Assessment of Titratable Acidity in Fruit Using Short Wave near Infrared Spectroscopy. Part B: Intact Fruit Studies

Interactance spectra (700–1100 nm) of intact fruit possess features interpreted as being due to the dilution and perturbation of water OH stretching and combination bands (with the second and third overtones of OH stretching at 960 nm and 740 nm, respectively), while the contribution from citric acid CH₂ and OH stretching and combination bands are not obvious. A model developed using interactance spectra collected from the cut surfaces of lime fruit (mean ± SD of 7.3 ± 0.51 g citric acid equivalents 100 mL⁻¹, units subsequently presented as% w/v) achieved prediction results of RMSEP=0.16%, r_p² = 0.79, bias = −0.03%. However, for intact lime fruit, model calibration results (RMSECV=0.16, r_cv² = 0.85) were markedly better than prediction results (RMSEP=0.30, r_p² = 0.49). For a low total titratable acidity (TTA) product, peach (with spectra collected across fruit maturity stages; mean ± SD of 0.88 ± 0.17%), calibration results were relatively poor (RMSECV=0.09%, r_cv² = 0.79) and the model failed in prediction (RMSEP=0.10%, r_p² = 0.00, bias = 0.02%). It is concluded that interactance geometry shortwave (700–1100 nm) near infrared spectroscopy using diode array instrumentation and an interactance optical geometry suited to on-line or field-portable instrumentation used for Brix and DM assessment is not appropriate for assessment of the acidity of intact low TTA fruit and has limited use for high TTA fruit.