Combined effects of NaCl, NaOH, and biocides (monolaurin or lauric acid) on inactivation of Listeria monocytogenes and Pseudomonas spp.

This study highlighted combinations of chemical stresses that could decreas e or eliminate Listeria monocytogenes and Pseudomonas spp. surviving in foo d processing plants. Strains of L. monocytogenes, Pseudomonas fragi, and Ps eudomonas fluorescens isolated from processing environments (meat and milk) were grown at 20 degreesC up to the early stationary phase. The strains we re then subjected to 30 min of physicochemical treatments. These treatments included individual or combined acid (acetic acid), alkaline (NaOH), osmot ic (NaCl), and biocides (fatty acids) challenges. Survival of the strains w as studied after individual or combined acid (acetic acid), alkaline (NaOH) , osmotic (NaCl), and biocides (monolaurin, lauric acid) challenges. Indivi dual pH shocks had lower efficiencies than those used in combinations with other parameters. The treatment pH 5.4 followed by pH 10.5 had a low effici ency against L. monocytogenes. The opposite combination, pH 10.5 followed b y pH 5.4, led to a 3-log reduction of the L. monocytogenes population. Pseu domonas spp. strains were much more sensitive than L. monocytogenes, and po pulation reductions of 5 and 8 log (total destruction), respectively, were observed after the same treatments. As for L. monocytogenes, the combinatio n pH 10.5 followed by pH 5.4 is more deleterious than the opposite. Whateve r the bacterial species, the most efficient treatments were combinations of alkaline, osmotic, and biocide shocks. For instance, the combination pH 10 .5 and 10% NaCl plus biocides showed reductions of 5 to 8 log for both bact eria. The origins of the observed lethal effects are discussed.