This work aims to investigate the emergence of aggregates caused by redundant plasticizers in the protein matrix of casein based biopolymers using chemical imaging techniques. Near infrared (NIR) images (950-1671 nm) were first acquired and the spatial variations on macroscale with a pixel size of 0.4 mm × 0.5 mm were visualized. The introduction of plasticizers resulted in a strong hydrogen bonding matrix in the protein polymeric film as evidenced by analysis of Fourier transform near infrared (FT-NIR) spectral profiles in the range of 7500-4000 cm-1. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) images (4000-650 cm-1) coupled with principal components analysis (PCA) and multivariate curve resolution alternating least squares (MCR-ALS) analysis suggested the existence of sorbitol re-crystallization after 5 months storage in the ambient condition. Raman images with a higher pixel size of 1.2 µm × 1.2 µm indicated an uneven film surface caused by sorbitol migration and re-crystallization. A partial least squares (PLS) regression model was developed to predict plasticizer concentration based on the mean spectra of FT-NIR hypercubes, producing coefficient of determination in calibration ( [Formula: see text] ) of 0.93, cross-validation ( [Formula: see text] ) of 0.92 and prediction ( [Formula: see text] ) of 0.89. Visualization of aggregates in the image field was obtained by applying the developed PLS model in a pixel-wise manner using single-element and array detectors. The combined information from NIR and FT-NIR evidenced the occurrence of high plasticizer-concentrated regions in the film sample, while the combined information from FT-NIR and ATR-FTIR further confirms the phenomenon of sorbitol re-crystallization.
Keywords: ATR-FTIR; Biopolymer; FT-NIR; Hydrogen bond; Raman imaging.
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