Unveiling the compositional remodelling of Arbutus unedo L. fruits during ripening

Abstract

• The architecture of the cell wall of strawberry tree ( Arbutus unedo L.) fruits, during four ripening stages, is herein elucidated for the first time. • Glucose and xylose are the major neutral sugars in the cell wall, similarly to other Ericaceae. • A. unedo fruits are unusually rich in lignin. • Multiple structures of aggregated sclereids are observed in the fruits, thus unveiling their peculiar gritty texture. • Sclereids are the source of high levels of xylose and lignin. Arbutus unedo is a Mediterranean shrub or tree that produces edible fruits with promising applications for the food industry and nutraceutical applications. A deep understanding of the cell wall structure and modifications of its components during fruit softening would benefit the development of new food products and retail of fresh fruits. Aiming to enhance the valorization of A. unedo fruits, its cell wall was characterized, for the first time, during ripening. In this study, four distinct ripening stages (green, yellow, orange and red) were considered. Several preliminary quality parameters were determined. The nature of the compounds making part of the cell walls were elucidated through Fourier‑transform infrared spectroscopy in attenuated total reflectance mode (FTIR-ATR) and spectral datasets were analyzed using unsupervised principal component analysis (PCA). High-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) was employed to assess monosaccharide composition, and lignin measurements were performed following the acetyl bromide soluble lignin methodology. In addition, scanning electron microscopy (SEM) analysis allowed to observe some relevant aspects of the microstructure of fruits. Significant modifications on the composition of these fruits were mostly found by the end of the ripening process when the fruits reached the red-ripe stage. Glucose, xylose and arabinose were determined as the major neutral cell wall sugars. Furthermore, the microscopy assessment unveiled the abundance of aggregated sclereids, and lignin measurements showed that A. unedo fruits are highly lignified. The richness in lignin and monosaccharides, such as glucose and xylose, may suggest the role displayed by that polyphenol and hemicelluloses like xylans and xyloglucans, which is supported by FTIR-ATR spectral assignments. It is hypothesised that the high amounts of xylose and lignin herein determined are highly associated with the sclereids. Relevant data were also obtained for the fundamental understanding of the ripening of other Ericaceae fruits and other berries.