Correlating optical maturity indices and firmness in stored 'Braeburn' and 'Cripps Pink' apples

Samenvatting

Non-destructive measurement of firmness is required by fruit industry for better fruit management. Aiming at non-destructively estimating firmness in apples, three optical maturity indices based on chlorophyll content were compared: the absorption coefficient measured at 670 nm (μa670) by time-resolved reflectance spectroscopy (TRS), the IAD measured by the DA-Meter and the NDVI measured by Pigment Analyzer. While the TRS probes the inner fruit tissue at a depth of ∼2 cm, the DA-Meter and Pigment Analyzer probe skin and the outer cortex of the apple fruit. 'Braeburn' and 'Cripps Pink' (Pink Lady®) apples were harvested at three dates (H1, H2, H3), stored for 6 months in controlled atmosphere and measured after 1 and 15 days of shelf life at 20°C. Thirty fruits per cultivar/harvest/day were individually analyzed for μa670, IAD and NDVI and for flesh firmness. The value of the optical indices was higher in 'Braeburn' apples for all harvest dates. The IAD and NDVI in both cultivars and the μa670 in 'Braeburn' were the highest for H1 and the smallest for H3, whereas μa670 in 'Cripps Pink' did not differ among the harvests. All the three indices decreased at the end of shelf life to the same value in 'Cripps Pink' whatever the harvest date, while in 'Braeburn' H1 apples kept the highest values. For firmness, the same trend of optical indices was observed for both harvest dates and shelf life in 'Braeburn', while in 'Cripps Pink' no differences in firmness were found among harvest dates and during shelf life. The μa670 showed a logarithmic correlation with the IAD and NDVI in 'Braeburn' apples while in 'Cripps Pink' these correlations were linear. The IAD and NDVI were highly and linearly correlated in both cultivars, indicating that they assessed the properties of the same tissue (skin) while μa670 explores deeper tissue layers, the pulp. In 'Braeburn', the firmness was well correlated with the μa670 and, with lower r, with the IAD and NDVI. In contrast, in 'Cripps Pink' no correlations were found between firmness and the optical indices.