Non-destructive assessment of pulp colour in mangoes by time-resolved reflectance spectroscopy: problems and solutions

Pulp colour in mangoes turns from greenish to deep orange with maturity, and is one of the most reliable maturity indices used in several production regions, even if it is a destructive measurement. Time-resolved reflectance spectroscopy (TRS) is a non-destructive technique which simultaneously quantifies the internal optical properties of fruit related to pigments (absorption coefficient, mu(a)) and to structure (reduced scattering coefficient, mu(s)') by probing the pulp at a depth of 1-2 cm with no or limited influence from the skin. TRS has been used to study the internal fruit attributes related to maturity; the mu(a) measured at harvest at 630-670 nm has been used to predict softening rate during shelf life in nectarines and mangoes. The results obtained in 2011-2012 seasons with Brazilian mangoes are presented analysing various aspects and problems of the TRS measurement for this type of fruit. The relationships between maturity at harvest measured by mu(a), TRS optical properties in shelf life, fruit softening and pulp colour were studied in 'Haden', 'Palmer' and 'Tommy Atkins' mangoes. 'Haden' and 'Palmer' fruit (less advanced maturity) could be sorted using mu(a)650-mu(a)690, near the chlorophyll absorption peak. In contrast, 'Tommy Atkins' fruit, which were selected for shipping (more advanced maturity), were successfully sorted according to mu(a)540, near the carotenoids absorption peak. It was confirmed that TRS non-destructively measures pulp colour, as shown by the high correlations between mu(a)540 and a*, hue and yellowness index (I-Y) and the good performances of PLS models based on mu(a)540-900 spectra for a*, hue and I-Y prediction.