Optimisation of stimuli-responsive films based on dynamic α,β-unsaturated imines from chitosan and trans-2-hexenal to enhance the antimicrobial acid-response.

Trans-2-hexenal (HX) is a potent antimicrobial which can be reversibly stabilised in chitosan (CS) films forming α,β-unsaturated imines. The hydrolysis of the imines promoted by acid environments triggers the release of HX to the media exerting its antimicrobial activity. It is known that besides imines, the electrophilic β-alkene carbon of HX can form Michael adducts with primary amino groups of chitosan. However, the formation of nucleophile-C bonds is undesired since these bonds are barely hydrolysed and limit the release of HX and by hence, the effectivity of the film. Thus, the aim of this work has been to optimise the formation of trans-2-hexenal-imine-chitosan films employing response surface methodology in order to favour the formation of conjugated imines avoiding Michael adducts. The optimisation of the reaction parameters indicated that synthesis temperature of 10 °C and without the use of an acid catalyst favours the formation of conjugated imines. Spectroscopic techniques, elemental analysis and swelling behaviour in various media were used to characterise the optimised films. The release kinetics of HX and the antimicrobial activity of the films were also studied. The present work provided relevant information to increase the antimicrobial efficacy of trans-2-hexenal-imine-chitosan films for the development of active food packaging.