Efficacy of phage P100 on Listeria monocytogenes in refrigerated vacuum packaged cooked ham

Authors: Frank Devlieghere1 & Lieve Vermeiren2 Key words: phages, cooked ham, Listeria monocytogenes, safety, storage, shelf life, Abstract In this study we investigated the effect of an anti-Listeria phage product on outgrowth of Listeria monocytogenes in vacuum packaged cooked ham free from vegetative background flora. Cooked ham was made without organic acids such as lactate or acetate and aseptically sliced and packaged. The inoculation level was low (~17 CFU/g). Phage treatment was performed between slicing and packaging at two levels (1 x107 and 5 x107 PFU/cm2). Phage treatment kept Listeria monocytogenes below or at the detection level of 1 CFU/g after 28 (low treatment level) and after 42 days (high treatment level) whereas in the control levels exceeded 1 x102 CFU/g already after 14 days. In the treated group first outgrowth occurred after 8 weeks with high variation in contamination levels as some samples reached levels of ~107 CFU/g after 120 days. The research shows however that phages can be an effective food safety tool for cooked meat products.