Influence of culture conditions on esterase activity of five psychrotrophic Gram negative strains selected from raw Tunisian milk

The contamination of milk by spoilage bacteria is undesirable, particularly when Gram negative bacteria which produce thermo-resistant protease and lipase can grow. In this work, spoilage bacteria in refrigerated raw milk were identified, using API 20NE System. Five dominant species were found:Pseudomonas fluorescens (20%),Aeromonas hydrophila (16%),Pseudomonas cepacia (13%),Pseudomonas putida (6%) andChryseomonas luteola (5%). On the basis of agar diffusion assays, five strains harbouring the strongest lipases activities were selected. It has been found that esterase activities are higher for each one. Effects of main environmental and nutritional factors on the esterase activity of those psychrotrophic strains were investigated. Biomass level, pH, lactose concentration and permanent agitation affected positively esterase activity of each strain. However, the addition of Tween 20 influenced it negatively. Finally, and in order to extract information from the data sets, principal components analysis was applied to the data sets. The first two principal components showed a clear discrimination betweenPseudomonas fluorescens andPseudomonas cepacia.