NONDESTRUCTIVE DETERMINATION OF SOLUBLE SOLIDS IN APPLE FRUIT BY NEAR-INFRARED SPECTROSCOPY (NIRS)

The viability of a near infrared (NIR) method based on a dual-beam, fi bre-optic portable spectrophotometer was tested to determine soluble s olids non-destructively in apples (Malus domestica Borkh,). A total of 340 apples (190 of cv. 'Golden Delicious' and 150 of cv. 'Jonagold') sampled from two positions in the canopy to increase variation in solu ble solids concentration, were measured after 5-months storage and the measurements randomly assigned to a calibration data set and a predic tion data set. Thus the calibration set and the prediction set represe nted exactly the same distribution. The calibration data set was used to select the wavelengths best correlated with degrees Brix and to fit a multiple linear regression (MLR) equation that was applied to calcu late the degrees Brix value in the prediction data set. The most signi ficant R-2 (0.56) was found with the first derivative of log(1/R) (whe re R = reflectance), yielding standard error of calibration (S.E.C.) = 1.01 degrees Brix, standard error of prediction (S.E.P.)= 1.14 degree s Brix and bias = - 0.13 degrees Brix. When MLR was carried out separa tely on each cultivar, the R-2 value was higher for 'Golden Delicious' (0.65), but not for 'Jonagold' fruit. Analysis of variance performed on the actual and the predicted degrees Brix showed no difference in s tatistical significance, although the bias was higher (+/- 5% of actua l degrees Brix) at the extremities of the degrees Brix range. The scor ing of the fruits according to soluble solids content measured by NIR enabled correct classification of 72 and 76% of the fruits, with a thr eshold of 12 and 13 degrees Brix, respectively. This NIR method seems reliable for determining soluble solids contents non-destructively, an d could prove useful in the orchard. (C) 1998 Elsevier Science B.V. Al l rights reserved.