Incorporation of gelatin and Fe2+ increases the pH-sensitivity of zein-anthocyanin complex films used for milk spoilage detection

In this study, blueberry anthocyanins, gelatin and Fe2+ were incorporated into zein matrix via electrospinning method to prepare colorimetric indicator films for monitoring milk freshness. Gelatin and Fe2+ were incorporated into the film to improve visual discrimination of indicator films' color changes in milk with different freshness degrees and in solution with pH 3–7. Results of SEM, FT-IR and XRD showed that there were intermolecular hydrogen bonds among components, which associated with the larger color difference of indicator films. UV–vis spectral analysis showed that blueberry anthocyanin solutions containing both gelatin and Fe2+ displayed the highest intensity absorption peaks. The optimal ability to distinguish the pH (3–7) of solutions was presented by the indicator film incorporating gelatin (1% (w/v)) and Fe2+ (0.07 mg/mL). Gelatin and Fe2+ increased the color-responsive sensitivity of the indicator film to pH. The film could be successfully used to detect the freshness of milk, whose color changes were visually perceivable: from purple black (fresh milk) to royal purple (spoiling milk) and then to violet red (spoiled milk). The color parameters (L*, a*, R, G and B) of the film revealed a high correlation with the pH/acidity of the milk during storage. The successful application of the indicator film embedding gelatin and Fe2+ for monitoring milk quality changes indicated that the addition of special substances could provide great potential for monitoring freshness and preparing intelligent packaging of food.