On-line detection of Brownheart in Braeburn apples using near infrared transmission measurements

Two prototype on-line NIR transmission systems were used to non-destructively measure the percentage of internal tissue browning (ITB) in 'Braeburn' apples (Malus domestica Borkh.) afflicted with Brownheart. One system was based on the principle of time-delayed integration spectroscopy (TDIS) in which light transmitted through a moving object was electronically tracked as it moved through the spectrometer's field-of-view. The other, a large aperture spectrometer (LAS), was a more conventional design in which the light from the object is accumulated in a series of one-shot measurements as the fruit progresses through the field-of-view. The systems were each optimally configured to operate at typical grader speeds (500 mm s−1 or approximately five fruit per second) and detect the low levels of light diffusely transmitted through apples in the wavelength range 650-950 nm. Regression models developed by PLS calibration methods gave reasonable correlations with ITB (R2 ∼ 0.7-0.9) and low prediction errors (RMSECV ∼ 4-7%). The LAS system was superior in every case with the best results (R2 ∼ 0.9, RMSEP ∼ 4.1%) being obtained when two separate spectral measurements, made around the circumference of the fruit, were averaged. Multiple measurement LAS systems are recommended for fast on-line measurement of ITB in apples.