Tracking post-storage apple age with polarized light measurements

A rotating retarder fixed polarizer (RRFP) polarimeter operating in the 600 – 1000 nm range was developed and used to measure the polarized reflected light response of post-storage apples. The degree of polarization (DOP) changes on two apple cultivars, Malus x domestica Borkh. (‘Scired’, marketed as Bay Queen®) and Malus x domestica Gala (‘Royal Gala’), was measured as the apples aged at room temperature and humidity (∼22 °C and 65%RH respectively) over an 18-day period. The DOP values were highest at 600 nm with observable peaks around the chlorophyll-related absorption at 675 nm, especially for the Bay Queen cultivar, and around the water-related absorption at 970 nm. The values between 700 and 900 nm were featureless and flat at half the peak values. The DOP values for circularly polarized light were about 50% lower over most of the spectral range than those for linearly polarized light. Overall, the DOP values decreased with apple age. There was a strong linear trend for individually tracked apples that was well correlated with non-destructive impact probe firmness measurements. The DOP loss rate was highest below 700 nm, peaking notably for Bay Queen at 675 nm, and is consistent with a probable loss with aging of visible-range pigment absorbers in the near surface region. In contrast, the DOP loss rate was spectrally near flat above 700 nm, suggesting there are no near-infrared absorber losses in the near surface region of the fruit. There was no evidence of an increased DOP loss rate near the water absorption peak at 970 nm, suggesting the water concentration in the near surface region is effectively constant over the 18-day aging period and despite a 5% total loss of water from the whole fruit. While the individually tracked apples trended linearly with age and correlated with non-destructive firmness, the relationships were very inconsistent between the apples. Also, no trends with age or correlations with firmness could be established for measurements on other untracked apples sampled independently from the same aging populations and tested destructively at each day. It is surmised that DOP reflectance measurements are unlikely to be useful for the direct and non-destructive prediction of internal fruit quality parameters such as firmness.