Mechanism of the antimicrobial activity of whey protein-ε-polylysine complexes against Escherichia coli and its application in sauced duck products

epsilon-Polylysine (epsilon-PL) is a natural and highly effective cationic antimicrobial, of which antibacterial activity is limited in food matrix because of epsilon-PL's charged amino groups that form complexes with food polyanions. Whey protein-epsilon-PL complexes delivery system was found to be able to solve the problem and keep the antibacterial activity. This study investigated the antibacterial activity of the complexes and its mechanism against Escherichia coli. The minimal inhibitory concentration of epsilon-PL was in the range 11.72-25.00 g/mL for the complexes containing different amount of epsilon-PL and was similar to that of free epsilon-PL. The results of scanning electron microscopy showed that the complexes could destroy the structure of E. coli cell membrane surface, leaving holes on the surface of the bacteria, leading to the death of the bacteria. The molecular dynamics simulation results showed that the mechanism of the antibacterial activity of the complexes was as follows: under electrostatic interaction, the complexes captured the phospholipid molecules of the bacterial membrane through the hydrogen bonds between the positively charged amino groups of epsilon-PL and the oxygen atom of the phosphate head groups of the membrane, which could create holes on the surface of the bacteria and lead to the death of the bacteria. The results of activity on real food systems showed that the complexes kept the number of E. coli within 5.8 log(10) CFU/g after 7 d storage in sauced duck products, while the positive control (epsilon-PL) was 6.5 log(10) CFU/g and negative control (sterile water) was 7.8 log(10) CFU/g. Overall, this study confirmed the antibacterial activity of the complexes and provided fundamental knowledge of its antibacterial activity mechanism.