Date of Award
Doctor of Philosophy (PhD)
Dawson, Paul L.
Pometto III , Anthony
Rieck , James
Jiang , Xiuping
Escherichia coli O157:H7 and Listeria monocytogenes are pathogens that have received special attention by federal agencies, food safety researchers and food industries due to their economic and human health impact. To reduce the presence of these pathogens, alternative interventions have been studied. However, increasing consumer's demand for natural ingredients has made the investigations of effectiveness of natural antimicrobials necessary. In this study, in vitro antimicrobial activity of bromelain and papain against E. coli JM109 and L. monocytogenes was investigated. Furthermore, actinidin and papain were evaluated to reduce populations of L. monocytogenes strain and three mixed strains of E. coli O157:H7 in cooked meat media and on beef when held at three different temperatures.
In vitro, bromelain levels of 4 mg/ml and 1 mg/ml were the most effective concentrations tested against E. coli JM 109 and L. monocytogenes, respectively, at 25 and 35 °C, reducing the populations by (3.37, 5.02) and (5.7, 4.1 ) log CFU/ml after 48 h, respectively. While papain levels of (0.0625 mg/ml) and (0.5 mg/ml) were the most effective concentration tested at 25 and 35°C against E. coli and L. monocytogenes, respectively, reducing populations by (4.94, 5.64) and (6.58, 5.78) log CFU/ml after 48h, respectively.
While significant enzyme effects on bacterial populations in cooked meat media were found in this study, significant differences (P ≤ 0.05) were sometimes ≤ 1‐log unit, which are not typically considered of practical significance. However, due to the highly controlled nature of the study and that meat broth media exposes enzymes and bacteria to concentrated amounts of meat protein, the results may indicate positive results are possible when enzymes are applied to foods surfaces.
In cooked meat media, at 25 and 35°C, for all actinidin and papain concentrations there were not significant reductions found in E. coli O157:H7 and L. monocytogenes populations for any time-temperature combination. Moreover, there was bacterial growth from 1 to 3 h for 25 and 35°C. The bacterial growth at 35 °C was significantly higher than that at 25 °C. At 5°C, actinidin and papain did not significantly reduce the populations of E. coli O157:H7 and L. monocytogenes except for actinidin at 50 mg/ml on L. monocytogenes at 24 h. Also, no difference was found between bacterial populations at 6 and 24 h for both pathogens except for L. monocytogenes at 24 h where there was bacterial growth for both papain levels tested.
On beef, the average reduction of E. coli O157:H7 was ≥ that of L. monocytogenes and higher concentrations of either proteases yielded greater reduction in bacterial populations. For instance, actinidin at 700 mg/ml significantly (p ≤ 0.05) reduced the population of L. monocytogenes by 1.49 log cfu/ml after 3 h at 25 & 35°C, and by 1.45 log cfu/ml after 24 h at 5°C. Also, the same actinidin concentration significantly (p ≤ 0.05) reduced the populations of three mixed strains of E. coli O157:H7 by (1.81 log cfu/ml) after 3 h at 25 & 35°C, and by (1.94 log cfu/ml) after 24 h at 5 °C. While papain at 10 mg/ml reduced the population of L. monocytogenes by 0.56 log cfu/ml after 3 h at 25 & 35°C and by (0.46 log cfu/ml) after 24 h at 5°C. Also, the same papain concentration significantly (p ≤ 0.05) reduced the populations of three mixed strains of E. coli O157:H7 by 1.48 log cfu/ml after 3 h at 25 & 35°C, and by 1.57 log cfu/ml after 24 h at 5 °C. These findings suggest that, in addition to improving the sensory attributes of beef, proteases can enhance meat safety and shelf life when stored at suitable temperatures. The findings also propose a promising approach in developing antimicrobial systems for beef products. If these enzymes are combined with current antimicrobial technologies, higher pathogen reductions may be achieved if present.
Eshamah, Hanan, "Antibacterial effects of proteases on different strains of Escherichia coli and Listeria monocytogenes" (2013). All Dissertations. 1177.