Mélika Mankai

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.

Characterisation and technological properties of psychotropic lactic acid bacteria strains isolated from Tunisian raw milk

A total of 102 wild lactic acid bacteria (LAB: 32%Lactococcus ssp.lactis, 23%Lactobacillus plantarum, 22%Lactobacillus pentosus, 7%Leuconostoc mesenteroides, 5%Lactobacillus brevis), isolated from Tunisian refrigerated raw milk obtained from collected centres, were screened according to their antimicrobial activities against spoilage and undesirable micro-organisms. Nineteen isolates having the most important antimicrobial activities were selected. Amplification of 16S/23S spacer regions of these selected LAB yielded two amplicons forLactobacillus genus and only one amplicon for bothLactococcus andLeuconostoc. However, three strains ofLactococcus and two strains ofLeuconostoc were eliminated because they present two amplicons. Hence, only fourteen strains were retained, they were also characterised in respect to their technological properties (lipolytic activities, proteolytic activities, enzymatic activities, acidifying capacity and antibiotic resistance). Acidification (degree Dornic) varied from 24.5±0.07 to 37.5±0.07 after 10 h. Low proteolytic capacity was detected for all strains using different substrates (oPA, TNBS and azocasein methods). Lipolytic capacity didn’t surpass 0.3±0.03% fatty acids for all strains. Lipase and esterase lipase activities were practically non-existent for most selected LAB. The behaviour of these strains against various antibiotics was also found to be similar.

Effects of Lactic Acid Bacteria and Citrus Essential Oil on the Quality of Vacuum-Packed Sea Bass (Dicentrarchus labrax) Fillets During Refrigerated Storage

ABSTRACT The preservative effect of refrigerated and vacuum-packed sea bass (Dicentrarchus labrax) fillets inoculated with four mixtures of lactic acid bacteria (LAB) (Lactococcus lactis, Lactobacillus plantarum, and Carnobacterium piscicola) and incorporated with citrus essential oil (CEO) was evaluated on the basis of microbiological and biochemical analysis. Initially, sea bass fillets showed high nutritional quality. During refrigerated storage, lipid contents did not show a significant decrease in any fish fillets; meanwhile, important proteolysis was observed in untreated control. In addition, results indicated that both CEO and LAB strains exhibited antimicrobial activity against spoilage, pathogenic, and fungi flora. Moreover, the total volatile bases (TVB-N) values were higher in control fillets, and the lowest TVB-N values were observed in Control CEO and C3 + CEO (30.47 ± 0.00 and 32.29 ± 1.12 mg TVB-N/100 g, respectively). Also, the levels of biogenic amines increased in all fillets without exceeding the upper limit of acceptability except for untreated control (sum of about 1396.63 ppm). Furthermore, this combined treatment ameliorated the muscle liquid-holding capacity, which improves technological properties. Overall, this treatment may open new promising opportunities for the biopreservation of fish products by enhancing the period of storage of refrigerated and vacuum-packed sea bass fillets.