Graduate Thesis Or Dissertation
 

Survival Assessment, Inactivation Kinetics Models, and Farm-to-Table Quantitative Risk Analysis of Food Pathogens

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  • About 48 million cases of Foodborne illnesses (FBIs) are estimated to occur every year in the US. These are diseases caused by contamination of foods with bacteria, viruses, protozoa, parasites and toxic chemicals. Clostridium difficile infections (CDIs) are an important health-care and community associated problem. Since C. difficile spores have been frequently isolates from meat product, this pathogen should be considered an emerging food pathogen. Low temperature and reduced water activity (a[subscript w]) are commonly used methods to preserve meats and meat products. Chapter 3 covers studies on the viability of C. difficile spores during 4 months at -80°C, -20°C, refrigeration (4°C) and room temperature (20°C), and after ten freeze-thaw cycles (10 cycles). Two epidemic strains, R20291 (PCR ribotype 027, human isolate) and M120 (PCR ribotype 078, animal isolate), were used in this published study. At 20°C and -80°C, the spore viability of both strains declined with storage time. The viability of M120 spores also declined during storage at 4°C and 20°C. Although no significant changes were observed in the viability of R20291 spores stored at 4°C, their significant viability increase when stored at 20°C is most remarkable and raises food safety concerns. Finally, the viability of both strains decreased when subjected to ten freeze-thaw cycles. In summary, this study provides evidence that C. difficile spores can survive the room and low temperature conditions found in food processing, storage and distribution. Chapter 4 covers studies on the aw effect on the viability of C. difficile spores stored at room temperature. In addition to the spores of the strains previously described, spores of a food isolate strain, C. difficile DK1 (unidentified ribotype), were included in this study. The viability of their spores during 3 months at room temperature in phosphate buffer saline (PBS) at a[subscript w] of ~1.00, 0.82 and 0.72, and in commercial samples of beef jerky with a[subscript w] 0.82 and 0.72 was investigated. Spores of the strains M120 and DK1 showed a ~1 decimal reduction after 3 months in PBS kept at room temperature, while no significant changes in spore viability were observed in the reduced a[subscript w] beef jerky. The steady and significant 2 decimal increase in the viability of R20291 spores in PBS and ~1 decimal increase in the beef jerky during the 3-month storage raises again food safety concerns. This study provides evidence that beef jerky enhanced spore survival and viability during room temperature storage. Furthermore, it suggests that R20291 spores lost superdormancy but the mechanism involved remains to be determined. Changes in the surface hydrophobicity of C. difficile spores have been reported to be associated with the loss of the exosporium and an increase in their viability. In this study, the R20291 spore viability increase during room temperature storage was not associated with a change in surface hydrophobicity. Chapter 5 covers the determination and modeling of the thermal resistance of spores of the C. difficile strains M120 and DK1. They were treated at 70, 75, 80 and 85°C in PBS buffer and at 80°C in sterile ground beef. Linear 1.5 and 0.8 decimal reductions for strains M120 and DK1, respectively, were observed in PBS after 60 min at 70°C. At higher temperatures, non-linear inactivation reached 3.2 and 1.6 decimal reductions in 32 min at 75°C, and 3.1 and 2.9 decimal reductions in 24 min at 80°C, respectively. During come-up time to 85°C, a 1.1 decimal reduction was observed for M120 but none for DK1. Spores of M120 and DK1 strains in ground beef showed a 1.6 and 1.4 decimal ¬reductions, respectively, after 24 min at 80°C, i.e., a much lower value than in PBS. The modified Gompertz model for nonlinear microbial activation fitted the spore inactivation in PBS data better than the Weibull model yielding smaller maximum inactivation levels (A), faster maximum inactivation rates (μ[subscript max]), and shorter lag times (λ) for strain M120 than DK1. This study confirmed the high resistance of C. difficile spores suggesting the need to lower their presence in animal meat products. Chapter 6 focuses on the development of a model for the microbial inactivation achieved by a novel food processing technology, high pressure carbon dioxide (HPCD). The non-linear survival curves for the microbial inactivation by HPCD depend on the processing temperature and CO₂ pressure. In this study, the nonlinear inactivation of Escherichia coli CGMCC1.90 in apple juice by HPCD treatments was best described using a modified Gompertz primary model and the secondary models for its parameters b(T,P)=14.21+0.011P-0.67T+0.0085T² and c(T,P)= -0.10 +0.0023P+0.0037T (T and P in °C and MPa). Monte Carlo simulations were used to incorporate the variability and uncertainty of the parameter b and c estimates, which were then used to predict microbial inactivation values for a given time, temperature and CO₂ pressure combination and desired confidence boundary. The model predicts that HPCD processes can meet 5 decimal reduction at 95% confidence but at relative long apple juice processing times, i.e., 35-124 min treatments in the experimental temperature and pressure range of 32-42°C and 10-30 MPa, respectively. Risk assessments of food microbial safety have become a valuable tool to understand, analyze and control microbial risks. They address the pathogen source, load reduction by processing, and pathogen survival during storage and distribution in a quantitative manner. Chapter 7 describes a Monte Carlo based quantitative microbial risk assessment tool developed for the consumption of raw oysters potentially contaminated with the pathogen Vibrio vulnificus. It predicted that depuration at 15°C for 47 and 16 h would reduce the risk of oysters harvested during the warm (Jun-Aug) and transition seasons (Apr-May, Oct-Nov), respectively, to an acceptable level. Furthermore, the model predicted that the consumption of untreated raw oysters harvested during the cold season (Dec-Mar) can be considered an acceptable and very low risk. Moreover, the model can be adjusted by an individual processor to reflect the conditions at their specific location. This study demonstrated that Monte Carlo based models are an effective approach to use in quantitative risk management when considering the variability of multiple farm-to-table factors.
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