An examination of the behaviour of Listeria monocytogenes during the storage and heat processing of shrimp (Pandalus borealis)

Perry, Lesley (2001) An examination of the behaviour of Listeria monocytogenes during the storage and heat processing of shrimp (Pandalus borealis). Masters thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (3MB)

Abstract

L. monocytogenes is ubiquitous in nature, has been isolated from many different types of food, has human health implications and can survive at very low temperatures. Additionally, the infectious dose of the pathogen is unknown, thus this bacteria is a concern in ready-to-eat seafood products. Currently, there is a lack of available information on the thermal resistance of Listeria monocytogenes in seafood, particularly in shrimp (Pandalus borealis), and on the pathogen's behaviour under different conditions of storage in seafood products. Furthermore, due to the lack of available data, it has been quite difficult for guidelines, regarding the processing times and temperatures required to control the presence of L. monocytogenes in foods, to be established. Hence, this study investigated the behaviour of L. monocytogenes on inoculated raw shrimp stored at -20°C, on ice (0-1°C), 4°C and 10°C and examined its thermal resistance in shrimp. Additionally, the total aerobic colony count and incidence of L. monocytogenes on raw shrimp was examined. -- Low levels of total aerobic bacteria, averaging 429.8 CFU/g, were found on the raw shrimp and no L. monocytogenes cells were isolated from the samples. For the storage experiments, shrimp samples were inoculated with low (10² CFU/g) and high (10⁶ CFU/g) levels of the bacteria and stored at the various temperatures in sealed plastic bags. Overall, L. monocytogenes was able to survive, but not grow, when stored on shrimp at -20°C and on ice (0-1°C). However, the pathogen was able to grow, quite significantly, when stored at temperatures of 4°C and 10°C, with generation times of 3.60 and 3.08 min at 4°C and 1.1 and 1.96 min at 10°C, for low and high inoculation levels, respectively. The highest increase in L.monocytogenes numbers was on shrimp inoculated with 10² CFU/g of the bacterium and stored at 10°C. Hence, refrigeration and freezing are no safeguards against this pathogen. -- For the thermal resistance portion of this study, the capillary tube method was used. Triplicate thermal inactivation trials were carried out and D and z values determined for temperatures of 65, 68, 70 and 72°C. L. monocytogenes were enumerated using both Listeriaselective agars, Oxford and PALCAM, and a non-selective agar, TSA. D values were calculated using D = -slope⁻¹, obtained from the linear regression analysis of the survivor curve (log₁₀ survivors vs. time). Average D values obtained in this study for L.monocytogenes in shrimp were 0.591, 0.266, 0.144 and 0.0202 min, for temperatures of 65, 68, 70 and 72°C, respectively. A z value of 5.07°C was also determined. Also, D values of 5.73, 2.31, 0.0063 and 0.00065 min were predicted for 60, 62, 75 and 80°C, using the D values and z value obtained from the experiments. The results of this study are compared to those of other studies.

Actions (login required)

View Item