Bromothymol blue (BTB) was fixed on the cationic cellulose fibers (CCFs) to prepare pH sensitive fibers (pH-SFs). The pH-SFs as intelligent indicator were added into the semen cassiae gum (SCG) as a weakly acidic matrix to prepare a visual-olfactory film. The 13C NMR results show that the CCFs were successfully obtained by introducing hydroxypropyltriethylamine groups which showed strong affinity to BTB molecules. Rheology results demonstrated that all of the film-forming solutions were shear-thinned fluids with non-Newtonian behavior. Scanning electronic microscopy showed that the addition of pH-SFs makes the film surface rougher and rougher. The addition of pH-SFs < 3% improved the tensile strength of the film. The visual-olfactory film was sensitive to ammonia with a highly visible color change from pale yellow to blue-green. The SCG-3SFs film pre-treated in NaOH solution changed from blue to pale yellow while the raw milk was close to spoilage. The light yellow SCG-3SFs film changed to blue-green as the freshwater shrimp changed from fresh to spoilage. The results indicate that the visual-olfactory film can be used for perceiving the freshness of milk and freshwater shrimp.
Keywords: Ammonia response; Food freshness; Semen cassiae gum; Visual–olfactory film; pH-sensitive fibers.
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