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    Factors contributing to biofilm formation of Yersinia enterocolitica : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology, Massey University, Palmerston North, New Zealand
    (Massey University, 2016) Wang, Haoran
    Biofilms of pathogenic bacteria are recognised as a threat to food safety. The aim of the present study was to investigate the potential of Yersinia enterocolitica to form biofilms in the pork processing environment and identify the resistance of these biofilms to sanitation. The biofilm formation by Y. enterocolitica was monitored at conditions simulating pork processing environment under daily cleaning routine using an impedance method established in this study. Results showed that Y. enterocolitica had the potential to form biofilm and become resistant to sanitation in a pork processing environment. An investigation into the factors influencing biofilm formation of Y. enterocolitica indicated that the Ca2+ ion increased the level of biofilm formation. In addition, the presence of the virulence plasmid pYV is essential for the biofilm Ca2+ response. Further analysis of the bacterial cell surface properties and extracellular polymeric substance (EPS) composition suggested that the pYV+ cell surfaces are more negatively charged and more hydrophobic than the pYV- cells although no significant difference was observed with the addition of Ca2+. The pYV+ cells appear to produce more exopolysaccharide than the pYV- cells regardless of Ca2+ concentration. Ca2+ was able to increase the yield of extracellular DNA while the presence of pYV appeared to be dispensable in terms of extracellular DNA release. Analysis of cell wall protein revealed one protein expressed in the pYV+ cells but absent in the pYV- cells.
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    The effect of internal and external roasting temperatures on pork sensory properties, physical measurements and consumer liking : a thesis presented in fulfillment of the requirements for the degree of Master of Technology (Food Technology) at Massey University, Palmerston North, New Zealand
    (Massey University, 2001) Cumarasamy, Ravishankar
    The objectives of this research were twofold. Initially it was to quantify the effects of external (roasting) temperature and meat internal (end-point) temperature on the sensory and physical characteristics of selected cuts of pork. Secondly, to investigate Australian consumer preferences to selected cut and cooking condition combinations, and determine the sensory attributes that are most important for preference formation. A two factor central composite rotatable design with independent variables external temperature (120°C-200°C) and internal temperature (65°C-100°C) was used in this trial. A trained sensory panel evaluated the sensory differences of selected cuts (C-loin chop; F-fillet; LE-leg; LO-loin, SH-shoulder, SC-scotch) of cooked pork. Using response surface analysis the effects of these cooking conditions on pork sensory properties (initial and sustained juiciness, pork flavour, hardness, cohesiveness, chewiness) and physical measurements (evaporation loss (%), drip loss (%), cooking time (min/kg), Instron shear force (N), Hunter colour L*, a*, b*) were studied. Sensory attributes initial juiciness (C, F, SC), sustained juiciness (C, F, LE, SC), pork flavour (C, F), hardness (LE, LO, SH), cohesiveness (LE, LO, SH, SC), and chewiness (LO) showed a significant linear relationship with internal temperature. Except for hardness (C) and pork flavour (C, F) all the other sensory attributes showed no significant linear relationships with external temperature. Relationships were also observed between physical measurements and relevant temperatures depending on the cut used. The second stage of consumer evaluation (degree of liking) of selected pork samples was done in Brisbane, Australia and internal preference mapping was used to correlate the trained panel data with consumer data. The results from preference mapping indicated tenderness (hardness) to be the most important sensory attribute driving consumer liking. This segment of Australian consumers primarily liked tender meat that was also flavourful and juicy. Tenderness of pork is achieved at lower internal temperatures for smaller cuts and at higher internal temperatures for larger cuts. Increasing internal temperature also significantly increases cooking time. Therefore, the recommended internal temperatures for smaller cuts should be within the range 68-70°C and for larger cuts within the range of 80-85°C to optimise the sensory properties in accordance with the liking of this segment of Australian consumers. The recommended external (ET) and internal (IT) temperatures from this research are: Chop Roast- ET 160°C-170°C, IT 68°C-70°C; Fillet Roast- ET 160°C-170°C, IT 68°C-70°C; Leg Roast- ET 180°C-190*C, IT 80°C-85°C; Loin Roast- ET 180°C-190°C, IT 80°C-85°C; Shoulder Roast- ET 180°C-190°C, IT 80°C-85°C; Scotch Roast- ET160°C-170°C, IT 68°C-70°C.
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    A novel model developed for quantitative microbial risk assessment in the pork food chain : a dissertation presented in partial fullfilment [sic] of the requirements for the degree of Doctor of Philosophy at Massey University, Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand
    (Massey University, 2007) Titus, Simone Megan
    Food-borne diseases contribute substantially to morbidity and mortality rates worldwide. The deleterious impact of these diseases on human health, concurrent with the associated socioeconomic cost has led to an increased demand for the production of safe food globally. Consequently, agencies such as the World Health Organization (WHO) and the Food and Agriculture Organization (FAO) have resolved to address this issue. In this vein, scientific, risk-based approaches which facilitate estimation of the probability of disease occurrence, the magnitude of the disease and efficacious control measures have been recommended for use internationally. Many pathogens have been implicated as aetiological agents of food-borne disease. The WHO has identified non-typhoidal Salmonella, Escherichia coli and thermophilic Campylobacter as zoonotic food-borne pathogens of greatest importance. These pathogens can be transmitted to humans through pork consumption. This thesis therefore proposes a suite of novel, mechanistic, semi-stochastic, quantitative, modular process risk models describing the propagation of these three pathogens from the live pig at the abattoir, to pork chops sold at retail. The model is developed for use in risk-based, quantitative microbial exposure assessments in New Zealand and can be employed to explore different intervention strategies targeted at mitigating contamination levels of these pathogens on pork chops. The models comprise multiple, coupled, differential and difference equations. These equations explicitly describe bacterial growth, inactivation, removal, cross-contamination and food partitioning occurring in continuous and discrete time in abattoirs and at retail. Distributions of pathogen numbers on the surface of carcasses, and prevalence levels are output by the models at different stages of abattoir processing and pork chop production. Both dressed pork carcases exiting abattoirs in New Zealand and pork chops at retail are predicted to contain low surface contamination levels of the pathogens under consideration, while a small percentage is estimated to be highly contaminated. Median contamination levels on dressed pork exiting the abattoir are predicted to be less than one cfu/cm2. Generally, there are large reductions in surface bacterial numbers for all three organisms from the time the live pig enters the abattoir, to sale of the pork chop at retail. The introduction of a second singeing procedure immediately postevisceration in the abattoir is predicted by our models, to be an effective mitigation strativ egy, with estimated reductions in median pathogen levels of 100%. This control measure is considered to be more effective than coverage of the anal region of the pig during evisceration. This latter mitigation strategy was predicted to result in 10% – 44% reduction of median pathogen contamination levels. At retail, pork chops are also estimated to contain low numbers of these pathogens. Therefore handling of the raw pork chop soon after purchase from retail outlets may be associated with a low risk of contracting salmonellosis, colibacillosis and campylobacteriosis. This risk can be further reduced by placing pork chops in a blast chiller for 12 hours prior to display. When this mitigation strategy was modelled the outputs indicated a 15% – 61% reduction in the maximum pathogen levels on pork chops, 44 – 100% reduction in the 10th – 90th range and 14% – 50% reduction in pathogen prevalence levels. Detailed investigation revealed the limitations of a specific modelling approach. We determined that the population-based modelling approach is not an appropriate alternative to the individual-based modelling approach when there is a large disparity in contamination levels between processed carcasses. Therefore the former technique should not be used in the presence of large heterogeneity with respect to the number of bacteria on the food unit of interest, or when bacterial populations input into the model are described with large variances. This thesis demonstrates the application of a suite of novel risk models in the pork food chain. We propose use in quantitative microbial exposure assessments. The applicability of these models is not only limited to the pork chain or to the above mentioned pathogens, but by modification of parameters, the entire model, or portions thereof can be extrapolated to other animal species undergoing similar abattoir procedures with pathogens of analogous epidemiological patterns. Finally the information provided by the models can be instrumental in assisting risk managers in their decision-making and policy development undertakings and provide guidance to effectively and strategically funnel limited resources.