Development of pH indicator and antimicrobial cellulose nanofibre packaging film based on purple sweet potato anthocyanin and oregano essential oil

A pH indicator and antimicrobial cellulose nanofibre packaging film was developed based on cellulose nanofibres as film matrix, anthocyanins from purple sweet potato as a natural dye and oregano essential oil as an antimicrobial agent. The colour of the anthocyanin solution was noticeably different from pH 2 to pH 12. The developed film sensitively responded to a change of pH and showed an obvious colour change from red to yellow, with pH ranging from 2 to 12, which indicated that the film had a superior colorimetric performance to pH change. Moreover, the film showed excellent antimicrobial activity and the inhibition rate against Escherichia coli and Listeria monocytogenes reached 99.99%. Bio-compatibility between anthocyanins and cellulose nanofibre matrix was confirmed by the analysis of cross-section morphology of the film. The XRD pattern revealed that the crystallinity index of the film decreased with the addition of anthocyanins and oregano essential oil. The film had a good barrier performance to ultraviolet and visible light; its tensile strength, elongation at break, and elastic modulus was 9.28 MPa, 9.87%, and 0.15 GPa, respectively. The developed pH indicator and antimicrobial film could intuitively indicate the change of food quality during storage and effectively prolong food shelf life.