Effects of nisin, EDTA and salts of organic acids on Listeria monocytogenes, Salmonella and native microflora on fresh vacuum packaged shrimps stored at 4 °C.

Nisin (500 IU ml⁻¹), EDTA (0.02 M), potassium sorbate (PS) (3%, w/v), sodium benzoate (SB) (3%, w/v) or sodium diacetate (SD) (3%, w/v); alone or in combination were used to dip uninoculated shrimps and shrimps inoculated with Listeria monocytogenes or Salmonella (∼4.0-5.0 log CFU g⁻¹). Shrimps were then drip-dried, vacuum packaged and stored at 4 °C for 7 days. Untreated shrimps were used as a control. Numbers of L. monocytogenes, Salmonella and native background microflora were determined on uninoculated and inoculated shrimps on days 0, 3 and 7. Nisin-EDTA-PS and nisin-EDTA-SD significantly reduced (p < 0.05) L. monocytogenes numbers by 1.07-1.27 and 1.32-1.36 log CFU g⁻¹, respectively, on day 0 and 3. However, all treatments failed to significantly reduce (p > 0.05) Salmonella counts on shrimps throughout storage. On day 7, numbers of aerobic bacteria, psychrotrophic bacteria and Pseudomonas on combined nisin-EDTA-salt of organic acids treated shrimps were significantly lower (p < 0.05) by 4.40-4.60, 3.50-4.01, and 3.84-3.99 log CFU g⁻¹ respectively, as compared to the control. Dipping in organic acids solutions followed by vacuum packaging and chilled storage can help reduce L. monocytogenes and native microflora, but not Salmonella, on fresh shrimps.

The effects of temperature, chlorine and acids on the survival of Listeria and Salmonella strains associated with uncooked shrimp carapace and cooked shrimp flesh.

The purpose of this study was to assess the influence of the association of Listeria and Salmonella with shrimp surfaces on the effects of temperature, chlorine and acids on their survival. Planktonic, attached and colonized cells of Listeria monocytogenes Scott A, L. monocytogenes V7, Salmonella Senftenberg 1734b and S. Typhimurium ATCC 14028 were challenged with high (50 degrees , 60 degrees and 70 degrees C) and low (4 degrees C) temperature, 100 ppm sodium hypochlorite solution, and acetic, hydrochloric and lactic acids (pH 4.0). Attached and colonized Listeria and Salmonella showed significantly greater (p < 0.05) resistance to heat ( approximately 1.3-2.6 fold increase in D-values), hypochlorite ( approximately 6.6 >or= 40.0 fold) and acids ( approximately 4.0-9.0 fold) than their planktonic counterparts. There were no significant differences (p > 0.05) in the survival of planktonic, attached or colonized cells of Listeria and Salmonella stored under refrigerated conditions. The association of Listeria and Salmonella with shrimp surfaces enhances their resistance to heat, chlorine and acids. Both attachment to, and subsequent colonization of, shrimp surfaces by pathogens may reduce the efficacy of methods used in their control. Strategies to reduce attachment of these pathogens to shrimp are required to assure safety of this product.

Speciation of Gram-positive bacteria in fresh and ambient-stored sub-tropical marine fish.

This study identified Gram-positive bacteria in three sub-tropical marine fish species; Pseudocaranx dentex (silver trevally), Pagrus auratus (snapper) and Mugil cephalus (sea mullet). It further elucidated the role played by fish habitat, fish body part and ambient storage on the composition of the Gram-positive bacteria. A total of 266 isolates of Gram-positive bacteria were identified by conventional biochemical methods, VITEK, PCR using genus- and species-specific primers and/or 16S rRNA gene sequencing. The isolates were found to fall into 13 genera and 30 species. In fresh fish, Staphylococcus epidermidis and Micrococcus luteus were the most frequent isolates. After ambient storage, S. epidermidis, S. xylosus and Bacillus megaterium were no longer present whereas S. warneri, B. sphaericus, Brevibacillus borstelensis, Enterococcus faecium and Streptococcus uberis increased in frequency. Micrococcus luteus and S. warneri were the most prevalent isolates from P. dentex, while E. faecium and Strep. uberis were the most frequent isolates from P. auratus and M. cephalus. With respect to different parts of the fish body, E. faecium, Strep. uberis and B. sphaericus were the most frequent isolates from the muscles, E. faecium, Strep. uberis from the gills and M. luteus from the gut. This study showed a diversity of Gram-positive bacteria in sub-tropical marine fish; however, their abundance was affected by fish habitat, fish body part and ambient storage.

Effects of bilimbi (Averrhoa bilimbi L.) and tamarind (Tamarindus indica L.) juice on Listeria monocytogenes Scott A and Salmonella Typhimurium ATCC 14028 and the sensory properties of raw shrimps.

The potential of using juice of bilimbi (Averrhoa bilimbi L.) and tamarind (Tamarindus indica L.) to reduce Listeria monocytogenes Scott A and Salmonella Typhimurium ATCC 14028 populations on raw shrimps after washing and during storage (4 degrees C) was investigated. The uninoculated raw shrimps and those inoculated with approximately 9 log cfu/ml of L. monocytogenes Scott A and S. Typhimurium ATCC 14028 were washed (dipped or rubbed) in distilled water (SDW) (control), bilimbi or tamarind juice at 1:4 (w/v) concentrations for 10 and 5 min. Naturally occurring aerobic bacteria (APC), L. monocytogenes Scott A and S. Typhimurium ATCC 14028 counts, pH values and sensory analysis of washed shrimps were determined immediately after washing (day 0), and on days 3 and 7 of storage. Compared to SDW, bilimbi and tamarind juice significantly (p<0.05) reduced APC (0.40-0.70 log cfu/g), L. monocytogenes Scott A (0.84-1.58 log cfu/g) and S. Typhimurium ATCC 14028 (1.03-2.00 log cfu/g) populations immediately after washing (0 day). There was a significant difference (p<0.05) in bacterial reduction between the dipping (0.40-0.41 log for APC; 0.84 for L. monocytogenes Scott A and 1.03-1.09 log for S. Typhimurium ATCC 14028) and rubbing (0.68-0.70 log for APC; 1.34-1.58 for L. monocytogenes Scott A and 1.67-2.00 log for S. Typhimurium ATCC 14028) methods. Regardless of washing treatments or methods, populations of S. Typhimurium ATCC 14028 decreased slightly (5.10-6.29 log cfu/g on day 7 of storage) while populations of L. monocytogenes Scott A (8.74-9.20 log cfu/g) and APC (8.68-8.92 log cfu/g) increased significantly during refrigerated storage. The pH of experimental shrimps were significantly (p<0.05) decreased by 0.15-0.22 pH units after washing with bilimbi and tamarind juice. The control, bilimbi or tamarind-washed shrimps did not differ in sensory panellist acceptability (p>0.05) throughout the storage except for odour (p<0.05) attributes at 0 day when acidic or lemony smell was noticed in bilimbi- and tamarind-washed shrimps and not in control shrimps.

Relationship between the physicochemical properties of nonchitinolytic Listeria and Salmonella and their attachment to shrimp carapace.

Listeria and Salmonella are important foodborne pathogens normally associated with the shrimp production chain. This study investigated the potential of Salmonella Typhimurium, Salmonella Senftenberg, and Listeria monocytogenes (Scott A and V7) to attach to and colonize shrimp carapace. Attachment and colonization of Listeria and Salmonella were demonstrated. Shrimp abdominal carapaces showed higher levels of bacterial attachment (P < 0.05) than did head carapaces. Listeria consistently exhibited greater attachment (P < 0.05) than did Salmonella on all surfaces. Chitinase activity of all strains was tested and found not to occur at the three temperatures (10, 25, and 37 degrees C) tested. The surface physicochemical properties of bacterial cells and shrimp carapace were studied to determine their role in attachment and colonization. Salmonella had significantly (P < 0.05) more positive (-3.9 and -6.0 mV) cell surface charge than Listeria (-18 and -22.8 mV) had. Both bacterial species were found to be hydrophilic (<35%) when measured by the bacterial adherence to hydrocarbon method and by contact angle (theta) measurements (Listeria, 21.3 and 24.8 degrees, and Salmonella, 14.5 and 18.9 degrees). The percentage of cells retained by Phenyl-Sepharose was lower for Salmonella (12.8 to 14.8%) than it was for Listeria (26.5 to 31.4%). The shrimp carapace was found to be hydrophobic (theta = 74.5 degrees), and a significant (P < 0.05) difference in surface roughness between carapace types was noted. There was a linear correlation between bacterial cell surface charge (r2 = 0.95) and hydrophobicity (r2 = 0.85) and initial attachment (P < 0.05) of Listeria and Salmonella to carapaces. However, the same properties could not be related to subsequent colonization.

Storage Life of Chilled Scallops Treated with Low Dose Irradiation.

The effect of low dose irradiation on the chilled storage life of saucer scallops ( Amusium balloti ) was investigated. Scallops were packed in polythene bags and treated with doses of 1, 0.5, 1.5, and 3 kGy, then stored at 0°C. Sensory assessment indicated that nonirradiated scallops had a storage life of 13 d and this was extended to 18, 23 and 42 d when scallops were treated with 0.5, 1.5, and 3 kGy, respectively. Irradiation resulted in a 2 to 4 log reduction in bacterial numbers from an initial total viable count of 2.5 × 107/g. No irradiation-induced off-odors or off-flavors were detected, but a naturally occurring garlic-like off-flavor was reduced in intensity by irradiation treatment.