Green tea and grape seed extracts to control Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella Typhimurium in whey protein edible film and mechanism of action of the extracts in inhibiting Listeria monocytogenes and Escherichia coli O157:H7

Foodborne diseases caused by Listeria monocytogenes, E. coli O157:H7 and Salmonella Typhimurium have been associated with the consumption of a range of food products including ready-to-eat poultry and meat products. There is an increasing interest in natural therapies and consumer demand for effective, safe, natural products. Hence, it is important to investigate antimicrobial activities of plant extracts as potential sources of novel antimicrobials. Combinations of natural antimicrobials can serve as potential intervention strategies for controlling foodborne pathogens. Also, understanding mode of action of grape seed extract (GSE) and green tea extract (GTE) is important if they are to find commercial application as natural preservatives. Therefore, the objectives of this study were to evaluate the inhibitory effect of GSE and GTE combined with nisin (N) and malic acid (MA) in whey protein edible films against the three major pathogens in the turkey frankfurter system and to investigate the mode of action of GTE and GSE in inhibiting the pathogens. GSE and GTE had synergistic effects when combined with N and MA and lowered the Listeria monocytogenes population by 4.6 log cycles after 28 days in turkey frankfurters stored at 4 °C. No such synergistic effects between the extracts and N and MA were observed in Salmonella Typhimurium and E. coli O157:H7. MA alone lowered the Salmonella Typhimurium population by 3.3 log cycles compared to the control, while the combination of N, MA, GSE and EDTA lowered the counts by 2.0 log cycles only compared to the control. The E. coli O157:H7 population was lowered by 4.6 log cycles compared to the control, while combination of N, MA, GSE and EDTA lowered the counts by 3.4 log cycles. GSE and GTE had bactericidal effects on Listeria monocytogenes and E. coli O157:H7 at concentrations of more than 4 and 6 mg/ml, respectively. At bactericidal concentrations GSE and GTE resulted in a high percentage (90%) of cells with propidium iodide uptake in Listeria monocytogenes and E. coli O157:H7, respectively. Cell killing was accompanied by depletion in the ATP pools of the cells due to cellular cytoplasmic leakage, indicating that the bactericidal activity of GSE and GTE results from damage to the cytoplasmic membrane. Both GSE and GTE inhibited the membrane bound ATPase activity significantly at concentrations of more than 2 and 6 mg/ml in Listeria monocytogenes and E. coli O157:H7, respectively.