Enhanced antimicrobial activity of starch-based film impregnated with thermally processed tannic acid, a strong antioxidant

Starch-based films impregnated with fresh tannic acid (FTA/starch film) and thermally processed tannic acid (PTA/starch film) were assessed for inhibition of Escherichia coli O157:H7 and Listeria monocytogenes. Disc-diffusion assay revealed that the PTA/starch film showed larger clear zone around the film on the bacterial lawn than the FTA/starch film at the same tannic acid concentrations (0.45 to 4.5mg per disc). Viable cell count assays in tryptic soy broth showed that the PTA/starch film also had a stronger antimicrobial activity on these foodborne pathogens than the FTA/starch film. L. monocytogenes did not replicate in trypic soy broth containing the FTA/starch film for the first 8h but multiplied up to 9.22 log CFU/ml at 48 h of incubation. The PTA/starch film caused a 2.72-log decrease in L. monocytogenes cells over the same time period. While 5-log E. coli O157:H7 cells were inactivated by the FTA/starch film within 48 h, more than 7-log E. coli O157:H7 cells were killed by the PTA/starch film over the same period. The antimicrobial activity of FTA/starch and PTA/starch film was primarily pH independent. HPLC measurement of the FTA or PTA release from starch film in water revealed that their release kinetic curves were in well match with their inactivation curves for E. coli O157:H7 and L. monocytogenes in 0.1% peptone water. In addition to antimicrobial activity, FTA showed antioxidant activity on soybean oil by doubling the induction time of oil oxidation. PTA further enhanced the oxidative stability of the oil by 17%. These results suggested that the use of processed tannic acid in starch films could improve the safety and quality of foods.