatments, as well as of the products according to the TRS maturity class at harvest was obtained analyzing by principal component analysis (PCA) microstructure parameters and texture and acoustic parameters. The differences in mechanical and acoustic characteristics between OSMO and noOSMO rings were due to the different subsurface structure as found with OCT analysis.Industrial relevanceThere is an increasing demand of dried crispy fruit as they are considered by consumers healthy, natural and tasty foods. The textural characteristics, exerting a strong effect on crispy and crunchy sensory characteristics, have a great impact at consumption of dried crispy fruit. As their textural characteristics depend on both fruit maturity at processing and processing conditions, food industry is demanding nondestructive techniques which could be used for on-line/off-line sorting of fruit into classes each one more suited for obtaining a specific product. Furthermore, the textural properties of dried foods depend on microstructure, defined as the spatial arrangement of structural components and their interactions. Due to the microscopic complexity, unambiguous methodologies that relate quality to food microstructure do not exist today, in contrast to what already existing for several engineering materials. Hence there is the need of developing methods that measure directly the microstructural properties of dried foods.