Detection of bruises on apples by near infrared reflectance spectroscopy

The objective of the study was to evaluate the ability of near infrared reflectance spectroscopy (NIR) to detect the bruises caused on apples by a definite compression test. Two cultivars ('Elstar' and 'Gala') and six stages of fruit development (from mid May to mid September) were compared. Spectra were first acquired on intact fruit. Two successive compressions at constant speed were then applied on each apple to reach a strain equal to 10% (first samplings) or 5% (samplings at harvest) of the fruit diameter. Two hours after compression, a second spectrum was recorded on the same face. The spectra were acquired on a NIR spectrometer at wavelength ranging from 800 nm to 2200 nm. Discriminative analyses (FDA) with cross validation procedure were applied on the spectral collections. In most of the cases, these analyses showed good discrimination between apples before and after compression. Nevertheless, the capacity of NIR spectroscopy to detect bruised apples and the more discriminative spectral absorption bands varied according to the cultivar and the stage of development. The relationship between the spectral response and the characteristics of the bruising event or the properties of fruit were studied.