The approach of dynamic covalent chemistry can be useful to develop active films with novel properties and functionalities. Several types of covalent bonds can be reversed in response to external stimuli and environmental factors such as pH. Among them, imines or Schiff bases (C=N) formed by condensation between carbonyl groups with amino groups, are of considerable interest. The application of imines linkage formed in a polymer are especially attractive due to its reversibility under external stimuli, which can be useful in the field of active food packaging for sustainable and control release of antimicrobial agents.

The main objective of this research was the application of the 'dynamic covalent chemistry’ concept to the development of pH-responsive chitosan films for its use as carrier of antimicrobial volatiles for active food packaging. Chitosan is a good model for the creation of imine linkage by condensation of antimicrobial aldehydes due to the presence of numerous amino groups along the polymer chain.

Natural-occurring aldehydes with proven antimicrobial activity (citral, citronellal, cinnamaldehyde and hydrocinnamaldehyde) were successfully grafted to the primary amino groups of chitosan films via imine linkage. Formation of the imine group was corroborated by spectroscopic techniques, the imination degree varying from 27 to 73%. Citral and cinnamaldehyde, α,β-unsaturated aldehydes, were the most reactive. The resulting polymers were characterized and their antifungal activity was tested at several pH values against common fungal food pathogens: B. cinerea and P. expansum. The antifungal response of films showed more activity at pH 4 than at pH 7 due a higher imine hydrolysis and subsequently, a greater aldehyde release. Unsaturated aldehydes showed higher antifungal activity than its saturated homologous.

The pH-responsive films showed considerable potential for use as stimulus-responsive sustained release systems and hence, they could have application in the design of active food packages