Polyuronide content and correlation to optical properties measured by time-resolved reflectance spectroscopy in ‘Jonagored’ apples stored in normal and controlled atmosphere

Time-resolved reflectance spectroscopy (TRS) studies on fruit have shown that the absorption coefficient (μa) at 670 nm is a good maturity index and that the scattering coefficient (μs′) at 780 nm allows an insight into the textural properties of apples. This work aimed at studying the relationship between the polyuronide pattern, firmness and TRS optical properties (μa and μs′) at 630, 670, 750 and 780 nm measured in ‘Jonagored’ apples at harvest, after 6 months’ storage in normal (NA) and controlled atmosphere (CA), and after 7 days of post-storage shelf life at 20 °C. Results showed that fruit of different TRS maturity class had a different polyuronide content, even if their firmness was not different: ‘less mature’ (high μa630) fruit compared to ‘more mature’ (low μa630) ones showed at harvest higher total galacturonic acid (GA) content, residue insoluble pectin (RIP) and protopectin index (PI), and lower GA content in oxalate-soluble pectin (OSP) fraction, indicating a less advanced breakdown of insoluble protopectins to soluble pectins. The μa630 and μa670 were correlated to alcohol-insoluble substances (AIS) and GA content in RIP measured after storage: as maturity increased, GA decreased in RIP, showing pectin solubilisation.   AIS, water-soluble pectin (WSP), RIP and PI measured after storage were highly correlated to scattering coefficients measured after storage. Generally, with increasing μs′, AIS and GA in WSP increased while firmness, RIP and PI decreased. CA apples were characterised by lower μs′ values than NA ones, along with lower WSP and OSP and higher total GA, RIP and PI.