Abstract

Several near infrared (NIR) and mid-infrared (MIR) instruments were evaluated for the qualitative and quantitative analysis of raw apple juices prepared from different varieties and cultivars: (i) NIR with a dispersive monochromator using transmittance and a cuvette (NIR transmittance), (ii) NIR with a dispersive monochromator using transflectance (NIR reflectance), (iii) NIR with Fourier-transform interferometer and a transmittance fibre optic (FT-NIR), (iv) MIR Fourier-transform interferometer and a transmittance flow cell (FT-MIR). Principal component analysis (PCA) can be used to detect abnormal juices and outliers. The PCA scores can be used to provide a rough estimate of the characteristics of the juices. It is shown that the best analytical results are achieved by the FT-MIR technique and by NIR transmittance. The accuracy obtained in transflectance (NIR reflectance) or by the FT-NIR technique are lower. The analysis of the main components of apple juices (malic acid, total sugars, sucrose, glucose, fructose) leads to valuable analytical results [R² > 0.95 and standard deviation/standard error of calibration (SD/SEC) ratio ranging from 4.2 to 10.2]. Tannic acid is hard to predict whatever the technique of spectral acquisition. pH is also rather difficult to predict but can be used to perform a three groups classification (SD/SEC between 2.5 and 3.0). The specific gravity can also be predicted in most cases.

© 1997 NIR Publications

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