Relationship between tissue firmness and optical properties of ‘Royal Gala’ apples from 400 to 1050 nm

• The optical properties of ‘Royal Gala’ apples from 400 to 1050 nm are presented.
• Acoustic firmness was not related to reduced scattering or absorption.
• The reduced scattering coefficient increased as fruit softened (penetrometer).
• Carotenoid and chlorophyll absorption peaks correlated with penetrometer firmness.
• The IAD technique is suitable for measuring apple tissue optical properties.

Absorption and reduced scattering coefficients were measured between 400 and 1050 nm for ‘Royal Gala’ apples using the inverse adding-doubling technique. Relationships between penetrometer, acoustic firmness and the optical properties are identified. Carotenoid (400–500 nm), and to a lesser extent chlorophyll-a (680 nm), absorptions were found to correlate with penetrometer firmness, with correlation coefficients equal to −0.69 and 0.52 respectively. The reduced scattering coefficient across all wavelengths increased as fruit softened. An average correlation coefficient of −0.68 was found between the reduced scattering coefficient (550–900 nm) and penetrometer firmness. No correlation was found between the AWETA acoustic firmness and either of the optical properties. A correction was developed to improve the correlation between penetrometer firmness and carotenoid absorption. This compensated for errors introduced by light loss in the inverse adding-doubling system and improved the correlation from −0.68 to −0.78.