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Title: Modelling death and growth of Bacillus cereus spores in Sous Vide processing
Author: Li, Hao.
ISNI:       0000 0001 3609 3120
Awarding Body: Leeds Metropolitan University
Current Institution: Leeds Beckett University
Date of Award: 2007
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Spores of psychrotrophic strains of Bacillus cereus have been of more and more concern in so us vide products due to their high heat resistance and the capability of the organism to grow at low temperatures. Laboratory media and real foods have been commonly used but the effects of the nutrient content of foods on the growth and death of microorganisms have not been well researched. The purposes of this study were to: (1) investigate the effect of environmental factors on the death and growth of B. cereus spores in Brain Heart Infusion (BHI) broth and simulated food during heating, chilling and storage, and (2) model and validate death and growth curves of B. cereus spores in BHI and simulated food during heating, chilling and storage. Central composition experiment design was used and a total of 38 heating/chilling experiments and 30 storage experiments were performed in BHI broth, and 53 heating/chilling experiments and 53 storage experiments in simulated food. The range of factors investigated were: heating temperature (75 - 95°C), storage temperature (4 - 12°C), pH (5.0 - 7.5), sodium chloride concentration (0.5 - 6.5%, w/v) , sucrose concentration (0 - 7.5%, w/v) , fat concentration (0 - 20%, w/v) and protein concentration (0 - 20%, w/v). The death of B. cereus spores significantly increased with a rise of heating temperature. Increasing pH and sodium chloride concentration produced protective effects on the heat resistance of spores, and resulted in an increase in shoulder time and a decrease in death rate. Above a temperature of 85°C, increasing sucrose concentration can reduce the death rate in BHI broth. Increasing storage temperature and pH value and lowering sodium chloride concentration stimulated the growth of B. cereus. Once growth occurred, the sucrose concentration did not show a significant effect on the growth rate and the final population of B. cereus in stationary phase. Compared with results from BHI broth, a relatively lower death rate of B. cereus spores II was observed in simulated food, and its effects were food composition dependent, i.e. higher fat and protein concentrations led to a lower death rate and a longer shoulder time. In addition, a remarkable tail phase phenomenon was observed in simulated food. For growth experiments, longer lag times and slower growth rates were found in simulated food than in BHI broth. A high protein concentration, i.e. ~15%, had a markedly inhibitory effect on the growth rate. Increasing fat concentration from 0% to 20% could definitely prolong the lag time. The death and growth of B. cereus spores was modelled by neural networks and mathematical models. Neural networks cannot be employed as the one-step model in modelling the death and growth of microorganisms because they are incapable of producing smooth death or growth curves and unexpected fluctuations or over-fitting are often observed in the modelling. Two new primary mathematical models, i.e. proposed models A and B were developed to model the death and growth curves of B. cereus spores. This work demonstrated that in terms of R2 and RMSE, the two models produced better fitting than previously established death models, for example the Whiting model and the Xiong model and growth models, for example the modified Gompertz model and the Buchanan model. In combination with response surface models and neural network models as secondary models, proposed models A and B were effective in predicting the death and growth of B. cereus spores in full model analysis.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available