Hydrogels based on gelatin: Effect of lactic and acetic acids on microstructural modifications, water absorption mechanisms and antibacterial activity

Lactic and acetic acids are applied as sanitising agents in meat processing since they present a low cost and are classified as Generally Regarded as Safe. However, solid antimicrobial agents are in demand by the industry since they may be easily handled. In this work, lactic and acetic acids were incorporated into enzymatically-crosslinked gelatin to produce antimicrobial hydrogels. Data on the minimum inhibitory concentration (MIC) of the free acids against Salmonella Typhimurium, Staphylococcus aureus, Pseudomonas aeruginosa and Escherichia coli was used to determine the concentration to be incorporated into the hydrogels. Chemical interactions between the carboxylic groups and the lactic and acetic acids of the hydrogels were demonstrated by infrared spectroscopy. Consequently, lactic acid caused microstructural modifications in the hydrogel, increasing the porosity and compacting the matrix walls. Application of mathematical modelling to the swelling kinetics indicated that diffusion was the predominant mechanism when the acids concentration was equal to six-fold the MIC of each acid. However, the relaxation mechanism dominated at low concentration (equal to the MIC of each acid). The hydrogels presented antimicrobial activity against Gram-negative (Salmonella Typhimurium, Escherichia coli and Pseudomonas aeruginosa) and Gram-positive bacteria (Staphylococcus aureus).