Spoilage and the presence of pathogens, such as Listeria monocytogenes and Salmonella, are among the most common reasons for shrimp product detentions and recalls. In addition, both listeriosis and salmonellosis have been associated with the consumption of shrimp. For these reasons, there is interest in reducing Listeria and Salmonella contamination of shrimps. A review of literature indicated no detailed study that described the attachment of Listeria and Salmonella onto shrimps and their resulting persistence. In addition, information related to the control of these two pathogens on fresh shrimps is also limited. The work reported in this thesis aims to partially fill these gaps. These aims were met through four studies. In the first study, the initial attachment and colonization of Listeria and Salmonella onto fresh and cooked shrimp carapaces and tissue was examined. The chitinolytic activity and the physicochemical properties namely cellular surface charge (CSC), hydrophobicity and electron donor/acceptor potential were also determined for all the strains. CSC was determined using zeta potential measurements. Hydrophobicity was determined using three hydrophobicity determination methods, while microbial adhesion to solvents was used to determine the electron donor/acceptor potential. Attachment and colonization of Listeria and Salmonella were demonstrated. Abdominal carapaces showed higher levels of bacterial attachment (p<0.05) than head carapaces while Listeria consistently exhibited greater attachment (p<0.05) than Salmonella on all surfaces. Chitinase activity of all strains was tested and found not to occur at the three temperatures (10°, 25° and 37°C) tested. Salmonella had significantly (p<0.05) more positive CSC than Listeria. A significant difference (p<0.05) in surface roughness between abdominal and head carapaces was noted. From the results obtained, it is possible to conclude that certain bacterial physicochemical properties and carapace roughness were involved in the attachment to carapaces but not attachment to tissue or colonization to carapace and tissue. The influence of attachment and colonization of Listeria and Salmonella onto shrimp surfaces on the resistance against environmental stress was investigated in the second study. Planktonic, attached and colonized cells of Listeria and Salmonella were challenged with high (50°, 60° and 70°C) and low (4°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 (~1.3-2.6 fold increase in D-values), hypochlorite (~6.6->40.0 fold) and acids (~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 increase in resistance observed in attached and colonized cells could have an important implication for shrimp product’s safety in general. In the third study, bilimbi (Averrhoa bilimbi L.) and tamarind (Tamarindus indica L.) juices were used to reduce Listeria monocytogenes Scott A and Salmonella Typhimurium ATCC 14028 populations on raw shrimps after washing and during storage (4ºC). The uninoculated raw shrimps and those inoculated with L. monocytogenes Scott A and S. Typhimurium ATCC 14028 were washed (dipped or rubbed) in sterile distilled water (SDW) (control), bilimbi or tamarind juice. Naturally occurring aerobic bacteria (APC), L. monocytogenes Scott A and S. Typhimurium ATCC 14028 counts of washed shrimps were determined on days 0, 3 and 7 of storage. Compared to SDW, bilimbi and tamarind juice significantly (p<0.05) reduced APC, L. monocytogenes Scott A and S. Typhimurium ATCC 14028 numbers on day 0. There was a significant difference (p<0.05) in bacterial reduction between the dipping and rubbing methods. Regardless of washing treatments or methods, populations of S. Typhimurium ATCC 14028 decreased slightly while populations of L. monocytogenes Scott A and APC increased significantly during refrigerated storage. These results suggest that consumable household items could be adopted as a natural method of decontaminating shrimps just before preparation and consumption. In the final study, use of nisin alone and in combinations with EDTA and salts of organic acids (potassium sorbate, sodium benzoate, or sodium diacetate) to control L. monocytogenes, Salmonella and native microflora on fresh shrimps were evaluated. Uninoculated, and Listeria monocytogenes or Salmonella inoculated, shrimps were dipped in treatment solutions, vacuum packaged and stored at 4ºC for 7 days. Listeria monocytogenes, Salmonella and native microflora counts were determined on days 0, 3 and 7. Nisin-EDTA-potassium sorbate and nisin-EDTA-sodium diacetate significantly reduced (p<0.05) L. monocytogenes numbers by 0.94-1.20 and 1.07-1.29 log CFU/g respectively, as compared to the control. All treatments failed to reduce (p>0.05) Salmonella counts on shrimps. At the end of storage, the native microflora counts on all nisin-EDTA-organic acids salts treated shrimps were significantly lower (p<0.05) than the control. The results suggest that some of the treatments can be used to improve shrimp microbial safety and shelf-life. Through achieving the aforementioned aims the present thesis was able to enhance the knowledge and literature available concerning the initial attachment of Listeria and Salmonella on shrimps, their persistence as well as methods to control them
| Identifer | oai:union.ndltd.org:ADTP/285423 |
| Creators | Wan Norhana Md Noordin |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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