Fig. 3. The profile of selected spectral regions of 11995.4~7498.1 cm1 before and after pretreatment by straight line subtIn order to further evaluate the prediction capability of the
models, an external validation was conducted by applying FT-NIRS
models to the 40 samples of prediction set. Table 2 shows that
adequate small RMSEP values were obtained for alcohol strength
and titratable acidity (4.25 mL/L and 0.213 mg/L), meanwhile the R2p
values (0.993 and 0.973) are closed to 1. The rate of prediction to
deviation (RPD) could be an indicator of model validation and a
higher RPD would be deemed better for prediction capability of
model. A good calibration model should have a RPD value higher
than 3.0 (Williams, 2001). A RPD value of 6.5e8.0 is generally
considered as good for quality control purpose (Botelho, Mendes, &
Sena, 2013). As shown in Table 2, RPD for the calibration models of
alcohol strength and titratable acidity (7.12 and 6.86) were
considered satisfactory. Additionally, the correlation plots between
chemical reference value and FT-NIR prediction value for the two
parameters of apple wine in prediction set are shown in Fig. 5. The
results indicate that the calibration models are adequate as a tool to
determine the alcohol strength and titratable acidity with accurate
performance during apple wine fermentation. Similar results were
obtained by Ye et al. (2014) for the determination of total acidity
with the determination coefficients of 0.98 and the RMSEP of 0.021
in apple wine by FT-NIRS transmission mode.raction (SLS) for developing calibration model of titratable
acidity in apple wine fermentation process (a: the original selected spectra; b: the spectra after pretreatment by SLS).retreatment by SD).