Prediction of the optimal picking date of different apple cultivars by means of VIS/NIR-spectroscopy

The use of visible/near infrared (VIS/NIR) spectroscopy was evaluated to determine the internal quality and the optimal harvest dates of apples non-destructively. Calibration models were constructed with data from eight cultivars, three orchards and 2 years, in order to make the models as robust as possible for future use. The prediction of the maturity, defined as the number of days before commercial harvest, was reasonably accurate. The most robust model predicted the maturity with a validation correlation of 0.90 (SEP = 7.4 days). The prediction of maturity, according to the Streif index, showed a validation correlation of 0.84 (SEP = 0.18 kg/% brix x starch index) for one orchard. Maturity was orchard-dependent, however, and as a consequence, a combined prediction equation was not accurate. Individual quality characteristics (soluble solids, Streif index, acidity and firmness) were well predicted. The calibration model for soluble solids content resulted in a validation correlation of 0.84 (SEP = 0.73% brix) for the results over 2 years from one orchard, but like the Streif index, was orchard-dependent and appeared to account largely for the orchard dependence of the latter. Acidity and firmness were predicted with a validation correlation of 0.80 and 0.78 and SEPs of 2.07 ml NaOH and 1.13 kg for, respectively, two and three orchards over the 2 years. (C) 2001 Elsevier Science B.V. All rights reserved.